background statement for semi draft document 5313b...

Background Statement for SEMI Draft Document 5313B LINE ITEM REVISIONS OF SEMI MF1535-0707

TEST METHOD FOR CARRIER RECOMBINATION LIFETIME IN SILICON WAFERS BY NON-CONTACT MEASUREMENT OF PHOTOCONDUCTIVITY DECAY BY MICROWAVE REFLECTANCE

Notice: This background statement is not part of the balloted item. It is provided solely to assist the recipient in

reaching an informed decision based on the rationale of the activity that preceded the creation of this Document.

Notice: Recipients of this Document are invited to submit, with their comments, notification of any relevant

patented technology or copyrighted items of which they are aware and to provide supporting documentation. In this

context, “patented technology” is defined as technology for which a patent has issued or has been applied for. In the

latter case, only publicly available information on the contents of the patent application is to be provided.

Background

This standard is past due for 5-year review. After reviewing the document in its entirety, the task force found that

the titles of SEMI M1 and SEMI C28 need to be updated. In addition, the footnote referring to the cited DIN

standard is changed to indicate that it is no longer available from DIN. No other change was proposed. These

changes were balloted in cycle 5 of 2011. There were two negatives. The first objected to the retention of this

standard in preference to the new microwave decay standard developed in the Photovoltaic Committee (SEMI PV9).

The second had four sections: (1) use of the fit parameters to describe the carrier lifetime; (2) use of several

“definitions” of injection; (3) inclusion of a patented material for Passivation; and (4) failure to reference PV9 and

PV13, claimed to be the most up to date standards available on the topic.

At the Silicon Wafer Committee meeting on Tuesday, October 25, 2011, the first part of the second negative was

found to be persuasive by a vote of 3 non-persuasive to 2 persuasive (not a 2/3 majority) and the document was

returned to the task force. As a result the document was reviewed very extensively by both the Japan and NA

sections of the International Test Method task force. As a result of these reviews, it was concluded that:

1) TITLE: The Title of SEMI MF1535 is adequate, clear and appropriate, and there is no need to change it.

2) SCOPE: The scope section along with Note 1 is adequate in warning that even though S/N may be better at

high injection levels, value obtained in the test may be affected by the effects discussed.

3) TERMS: Injection and injection level are appropriate and well defined, and there is no need to change these.

4) REFERENCES: Both PV9 and PV13 relate to extensions of the measurement of carrier recombination on

multicrystalline and brick specimens, extensions not relevant to the high purity single crystal electronic

materials covered by this standard. So there is no reason to reference either of these in this standard. In fact,

PV9 references this standard twice, once in relation to the dependence of the measured carrier recombination on

injected carrier density and also in relation to surface recombination effects.

5) PASSIVATION TECHNIQUES: This standard does not require the use of a mentioned passivation technique,

which is patented. Because the patented technique is not required there is no patent issue in the document. In

fact, it is repeatedly mentioned in the published version of the standard that other passivation techniques may be

used and the criteria for their use are provided.

6) POSSIBLE NON PROPORTIONALITY BETWEEN MICROWAVE REFLECTIVITY AND

CONDUCTIVITY: A statement to this effect together with a reference to the paper by Schofthaler and Brendel

was added to ¶ A1-2.

This draft was balloted in cycle 3 of 2012 as document 5313A and reviewed at the meetings of the Test Method TF

and Silicon Committee during SEMICON® West in July 2012. Again one of the negatives was found persuasive

and the document was returned to the task force for further work.

A draft revision of Document 5313A was submitted for the review of the Japan Test Methods TF in August 2012.

The TF met on August 28 and reached the following conclusions about this draft:

This draft, described that the "recombination lifetime" is the response of limited condition with the microwave

decay lifetime measurement. The general description was given as "decay lifetime;” then "recombination

lifetime" is measured "decay lifetime" under the limited condition.

In contrast, the current document (MF1535-0707) is described "recombination lifetime" generally. Then, in this

document additionally noted that is described as "not recombination lifetime" in case of measurement under the

high level injection.

The JP TF felt that such a large revision was not necessary, because each document has a relation as both sides

of the shield. It was also felt that this revision has a large risk to giving the confusion to general user.

Therefore, the JP TF recommends no change from previous draft.

The present draft (document 5313B) has a few minor corrections and clarifications to the published standard,

including the addition of material in ¶3.8 that was discussed at SEMICON West, but no significant changes are

proposed. To emphasize the intent of the Task Force related to changes in certain sections of the document, the

ballot is being issued in the form of eight line items as follows:

Line item 1: Changes to ¶1.3.

Line item 2: Renumbering of certain paragraphs and notes in §2 Scope, and correction to Note 1.

Line item 3: Addition of ¶3.8 and reference 7.

Line item 4: Corrections in ¶¶4.1, 5.1, 6.1.2, 6.1.3, 6.4, 7.2, 8.4, 11.3.2, and 13.1.6, Footnote 9, and the

caption of Figure 3, and addition of reference 16 in §14.

Line item 5: Changes to Appendix 1.

Line item 6: Changes to Related Information 1.



Parts of the Document not specifically identified in the line items are not subject to comment or rejection. The

technical committee can determine that any comment or rejection that refers to material not included in the line-item

ballot(s) is ‘not related,’ but must either assign a not related negative to a TF or place it on the agenda of the current

committee meeting for consideration as new business.

Note: Additions are indicated by underline and deletions are indicated by strikethrough.

Review and Adjudication Information

Task Force Review Committee Adjudication

Group: International Test Methods TF Japan Silicon Wafer Committee

Date: 4th

December, 2012 6th

December, 2012

Time & Time zone: 09:00-12:00 13:00-17:00

Location: SEMI Japan Office, Tokyo Int’l Conference Hall, Makuhari Messe, Chiba

City, State/Country: Tokyo, Japan Chiba, Japan

Leader(s): Ryuji Takeda / Covalent Silicon Ryuji Takeda / Covalent Silicon

Standards Staff: Hirofumi Kanno Hirofumi Kanno

This meeting’s details are subject to change, and additional review sessions may be scheduled if necessary. Contact

the task force leaders or Standards staff for confirmation.

Telephone and web information will be distributed to interested parties as the meeting date approaches. If you will

not be able to attend these meetings in person but would like to participate by telephone/web, please contact

Standards staff.

This is a Draft Document of the SEMI International Standards program. No material on this page is to be construed as an official or adopted Standard or Safety Guideline. Permission is granted to reproduce and/or distribute this document, in whole or in part, only within the scope of SEMI International Standards committee (document development) activity. All other reproduction and/or distribution without the prior written consent of SEMI is prohibited.

Page 1 Doc. 5313B SEMI

Semiconductor Equipment and Materials International 3081 Zanker Road San Jose, CA 95134-2127 Phone: 408.943.6900, Fax: 408.943.7943

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DRAFT Document Number: 5313B

Date: 2012/10/19

SEMI Draft Document 5313B LINE ITEM REVISIONS TO SEMI MF1535-0707

TEST METHOD FOR CARRIER RECOMBINATION LIFETIME IN SILICON WAFERS BY NON-CONTACT MEASUREMENT OF PHOTOCONDUCTIVITY DECAY BY MICROWAVE REFLECTANCE

1 Purpose

1.1 If the free carrier density of a semiconductor is not too high, the carrier recombination lifetime is controlled by

impurity centers that have energies located in the forbidden energy gap. Many metallic impurities form such

recombination centers in silicon. In most cases, very small densities of these impurities (1010

to 1013

atoms/cm3)

reduce the carrier recombination lifetime and adversely affect device and circuit performance. In some cases, such

as very fast bipolar switching devices and high power devices, the recombination characteristics must be carefully

controlled to obtain the desired device performance.

1.2 This test method covers a procedure for measuring carrier recombination lifetime in a variety of types of silicon

wafers. Because electrical contact is not made to the wafer during the test, additional processing steps can be carried

out following the test if wafer cleanness is maintained.

Line Item 1: Update of ¶1.3

1.3 This test method is suitable for use in research and development, process control, and materials acceptance

applications. However, the results obtained by this test method depend on the measurement conditions utilized

including the degree of surface passivation. Therefore, when this test method is used for materials specification or

acceptance, the supplier and the purchaser need to agree on the measurement conditions including the surface

treatment for the test.

2 Scope

2.1 This test method covers the measurement of carrier lifetime appropriate to carrier recombination processes in

homogeneously doped, polished, n- or p-type silicon wafers. The room-temperature resistivity of the specimen

should be greater than a limit that is determined by the sensitivity of the detection system and is normally in a range

from 0.05–1 ·cm. This test method may also be applied to the measurement of carrier recombination lifetime in

as-cut, lapped, or etched wafers, provided that the sensitivity of the conductivity detection system is adequate.

2.2 In this test method, the decay of the wafer conductivity following generation of excess carriers with a light pulse

is determined by monitoring the microwave reflectivity of the wafer. Since no contact is made to the specimen, this

test method is nondestructive. If wafer cleanness is maintained, wafers may be further processed following testing

by this test method.

2.3 Measurement of the carrier recombination lifetime in the absence of surface recombination results in the

determination of the “ideal” bulk recombination lifetime (b). In general, however, it is very difficult to completely

suppress surface recombination, and the lifetime measurement is in most cases done without any surface passivation

or without confirming that the surface passivation is perfect. In addition, the injection level also affects the

measured lifetime value. If the injection level is low, the measured lifetime is not influenced by the injection level,

and the bulk recombination lifetime is the minority carrier lifetime (see Note 1). However, to enhance the signal-to-

noise (S/N) ratio, high-level injection is often adopted. Therefore, this test method also includes measurement of

carrier recombination lifetime under "non-ideal" conditions, where the surface recombination is finite and/or the

injection level is not low. Two such determinations are included:

2.3.1 The primary mode lifetime 1, which is influenced by both bulk and surface properties,

2.3.2 The 1/e lifetime e, which depends on both the injection level and the laser penetration depth as well as being

strongly influenced by the surface condition, because just after injection, the excess carriers are distributed near to

the surface. On the other hand, e is measured easily and quickly owing to good signal-to-noise ratio in the initial

part of the decay curve and simplicity of the data analysis.




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Line item 2: Changing Note 2 in §2 Scope to a paragraph, and correction to Note 1.

NOTE 1: Depending on the level of photoexcitation, the carrier recombination lifetime determined by this test method may be

the minority-carrier lifetime (low injection level) or a mixture of minority- and majority-carrier lifetimes (intermediate and high

injection levels). In the latter case, the minority and majority carrier lifetimes may be separated under some conditions if a single

recombination center that follows the Shockley-Read-Hall model, which applies for a small density of recombination centers, is

assumed. When the density of injected carriers greatly exceeds the density of the majority carriers (high injection), the

recombination lifetime is the sum of the minority and majority carrier capture time constants (see Related Information 1).

2.3.3 The above three lifetimes coincide (b=1=e) if the surface recombination is negligible and the injection level

is low enough. In the case that the surface recombination is negligible, b=1 even if the injection is not of low level

just after the laser irradiation.

2.4 NOTE 2: This test method is appropriate for the measurement of carrier recombination lifetimes in the range

from 0.25 s to >1 ms. The shortest measurable lifetime values are governed by the turn-off characteristics of the

light source and by the sampling frequency of the decay signal analyzer while the longest values are determined by

the geometry of the test specimen and the degree of passivation of the wafer surface. With suitable passivation

procedures, such as thermal oxidation or immersion in a suitable solution, lifetimes as long as tens of milliseconds

can be determined in polished wafers with thickness as specified in SEMI M1.

NOTE 3: NOTE 2: Carrier recombination lifetime of large bulk specimens can be determined by Method A or B of SEMI MF28.

These test methods, which are also based on measurement of photoconductivity decay (PCD), require electrical contacts to the

specimen. In addition, they assume large surface recombination on all surfaces and so the upper limit of measurable lifetime is

governed by the size of the test specimen. Method B of SEMI MF28 stipulates that the test be carried out under conditions of

low injection to ensure that the minority-carrier lifetime is determined. Minority carrier recombination lifetime can also be

deduced from the carrier diffusion length as measured by the surface photovoltage (SPV) method in accordance with Method A

or B of SEMI MF391. When carried out under low injection conditions, both the SPV method and the PCD method should yield

the same values of minority-carrier lifetime1 under certain conditions. First, it is required that carrier trapping does not occur.

Second, correct values of absorption coefficient and minority-carrier mobility must be used in analyzing the SPV measurements.

Third, surface recombination effects must be eliminated (as in the present test method) or properly accounted for (as in SEMI

MF28) in carrying out the PCD measurements. The generation lifetime, which is another transient characteristic of

semiconductor materials, is typically orders of magnitude larger than the recombination lifetime. Although SEMI MF1388

covers the measurement of the generation lifetime in silicon wafers, the recombination lifetime can also be deduced from

capacitance-time measurements made at temperatures above room temperature (70°C) using the same MOS capacitor structure.2

2.5 2.4 Interpretation of measurements to identify the cause or nature of impurity centers is beyond the scope of

this test method. However, some aspects of deriving this information from carrier recombination lifetime

measurements alone are discussed in the related information sections. Use of “injection level spectroscopy”3 is

discussed in Related Information 1 and use of the temperature dependence of the carrier recombination lifetime as

determined with low-injection level4 is discussed in Related Information 2. The identity and density of impurity

centers found to be present in the wafer by means of recombination lifetime measurements may usually be

determined more reliably from deep-level transient spectroscopy (DLTS) measurements made in accordance with

SEMI MF978 or from other capacitance or current transient spectroscopy techniques provided that a suitable catalog

of impurity characteristics is available.5

2.6 2.5 Metallic impurities that affect the carrier recombination lifetime may be introduced into the wafer during

various processing steps, especially those that involve high temperatures. Analysis of procedures for detection of

contamination sources (see ¶ 6.4) is beyond the scope of this test method. Although the test method is generally

nonselective, certain individual impurity species can be identified under very restricted conditions (see ¶ 6.3,

Related Information 1 and Related Information 2).

1 Saritas, M., and McKell, H. D., “Comparison of Minority-Carrier Diffusion Length Measurements in Silicon by the Photoconductive Decay

and Surface Photovoltage Methods,” J. Appl. Phys. 63, 4562–4567 (1988) 2 Schroder, D. K., Whitfield, J. D., and Varker, C. J., “Recombination Lifetime using the Pulsed MOS Capacitor,” IEEE Trans. Electron Devices

ED-31, 462–467 (1984) 3 Ferenczi, G., Pavelka, T., and Tüttô, P.,“Injection Level Spectroscopy: A Novel Non-Contact Contamination Analysis Technique in Silicon,”

Jap. J. Appl. Phys. 30, 3630–3633 (1991) 4 Kirino, Y., Buczkowski,, A., Radzimski, Z. J., Rozgonyi, G. A., and Shimura, F, “Noncontact Energy Level Analysis of Metallic Impurities in

Silicon Crystals,” Appl. Phys. Lett. 57, 2832–2834 (1990) 5 Schulz, M., ed, in Semiconductors: Impurities and Defects in Group IV Elements and III-V Compounds, Landolt-Börnstein, New Series III/22b,

(Springer Verlag, Heidelberg, 1989) § 4.2.3.1




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NOTICE: SEMI Standards and Safety Guidelines do not purport to address all safety issues associated with their

use. It is the responsibility of the users of the Documents to establish appropriate safety and health practices, and

determine the applicability of regulatory or other limitations prior to use.

3 Limitations

3.1 If the lifetime of the carriers is such that the carrier diffusion length is greater than 0.1 times the wafer thickness

the surface recombination influences the conductivity decay. Then the effects of recombination at the surfaces of

the wafer must be suppressed by thermal oxidation or by immersion in a suitable electrolyte (see § 11) to measure

the bulk recombination lifetime.

3.1.1 Treatments with electrolyte solutions must result in a stable surface for the test method to produce reliable

results.

3.1.2 A further caution is in order if thermal oxidation is employed. Particularly in high oxygen wafers, oxide

precipitates may form in the bulk of the wafer during oxidation. The presence of such precipitates can alter the

recombination properties of the wafer (see also ¶ 3.3) thus rendering the test specimen unsuitable for measurement

by this test method.

3.1.3 Externally gettered wafers may, under some conditions, yield erroneous values of carrier recombination

lifetime when measured by this test method. Results of measurements on such wafers should be checked very

thoroughly for validity.

3.2 The method is not suitable for measurement of recombination lifetime in very thin films of silicon. If the

thickness of the test specimen is comparable with or smaller than the inverse of the absorption coefficient of the

incident radiation, the decay curve may be distorted by the spatial dependence of the generation of excess carriers.

3.3 Variations in carrier recombination properties in the direction perpendicular to the wafer surface may result in

inaccurate determinations of the bulk recombination lifetime. These variations may arise because of the presence

(1) of p-n or high-low (p-p+ or n-n+) junctions parallel with the surface or (2) of regions of dissimilar recombination

characteristics (such as a wafer with oxide precipitates and a surface denuded region free of such precipitates).

3.4 The recombination characteristics of impurities in silicon are strongly temperature dependent. If comparisons

between measurements are to be made (i.e., before and after a process step or at a supplier and a customer), both

measurements should be made at the same temperature.

3.5 Different impurity centers have different recombination characteristics. Therefore, if more than one type of

recombination center is present in the wafer, the decay may consist of contributions with two or more time constants.

The recombination lifetime deduced from such a decay curve may not be representative of any of the individual

centers.

3.6 The recombination characteristics of an impurity center depend on the dopant type and density of the wafer as

well as the position of the energy level of the impurity center in the forbidden energy gap (see Related Information

R3-2).

3.7 Higher mode decay of photoinjected carriers influences the shape of the decay signal, particularly in its early

phases.6 These effects are minimized by measuring the decay after the higher modes have died away (beginning

below 50% of the maximum decay signal).

Line item 3: Addition of ¶3.8 and reference 7.

3.8 It has been shown by calculating microwave reflection transients after pulsed laser excitation7 that effective

carrier lifetime measurements require (a) low-injection conditions, (b) homogeneous carrier generation, and (c)

carefully adjusted reflector distance for linear response. When these precautions are observed, the contactless time-

resolved microwave reflection technique is a powerful and sensitive method for measuring effective carrier lifetimes

in semiconductors. The microwave decay signal may also be influenced by choosing a narrow spot size, as carrier

spreading may appear as a faster decay time.

6 Blakemore, J. S., Semiconductor Statistics, (Dover Publications, New York, 1987) § 10.4

7 M. Schofthaler, M., and Brendel R., “Sensitivity and Transient Response of Microwave Reflection Measurements”, J. Appl. Phys., 77, No. 7,

1 April 1995.




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4 Referenced Standards and Documents

Line item 4: Corrections in ¶¶4.1, 5.1, 6.1.2, 6.1.3, 6.4, 7.2, 8.4, 11.3.2, and 13.1.6, Footnote 9, and the caption of Figure 3, and addition of reference 16 in §14.

4.1 SEMI Standards

SEMI C28 — Specifications and Guideline for Hydrofluoric Acid

SEMI C35 — Specifications and Guideline for Nitric Acid

SEMI M1 — Specifications for Polished Monocrystalline Single Crystal Silicon Wafers

SEMI M59 — Terminology for Silicon Technology

SEMI MF28 — Test Methods for Minority-Carrier Lifetime in Bulk Germanium and Silicon by Measurement of

Photoconductive Decay

SEMI MF42 — Test Methods for Conductivity Type of Extrinsic Semiconducting Materials

SEMI MF84 — Test Method for Measuring Resistivity of Silicon Wafers With an In-Line Four-Point Probe

SEMI MF391 — Test Methods for Minority Carrier Diffusion Length in Extrinsic Semiconductors by Measurement

of Steady-State Surface Photovoltage

SEMI MF533 — Test Method for Thickness and Thickness Variation of Silicon Wafers

SEMI MF673 — Test Methods for Measuring Resistivity of Semiconductor Slices or Sheet Resistance of

Semiconductor Films with a Non-contact Eddy-Current Gage

SEMI MF723 — Practice for Conversion Between Resistivity and Dopant Density for Boron-Doped, Phosphorus-

Doped, and Arsenic-Doped Silicon

SEMI MF978 — Test Method for Characterizing Semiconductor Deep Levels by Transient Capacitance Techniques

SEMI MF1388 — Test Methods for Generation Lifetime and Generation Velocity of Silicon Material by

Capacitance-Time Measurements of Metal-Oxide-Silicon (MOS) Capacitors

SEMI MF1530 — Test Method for Flatness, Thickness, and Thickness Variation of Silicon Wafers by Automated

Noncontact Scanning

4.2 ASTM Standard8

D5127 — Guide for Ultra Pure Water Used in the Electronics and Semiconductor Industry

4.3 DIN Standard9

DIN 50440 — Measurement of carrier lifetime in silicon single crystals: carrier recombination lifetime at low

injection by photoconductivity method

4.4 JEITA Standard10

JEITA EM-3502 (JEIDA-53-1999) — Test Method for Carrier Recombination Lifetime in Silicon Wafers by

Measurement of Photoconductivity Decay by Microwave Reflectance

NOTICE: Unless otherwise indicated, all documents cited shall be the latest published versions.

5 Terminology

5.1 Definitions of terms used in silicon technology may be found Terms. acronyms, and symbols associated with

silicon wafers and silicon technology are listed and defined in SEMI M59.

8 Annual Book of ASTM Standards, Vol 11.01, ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428. Telephone:

610-832-9500; Fax: 610-832-9555; http://www.astm.org 9 This standard is no longer available in either a German or an English edition from Deutches Institut für Normung e.V., Beuth Verlag GmbH,

Burggrafenstrasse 4-10, D 10787 Berlin, Germany; http://www.din.de. The reference to this standard is included for historical reasons only. 10 Japan Electronics and Information Technology Industries Association, 3rd floor, Mitsui Sumitomo Kaijo Bldg. Annex, 11, Kanda-Surugadai

3-chome, Chiyoda-ku, Tokyo 101-0062, Japan. Telephone: 81.3.3518.6434; Fax: 81.3.3295.8727; http://www.jeita.or.jp

http://www.astm.org/

http://www.din.de/

http://www.jeita.or.jp/




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6 Summary of Test Method

6.1 Excess hole-electron pairs are locally created in the wafer for a very brief time by a short pulse (width 200 ns,

rise and fall times 25 ns) of light with energy slightly greater than the width of the forbidden energy gap at a

specified power density (injection level). The decay of the conductivity following cessation of the light pulse is

monitored by means of microwave reflectance, and the carrier recombination lifetime is determined as follows:

6.1.1 In the case that it is desired to obtain the bulk recombination lifetime, the specimen surface is conditioned in

such a way that surface recombination has a negligible effect on the decay of the conductivity, and the carrier

recombination lifetime is determined as the time constant of an appropriate portion of the exponential conductivity

decay.

6.1.2 The primary mode lifetime (τ1) is taken as the decay time constant determined from an exponential portion of

the decay curve.

6.1.3 The 1/e lifetime (τe) is taken as the time duration from the laser pulse injection to the instant that the excess

carrier density decreases to 1/e.

6.2 A narrow-beam light source may be used so that measurements may be made repeatedly at different localized

points on the wafer to obtain a map of the distribution of carrier recombination lifetime.

6.3 The measurement may be repeated at several different values of specific parameters, such as injection level

(light source intensity) or temperature in order to obtain more detailed information about the nature of the

recombination centers.

6.4 A process step that acts as a contamination source can sometimes be identified by comparing measurements of

carrier recombination lifetime (τe or τ1) made before and after the step.

7 Apparatus

7.1 Pulsed Light Source — A laser diode with wavelength between 0.9 and 1.1 m. Pulse length is nominally

200 ns, and the rise and fall times are 25 ns (see Note 3). It is preferred that the output power of the light source

be variable such that photon densities between 2.5 1010

and 2.5 1015

photon/cm2 are generated at the wafer

surface during the pulse.

NOTE 4: NOTE 3: The rise and fall times of the pulsed light source and the sampling time of the signal conditioner (see ¶ 7.4)

should be 0.1 of the shortest lifetime to be measured.

7.1.1 Photon Detector — It is often desirable that a suitable means, such as a semitransparent mirror in the light

path at an angle of 45° and a silicon photodetector, be used to provide feedback control to maintain the laser power

at a constant level appropriate to the specified injection level.

7.2 Microwave Pick-Up System — Including a microwave source operating at a nominal frequency of 10 ± 0.5 GHz

and an apparatus for measuring reflected power, such as a circulator, an antenna, and a detector (see Figure 1). The

sensitivity of the detection system shall be as great as possible to permit measurement of photoconductivity

microwave reflectance decay at low injection levels.

7.3 Wafer Mounting Stage — For holding the wafer (with vacuum hold down) in the desired position under the

pulsed light source. The stage may contain a heater for controlling its temperature over a small temperature range

above room temperature. It may be driven by computer-controlled motors to provide x-y or r- motion for mapping

capability over the wafer surface and may have automatic wafer loader and transport to facilitate automatic

sequential measurement of a group of wafers.

7.4 System for Analysis of the Decay Signal — Appropriate signal conditioners and display unit (real or virtual

oscilloscope with suitable time sweep and signal sensitivity). The signal conditioner shall have a bandwidth

40 MHz, or a minimum sampling time 25 ns (see Note 3). The display unit shall have a continuously calibrated

time base with accuracy and linearity better than 3%. The system shall be such that the time constant of user-

specified portions of the decay signal can be established independently.




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Figure 1

Example Block Diagram of Pulsed Light and Microwave Systems

7.5 Computer System — Although the measurement can be made manually, it is recommended that it be carried out

using a suitable computer system that controls the wafer loading, stage motion, the pulse and detector operation,

decay signal analysis, statistical analysis of the data, data logging and storage, and printing or plotting of results.

7.6 Facilities for Etching or Passivating Wafer Surfaces — If required.

7.6.1 For Chemical Passivation — A fume hood equipped with an acid-proof sink and suitable beakers or other

containers for holding wet chemicals, including hydrofluoric acid at room temperature and protective gear

appropriate to the chemicals used.

7.6.2 For Oxidation — A clean furnace capable of high quality dry oxidations at temperatures of 950–1050°C and

associated cleaning, drying, and wafer handling facilities.

7.7 Wafer Holder — If required. In some cases, it may be necessary to measure the wafer while it is immersed in a

passivating solution (see § 11). In this case, a flat chemically inert, optically transparent holder is required to

contain both the wafer and the passivating solution.

8 Reagents

8.1 Purity of Reagents — All chemicals for which such specifications exist shall conform to Grade 1 SEMI

specifications for those specific chemicals. Other chemicals shall conform to reagent grade, as specified in Reagent

Chemicals.11 Other grades may be used, provided it is first determined that the chemical is of sufficiently high

purity to permit its use without lessening the accuracy of the test.

8.2 Purity of Water — Reference to water shall be understood to mean Type E-3 or better water as described in

ASTM Guide D5127.

8.3 The recommended chemicals shall have the following nominal assays:

8.3.1 Ethanol (CH3CH 2OH) — Absolute, 99.9%.

8.3.2 Iodine (I2) — >99.8%.

8.3.3 Hydrofluoric Acid (HF) — Concentrated, 49.00 ± 0.25% in accordance with Grade 1 of SEMI C28.

WARNING: see ¶ 9.3 for warning statement.

8.3.4 Nitric Acid (HNO3) — Concentrated, 70.0–71.0% in accordance with Grade 1 of SEMI C36.

8.4 Iodine-Ethanol Passivating Solution — Iodine content 0.02 mol/L -– 0.2 mol/L.

11 Reagent Chemicals, American Chemical Society Specifications, American Chemical Society, Washington, DC. For suggestions on the testing

of reagents not listed by the American Chemical Society, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset, U.K., and

the United States Pharmacopeia and National Formulary, U.S. Pharmacopeial Convention, Inc. (USPC), Rockville, MD.




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NOTE 5: NOTE 4: Other passivating solutions may be utilized for measurement of the bulk recombination lifetime provided that

they (1) reduce the surface recombination velocity to a value at which surface recombination no longer interferes with the

determination of the bulk recombination lifetime (see ¶ 11.3), and (2) result in stable surfaces (see ¶ 6.1.1).

8.5 Bright Etching Solution, for Etching Non-Polished Surfaces — Mix 95 mL concentrated HNO3 with 5 mL

concentrated HF. WARNING: See ¶ 9.3 for warning statement.

8.6 Dilute HF Solution, for Etching Surface Oxide Films — To obtain 100 mL of a 2 % solution of HF, mix 4 mL

concentrated HF with 96 mL of water. WARNING: See ¶ 9.3 for warning statement.

9 Hazards

9.1 The laser illumination system should be interlocked so that direct observation of the laser beam is prevented.

WARNING: Do not operate the laser illumination system with the interlock disabled.

9.2 The microwave system should be shielded and interlocked so that personnel cannot come into contact with the

beam. WARNING: Do not operate the microwave system with the interlock disabled.

9.3 The chemicals used for etching and for some surface passivating solutions are potentially harmful and must be

handled in an acid exhaust fume hood, with proper protective gear including safety goggles, and with utmost care at

all times. WARNING: Hydrofluoric acid solutions are particularly hazardous. HF solutions should not be used by

anyone who is not familiar with the specific preventive measures and first aid treatments given in the appropriate

Material Safety Data Sheet.

10 Sampling

10.1 If the test method is not used on a 100% inspection basis, sampling procedures shall be agreed upon by the

parties to the test.

10.2 If sampling by lot is required, the determination of what constitutes a lot and the procedures for sampling and

by lot shall be agreed upon by the parties to the test.

10.3 Because the concentration of recombination centers in a wafer may be non-uniform, it is desirable to determine

the recombination lifetime at various points across the wafer surface. The point density and location of points

measured shall be agreed upon by the parties to the test.

11 Test Specimen Preparation

11.1 Prepare the test specimen only for making bulk recombination lifetime measurements. For other

determinations, proceed to § 12. For making bulk recombination lifetime measurements, the required test specimen

preparation depends on both the surface condition of the test specimen and the expected magnitude of the bulk

recombination lifetime, b, to be measured.

11.2 No test specimen preparation is required if the value of b is no greater than one-tenth of the surface

recombination lifetime, s. The surface recombination lifetime is composed of two terms, a diffusion term, diff,

which accounts for the diffusion of carriers to the surface, and a surface recombination term, sr, which accounts for

the recombination at the surface. The surface recombination lifetime may be computed from the following

approximate relation:12

S

L

D

Lsrdiffs

22

2

(1)

where:

D = Minority-carrier diffusion coefficient, in cm2/s,

L = wafer thickness, in cm, and

S = surface recombination velocity, in cm/s, assumed equal on both surfaces.

12 Horányi, T. S., Pavelka, T., and Tüttô, P., “In Situ Bulk Lifetime Measurement on Silicon with Chemically Passivated Surface,” Applied

Surface Science, 63, 306–311 (1993)




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Electron and hole surface recombination lifetimes are shown in Figure 2 as a function of surface recombination for

wafers with different thickness.13

NOTE 6: NOTE 5: If S is very large (>104 cm/s), excess carriers recombine immediately on striking the surface so the surface

recombination lifetime is dominated by diff. A well polished surface has a surface recombination velocity of ~104 cm/s12 while

for an abraded (lapped) surface the surface recombination velocity is even larger (~107 cm/s, the carrier saturation velocity). In

such cases, the maximum bulk recombination lifetime that can be measured to 10% accuracy in wafers of standard thickness is

about 1 s for p-type wafers and about 2 s for n-type wafers. In spite of this limitation of accurate determination of the bulk

recombination lifetime, it is possible to detect relative variations of bulk recombination lifetime on unpassivated polished wafers

that have bulk recombination lifetime as large as 0.5–1 ms provided that the following conditions are met:

1) the diffusion coefficient and surface recombination velocity are uniform over the wafer, and

2) the microwave system is sensitive enough to resolve measured lifetimes which differ by 1%.

Under these same conditions, relative measurements can be made on lapped wafers with bulk recombination lifetimes up to about

100 s. In this case, it may be necessary to etch the surfaces in bright etching solution (see ¶ 8.5) for about 1 min in order to

obtain sufficient uniformity of the surface recombination velocity.

11.3 The maximum bulk recombination lifetime that can be accurately measured is about 1/10 of the surface

lifetime. If bulk recombination lifetimes larger than about 0.1 s are to be measured, the wafer surfaces must be

passivated by one of the following methods (see Note 5) so that 0.1s > b is satisfied.

11.3.1 Oxidation — Bulk recombination lifetimes up to about 1 ms can be measured on wafers 0.5 mm thick that

have a very high quality (dry) thermal oxide (Dit < 1010

/cm2·eV) (see Note 6). Ensure that the oxidation conditions

are such that significant numbers of oxide precipitates do not form during the oxidation cycle (see ¶ 3.1.2). For

measurement of lifetimes between about 1 ms and 10 ms, strip the oxide in dilute HF (see ¶ 8.6) and make the

measurement within 15 min.

Figure 2

Surface Recombination Lifetime as a Function of Surface Recombination Velocity for Constant Diffusion

Coefficient and Selected Values of Wafer Thickness

11.3.2 Immersion in Passivating Solution — To measure bulk recombination lifetimes up to ~1 ms on a bare

polished wafer 0.5 mm thick, first pretreat the wafer in iodine-ethanol passivating solution (see ¶ 8.4) or an

alternative passivating solution (see Note 4). Then enclose the wafer in a small plastic bag or other fixture

containing enough of the passivating solution to coat the surface with a thin film while the measurement is being

made. Ensure that the passivation technique results in stable and repeatable measurements before proceeding with

the test (see ¶ 3.1.1).

11.3.2.1 If the wafer is oxidized, passivate it with iodine-ethanol, an alternative passivating solution, after first

removing the oxide by etching in dilute HF solution for a time that depends on oxide thickness; etch times range

from about 30 s for thin oxide (<5 nm) to about 10 min for thick oxide (~200 nm).

11.3.2.2 Again, ensure that the passivation technique results in stable and repeatable measurements before

proceeding with the test (see ¶ 3.1.1).

13 For these estimates, the diffusion coefficients were assumed to be constant at the following limiting values: Dn = 33.5 cm2/s and Dp = 12.4

cm2/s. These values are somewhat smaller than the limiting values given in the 1993 edition of DIN 50440, Part 1.




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NOTE 7: NOTE 6: Polished surfaces that have been oxidized or passivated with certain chemical solutions have much reduced

surface recombination velocity. For example, carefully prepared thermally oxidized silicon surfaces have surface recombination

velocity as low as 1.5–2.5 cm/s while the surface recombination velocity can be as low as 0.25 cm/s following stripping of the

oxide in hydrofluoric acid. 14 This reference also outlines a procedure for determining surface recombination velocity.

Immersion in the iodine-ethanol passivating solution (see ¶ 8.4) has been shown to reduce surface recombination velocity of a

chemically polished, oxide-free silicon wafer to 10 cm/s.12

NOTE 8: NOTE 9: The density of interface trapped charge (Dit) can be measured by a variety of techniques described in the

literature;15 however, none of these techniques has been standardized.

12 Procedure

NOTE 9: NOTE 8: The following procedures are given in sufficient detail for manual data collection and analysis. However it is

strongly recommended that instrument setup, data collection, and analysis be carried out using computer-controlled equipment,

with data storage and display capabilities. In such cases, the procedures and algorithms employed must be equivalent to those

given in this section.

12.1 If they are not known, determine the conductivity type in accordance with SEMI MF42, the center-point wafer

thickness in accordance with SEMI MF533 or SEMI MF1530, and the center-point resistivity in accordance with

SEMI MF84 or SEMI MF673. Convert the resistivity to the density of the majority carriers (nmaj, in carriers/cm3) in

accordance with SEMI MF723. Record these data together with the nominal diameter and the condition (polished,

etched, lapped, as-cut, etc.) of the front and back surfaces.

12.2 Record the temperature of the room, or if the stage is temperature-controlled, the temperature of the stage

surface.

12.3 Load the wafer onto the stage so that the light pulse strikes the desired region.

12.4 Switch on the pulsed laser light source (see ¶ 7.1).

12.5 Adjust the intensity as follows so that the injection level, , is at the specified value.

NOTE 10: NOTE 9 If the injection level has not been specified, preferably set it to less than unity. A higher injection level can

also be used if the injection level is accurately measured and reported. In general, a higher injection level has the advantage of

better signal-to-noise ratio.

NOTE 11: NOTE 10: If it is not adjusted automatically by the apparatus, the injection level may be set according to the

procedures in Appendix 1.

12.6 Turn on the microwave power source and view the photoconductivity decay on the display unit. Adjust the

time and voltage scales so as to display the desired portion of the decay signal.

12.7 Determine that the decay is exponential over the desired range. Determine the time constant by fitting an

exponential curve to the voltage, V, as a function of time, t, or (for manual data collection) a straight line to the

curve of lnV as a function of t.

12.8 Calculate recombination lifetime from the recorded decay curve, that is, the change in the reflected microwave

power V after the light injection (see Figure 3).

NOTE 12: NOTE 11: The lifetime may be obtained from a single measurement of the decay, but if the signal-to-noise (S/N) ratio

is not sufficiently high, it is recommended to repeat the measurement and average the decay curve.

12.8.1 Bulk Recombination Lifetime (b) — If the surface has been treated to reduce surface recombination velocity

in accordance with ¶ 11.3.2, calculate the time constant from a part of the decay curve that is exponential. In the

absence of indications to the contrary, use the portion of the decay signal in the range from 45–5% of the peak

voltage, V0.

NOTE 13: NOTE 12: The calculation is identical with that for primary mode lifetime (see ¶ 12.8.3).

14 Yablonovitch, E., Allara, D. L., Chang, C. C., Gmitter, T., and Bright, T. B., “Unusually Low Surface-Recombination Velocity on Silicon and

Germanium Surfaces,” Phys. Rev. Lett. 57, 249–252 (1986) 15 See, for example, Schroder, D. K., Semiconductor Material and Device Characterization (John Wiley & Sons, New York, 1990)

pp. 267–286




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12.8.2 1/e Lifetime (e) — Calculate the 1/e lifetime as the difference between t1 and t0, where t0 is the instant of

excess carrier injection into the specimen by the light pulse, and t1 is the instant that the reflected power decays to

1/e of the peak (see Figure 3).

12.8.3 Primary Mode Lifetime (1) — Calculate the primary mode lifetime as the time constant from a part of the

decay curve that can be regarded as exponential. If the reflected power is VA at t = tA and it decays exponentially to

VB=VA/e at t = tB, then 1 = tB – tA. Alternatively, calculate t2 – t1, where t1 is the instant that the reflected power

decays to 1/e of the peak and t2 is the instant that the power decays to 1/e2 of the peak, on condition that the curve in

this range does not largely deviate from an exponential curve (see Figure 3).

12.9 Record the type of recombination lifetime determined and its value.

12.10 If desired, move the wafer position and repeat ¶¶ 12.6–12.9 as required to obtain a wafer map, noting the

point spacing and pattern together with the radius of the mapped area.

12.11 Alternatively, if desired, repeat ¶¶ 12.2 and 12.6–12.9 at the same location for different temperatures or

repeat ¶¶ 12.2–12.9 at the same location for different values of injection level.

Figure 3

Decay Curve of Reflected Microwave Power and the Definition of Recombination Lifetime

13 Report

13.1 Report the following information:

13.1.1 Date and location of the test,

13.1.2 Instrument type, model number, and, if computer controlled, software version,

13.1.3 Portion of the decay signal from which the time constant was determined,

13.1.4 Injection level, , or the density of the incident photons, I, as established in ¶ 12.5,

13.1.5 Surface passivation procedure used (see § 11), and

13.1.6 Carrier recombination lifetime in s. It is required to clarify whether the measured lifetime is 1 or e.

13.1.7 If measurements were made at several injection levels, report the lifetime for each value of or I.

13.2 If necessary, report wafer description including any identification markings, center-point resistivity, center-

point thickness, conductivity type, surface condition (front and back), and nominal diameter.

time

Reflected microwave power

exponential

region

t0 tA t1 t2

VA

V0

tB

V1(=V0/e)

VB(=VA/e)

V2(=V0/e2)

1/e lifetime (e)

Primary mode lifetime (1)

time


exponential

region

time


exponential

region

t0 tA t1 t2

VA

V0

tB

V1(=V0/e)

VB(=VA/e)

V2(=V0/e2)

1/e lifetime (e)

Primary mode lifetime (1)




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13.3 If a wafer map was made, report the following information in addition to a density plot of the carrier

recombination lifetime:

13.3.1 Point spacing and pattern, and

13.3.2 Radius of the mapped area.

13.4 If measurements were made at a specific or several temperatures, report the temperature of each measurement.

14 Precision and Bias

14.1 Precision — The round robin test of this method was conducted from May 1996 to November 1996 to

establish JEIDA 53 now JEITA EM3502.16

The specimens were 150 mm-diameter CZ (100) thermally oxidized

wafers of both p- and n-type. Two conductivity levels were set between 3･cm and 47･cm and three lifetime

levels between 2 s and 1500 s, and total 12 kinds of wafers were characterized in the round robin test. 19

organizations (17 companies and 2 universities) joined the test. The center value of the standard deviation of errors

for each wafer is 9.6% (1) and 9.4% (e) for an injected photon density of 5 1013

photons/cm2.

14.2 Bias — No information can be presented on the bias of this test method because no material having an

accepted reference value of carrier recombination lifetime is available.

15 Keywords

15.1 contactless measurement; microwave reflection; photoconductivity decay; recombination lifetime; silicon

wafers

16 Miyazaki, M., Kawai, K., and Ichimura, M., “Measurement of Minority Carrier Recombination Lifetime in Silicon Wafers by Measurement of

Photoconductivity Decay by Microwave Reflectance: Result of Round Robin Test” Recombination Lifetime Measurements in Silicon, ASTM

STP 1340, D. C. Gupta, F. R. Bacher, and W. M. Hughes, eds (ASTM International, West Conshohocken, PA, 1998), PP. 347–366.




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APPENDIX 1 ADJUSTMENT OF THE INJECTION LEVEL

NOTICE: The material in this Appendix is an official part of SEMI [designation number] and was approved by full

letter ballot procedures on [A&R approval date].

Line item 5: Changes to Appendix 1.

A1-1 Injection Level Adjustment Procedure

A1-1.1 A1-1 If the test specimen is oxidized and the thickness of the oxide layer is not known, measure or estimate

it, using a method acceptable to the parties to the test. Determine and record the fraction of the incident light that

penetrates the oxide and is absorbed by the specimen from the dashed curve in Figure A1-1.

NOTE 13: For these calculations the wavelength of the incident radiation, is assumed to be 905 nm, the index of refraction of

silicon is taken as 3.610, and the index of refraction of SiO2 is taken as 1.462. Maximum absorption occurs at an oxide thickness

d = (2n+1)/4 while minimum absorption occurs at an oxide thickness d = n/2, when n = 0, 1, 2, etc. Therefore, if the

wavelength of the incident radiation, 1, differs from 905 nm, these curves can be used by determining the relative intensity for

an effective oxide thickness d 0 = 905 d1/ 1 where d1 is the actual thickness of the oxide.

NOTE 2: Figure A-1

Fraction of Incident Radiation Reflected from (solid line) or Absorbed in (dashed line) a Silicon Wafer

Covered with a Silicon Dioxide (SiO2) Layer between 0 and 1 m Thick

A1-1.2 A1-2 Adjust the light source intensity so that the photon density absorbed in the silicon during the pulse,,

is equal to nmaj, where is the desired injection level and nmaj is the density of majority carriers in the wafer as

determined in ¶ 12.1. The photon density, , in photons/cm3, is given by:

L

f

L

tf

I

t

I

p

0

d

(A1-1)

where:

f = the fraction absorbed found from Figure A-1

I = the intensity of the incident light, in photons/cm2·s,

tp = the length of the light pulse, in s, and

I = the photon density per pulse, in photons/cm2

L = wafer thickness, in cm.

A1-1.3 Typically, I is set to 5 x 1013

photons/cm2 ±20%, which corresponds to of the order of 10

14

photons/cm3 for a 1 mm thick wafer. For wafers with resistivity of 10 ･cm, for example, the majority carrier




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density is about 1015

cm3 and thus becomes of the order of 0.1. A higher value of I may also be adopted for

measurements of specimens with resistivity less than 1 ･cm.




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RELATED INFORMATION 1 INJECTION LEVEL CONSIDERATIONS

NOTICE: This Related Information is not an official part of SEMI [designation number] and was derived from the

work of the global [committee name] Technical Committee. This Related Information was approved for publication

by full letter ballot procedures on [A&R approval date].


R1-1 Effect of Injection Level on Carrier Recombination Lifetime

R1-1.1 R1-2 The carrier recombination lifetime is frequently associated with the minority-carrier lifetime. This

association is correct only if the lifetime is determined for low injection level when (the ratio of density of excess

photogenerated carriers to the equilibrium density of majority carriers) is much less than 1, and then only if certain

other conditions are also met (see Related Information 2 and Related Information 3). Nevertheless, the low-

injection (small-signal) value of the carrier recombination lifetime is independent of the exact value of provided

that << 1. However, in this test method, it is often neither possible nor convenient to make measurements in the

low-injection regime. When this is the case, the measured recombination lifetime is a function of the injection level.

R1-1.2 R1-2 The basic model for carrier recombination through defect centers in semiconductors was developed

independently by Hall17

and by Shockley and Read.18

This model has been thoroughly discussed by Blakemore.19

In the Shockley-Read-Hall (S-R-H) model, it is assumed (1) that the doping level of the semiconductor is not so

high that the semiconductor becomes degenerate, and (2) that the density of defect centers is small compared with

the majority carrier density.

R1-1.3 R1-3 The reader should refer to Blakemore’s text for a more complete treatment than is presented here,

including the derivation of the S-R-H expression (Equation R1-1) for carrier lifetime and discussion of the effect of

Fermi energy on the small-signal recombination lifetime. In addition, Blakemore goes on to discuss other

complexities that result when the density of defect centers is not small compared with the majority carrier density,20

and when carrier trapping occurs.21

R1-1.4 R1-4 Both the assumptions underlying the S-R-H model are generally appropriate for the specimens to be

measured by this test method. With these assumptions, the density of excess electrons (ne) is equal to the density of

excess holes (pe), and the electron (n) and hole (p) lifetimes for recombination through a defect center located at an

energy T within the forbidden energy gap are equal. This carrier recombination lifetime, , in s, is given as

follows:

)(

)()(

00

100100

e

epenpn

npn

nnnnpp

(R1-1)

where:

n0 = time constant for capture of an electron in an empty center, in s,

p0 = time constant for capture of a hole in a filled center, in s,

n0 = equilibrium density of electrons in a nondegenerate semiconductor, in electrons/cm3,

p0 = equilibrium density of holes in a nondegenerate semiconductor, in holes/cm3,

n1 = density of electrons in a nondegenerate semiconductor when the Fermi energy, F, = T, in electrons/cm3, and

p1 = density of holes in a nondegenerate semiconductor when the Fermi energy, F, = T, in holes/cm3.

R1-1.5 R1-5 In the low-injection limit, ne can be neglected and Equation R1-1 reduces to the small-signal

recombination lifetime, 0.

17 Hall, R. N., “Electron-Hole Recombination in Germanium,” Phys. Rev. 87, 387 (1952) 18 Shockley, W., and Read, W. T., “Statistics of the Recombination of Holes and Electrons,” Phys. Rev. 87, 835–842 (1952) 19 Blakemore, J. S., op. cit., § 8.3 20 Blakemore, J. S., op. cit., §§ 8.4 and 8.5 21 Blakemore, J. S., op. cit., § 8.2




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)(

)(

)(

)(

00

100

00

1000

pn

nn

np

pppn

(R1-2)

On the other hand, in the high-injection limit, ne is the dominant term and the recombination lifetime becomes:

00 pn (R1-3)

At intermediate injection levels the recombination lifetime can be expressed as a combination of 0 and:

1

)( 0

00

000

e

e

npn

npn (R1-4)

Therefore, a straight line is obtained when the quantity (1+) is plotted against . The zero intercept of this line is

0 and its slope is. The linearity of this function provides a test for the validity of the S-R-H model and also for

the presence of multiple defect centers in the test specimen.

Figure R1-1

Derivation of 0 and from Plots of Recombination Lifetime versus Injection Level

Table R1-1 Table 1 Parameters Used for Calculation of Recombination Lifetime versus Injection Level

Parameter Elemental Iron (Fe) Iron-Boron (Fe-B)

Temperature, K 300 300

Boron density (p0), cm3 1×1015 1×1015

n0, cm3 1.16×105 1.16×10 5

Iron density, atoms/cm3 5×1011 5×10 11

Defect energy, eV (above valence band edge) 0.400 0.100

n0, s 3.64 0.400

p0, s 30.3 3.33

n1, cm3 1.96×107 179

p1, cm3 5.91×1012 6.48×10 17

R1-1.6 R1-6 This linearity is illustrated in Figure R1-1 for recombination through the elemental iron defect center

in both n- and p-type silicon and through iron-boron pairs in p-type silicon. The parameters for the calculations are

listed in Table R1-1; in each case it is assumed that all of the iron is in the defect state listed. Note that for elemental

iron in p-type silicon, 0=n0, for the iron-boron pair,0>>n0; and for elemental iron in n-type silicon, 0=p0.

R1-1.7 R1-7 Injection level spectroscopy, which has been proposed3 as a method for identifying impurity levels,

relies on the relationship between 0 and as a function of the density of the impurity center for specific doping

conditions. This method is particularly useful for studying iron in p-type silicon because of the facts that (1) it is




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possible to treat the sample to ensure that essentially all of the iron is in either the elemental or paired state,22

and (2)

the injection level dependence is markedly different for the two species (see Figure R1-2).

Figure R1-2

Recombination Lifetime as a Function of Injection Level

R1-1.8 R1-8 However, it should be noted that if several contaminants are present at the same time at similar

concentrations, the measured ratio of 0 to may represent some average of the values for the various contaminants

because this technique is not impurity specific as is deep-level transient spectroscopy or other spectroscopic

techniques involving filling and emptying of defect centers in the space-charge layer.

22 Zoth, G., and Bergholz, W., “A Fast, Preparation-Free Method to Detect Iron in Silicon,” J. Appl. Phys 67, 6764–6771 (1990)




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RELATED INFORMATION 2 TEMPERATURE DEPENDENCE OF CARRIER RECOMBINATION LIFETIME





R2-1 Discussion

R2-1.1 The temperature dependence of the carrier recombination lifetime as determined under low-injection

conditions has been proposed4 as a means for identifying metallic impurities in silicon. However, this is possible

only for low injection and then only under very restricted conditions.

R2-1.2 R2-1.2 In the low-injection limit, the (S-R-H) carrier recombination lifetime is given by Equation R1-2. In

a nondegenerate semiconductor, the carrier densities, n0, p0, n1, and p1, are all exponential functions of temperature.

The equilibrium electron and hole densities, n0 and p0, respectively, are given as follows:

kTNn cF

c

exp0

and

kTNp Fv

v

exp0

(R2-1)

where:

Nc = the density of states in the conduction band, in states/cm3,

Nv = density of states in the valence band, in states/cm3,

F = Fermi energy, or the equilibrium electrochemical potential, in eV,

c = conduction band edge, in eV,

v = valence band edge, in eV,

k = Boltzmann's constant ( = 8.6173 × 105

eV/K), and

T = temperature, in K.

Similarly, the electron (n1) and hole ( p1) densities when the Fermi energy is at the defect center energy T are given

as follows:

kTn

kTNn FTcT

c

expexp 01

(R2-2)

and

kTp

kTNp TFTv

v

expexp 01

(R2-3)

R2-1.3 R2-1.3 From Equation R1-2, it is clear that the low-injection (or small-signal) carrier recombination lifetime,

0, can be calculated readily in terms of the electron and hole capture time constants, n0 and p0, as the sum of four

terms:

00

10

00

00

00

10

00

000

np

n

np

n

np

p

np

p ppnn

(R2-4)

In the temperature region between the freeze-out region and the intrinsic region where the majority carrier density is

equal to the net dopant density, the denominator of these terms is constant. If, in addition, the capture time constants

are assumed not to depend on temperature, the slope of the ln0 versus 1/T curve yields the defect center energy, T,

in those temperature regions where the defect centers are partially filled (that is, when a term in p1 or n1 dominates

the small-signal recombination lifetime). Although this assumption is usually not rigorously correct, the variation of

capture time constant with temperature is usually much less strong than the exponential dependence of the carrier

densities.




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R2-1.4 R2-4 Three examples, elemental iron in both n- and p-type silicon and iron-boron pairs in p-type silicon,

serve to illustrate these considerations. In each case the iron density is assumed to be 5 × 1011

atoms/cm3 and the

dopant density is assumed to be 1 × 1015

atoms/cm3, for p-type silicon this dopant density corresponds to a

resistivity 10–15 ·cm and for n-type silicon it corresponds to a resistivity 3–5 ·cm. The temperature

range considered is from 250–1000 K over which the dopant atoms may be assumed to be fully ionized. Elemental

iron is a donor center which, as shown in Table R1-1, lies well above the valence band edge in the bottom half of the

forbidden energy gap. The iron-boron pair is also a donor center but it lies much closer to the top of the valence

band. Consequently, in each case p1 >> n1, the difference being greater for the iron-boron pair.

R2-1.4.1 R2-4.1 Elemental Iron in p-Type Silicon (see Figure R2-1) — Below room temperature, p0 >> p1>> n1 >>

n0, so 0 = n0. Between about 150°C and about 200°C, p 1 > p0 > n0 and the term p1 is the largest single term.

However, because there is not much difference between p1 and p0, the slope of the 0 curve never quite reaches that

of the p1 term, and thus the energy of the elemental iron center cannot be determined accurately from the curve. At

still higher temperatures, n0 becomes comparable with p0 (approaching the intrinsic condition) and the carrier

recombination lifetime decreases; however, no single term dominates the expression and so the slopes have no

physical meaning.

Figure R2-1

Low-Injection Recombination Lifetime (solid curve) as a Function of

Reciprocal Temperature for Elemental Iron in p-Type Silicon

R2-1.4.2 R2-4.2 Elemental Iron in n-Type Silicon (see Figure R2-2) — Below about 100°C, n0 >> p1>> n1 >> p0, so

0 =p0. Between about 200°C and about 225°C, p1 > n0 > p0 and the term p1 is again the largest single term. In this

case, there is even less difference between the terms in p1 and p0, so the slope of the 0 curve is never dominated by a

single term, and thus the energy of the elemental iron center cannot be determined from the curve. At still higher

temperatures, p0 becomes comparable with n0 (approaching the intrinsic condition) and the carrier recombination

lifetime decreases; however, as for near-intrinsic p-type material, no single term dominates the expression and so the

slopes have no physical meaning.

R2-1.4.3 R2-4.3 Iron-Boron Pairs in p-Type (Boron-Doped) Silicon (see Figure R2-3) — In this case, p1 >> p0 >>

n0 >> n1, so that the term in p1 dominates the low-injection recombination lifetime at all temperatures from well

below room temperature to about 225°C. Since p0 = Nboron, 0 n0 (p1/Nboron), so that the negative slope of a plot of

ln0 against 1/T yields = FeB v, the activation energy of the iron-boron pair. At higher temperatures, the

material becomes near-intrinsic and the denominator increases, resulting in a decrease in 0.

R2-1.5 R2-5 These examples illustrate the limited range of conditions over which the activation energy obtained

from measurements of carrier recombination lifetime as a function of temperature slightly above room temperature

can be associated with the energy level of a defect center located near the middle of the forbidden energy gap, as are

most of the elemental metallic impurities.




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Figure R2-2


Reciprocal Temperature for Elemental Iron in n-Type Silicon

Figure R2-3


Reciprocal Temperature for Iron-Boron Pairs in p-Type Silicon




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RELATED INFORMATION 3 MINORITY-CARRIER RECOMBINATION LIFETIME





R3-1 Introduction

R3-1.1 R3-1The recombination of an excess electron-hole pair at a defect center is a two-step process. It involves

either (a) the capture of an electron in an empty defect center followed by the capture of a hole or (b) the capture of

a hole on a filled defect center followed by the capture of an electron. Thus, the recombination time depends on the

number of filled or empty defect centers as well as the capture time constants.

R3-2 Discussion

R3-2.1 R3-2The occupancy of the defect centers is controlled by the position of the Fermi energy as discussed by

Blakemore.19

If all of the defect centers are empty, the electron recombination time is governed by the electron

capture time constant, n0; this occurs when the Fermi energy is well below the defect center energy. If all of the

defect centers are filled, the hole recombination time is governed by the hole capture time constant, p0; this occurs

when the Fermi energy is well above the defect energy. Since the position of the Fermi energy is governed

primarily by the dopant density, the occupancy of the defect centers may be different for different resistivity wafers.

R3-2.2 R3-3There are two sets of equivalent cases. The defect center may be in the same half of the forbidden gap

as the Fermi energy or it may be in the other half. Thus, the case for the defect center in the lower half of the gap of

a p-type semiconductor is the same as for the defect center in the upper half of the gap in an n-type semiconductor

with the roles of electrons and holes interchanged. Similarly, the case for the defect center in the upper half of the

gap in a p-type semiconductor is the same as for the defect center in the lower half of the gap in an n-type

semiconductor.

R3-2.3 R3-4As examples, consider the three cases of elemental iron in both n- and p-type silicon and iron-boron

pairs in p-type silicon at room temperature (300 K). In each case the iron density is assumed to be 5 × 1011

atoms/cm3. Note that at for very low resistivity wafers, the nondegeneracy requirement of the S-R-H model may be

violated.

R3-2.3.1 R3-4.1Elemental Iron in p-Type Silicon (see Figure R3-1) — For all practical values of resistivity, the

Fermi energy lies many kT below the near mid-gap energy of the elemental iron center. Consequently, the defect

centers are empty and the limiting process is capture of excess minority electrons, there being ample numbers of

holes present to recombine with the electron immediately upon its capture by the defect center. Therefore, the

small-signal carrier recombination lifetime is equal to the electron (minority-carrier) capture time constant: 0 = n0.

R3-2.3.2 R3-4.2Elemental Iron in n-type Silicon (see Figure R3-2) — In this case, for all values of resistivity, the

Fermi energy is very far above the defect center energy and all the defect centers are filled, and hole capture is

governed by the hole capture time constant, p0. Because the Fermi energy is close to the conduction band edge,

there are large numbers of electrons present so that as soon as a hole is captured, the defect center is filled again.

Therefore, hole capture is the limiting process, and the small-signal carrier recombination lifetime is equal to the

hole (minority-carrier) capture time constant: 0 = p0.

R3-2.3.3 R3-4.3Iron-Boron Pairs in p-Type (Boron-Doped) Silicon (see Figure R3-3) — This defect center is

located quite close to the valence band edge. For all but very low resistivity wafers, the Fermi energy lies above the

defect center energy so that some of the defect centers are filled. In this case, the capture of minority electrons is

still the limiting process, but since only a fraction of the sites can capture an electron, the small-signal recombination

lifetime is larger than the electron capture time constant. Quantitatively, because n0 << p0, the small-signal carrier

recombination lifetime is given by 0 = n0(p1/p0).




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R3-2.3.4 R3-4.4The fourth possible case is one in which the defect center is located near valence band edge in n-

type material. The iron-boron pair, of course, cannot exist in n-type material; however, for the case of a hypothetical

defect center with energy near the valence band in n-type material, n0p1 >> p0n0 for all but the most heavily doped

material so this term dominates the small-signal lifetime and the minority-carrier properties do not influence the

recombination process.

R3-2.3.5 R3-4.5Thus, for deep lying defect centers, the low-injection (small-signal) carrier recombination lifetime

is equal to the minority-carrier capture time constant (or minority-carrier lifetime), while for centers close to the

band edge, the small-signal carrier recombination lifetime can be much larger than the minority-carrier capture time

constant.

Figure R3-1


Resistivity for Elemental Iron in p-Type Silicon at Room Temperature

Figure R3-2

Low-Injection Recombination Lifetime (solid curve) as a Function of Resistivity for Elemental Iron in n-Type

Silicon at Room Temperature




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Figure R3-3


Resistivity for Iron-Boron Pairs in p-Type Silicon at Room Temperature

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