bibliography - gistmaster/lecture/tft2009/c5-2(41-plasma... · plasma etching k - jistk - jist...

Plasma etchingPlasma etching

K - JISTK - JISTMaterials Science & EngineeringNanophotonic semiconductor Materials Lab.1

Bibliography1. B. Chapman, “Glow discharge processes”, (Wiley, New York, 1980).

- Classical plasma processing of etching and sputtering2. D. M. Manos and D. L. Flamm, “Plasma etching; An introduction”,

(Academic, Boston, 1989).- Most helpful textbook for the researcher majoring the dry etching.

3. M. Sugawara, “Plasma etching; Fundamentals and applications”, (Oxford Univ. Press, New York, 1998).- Mostly dedicated to the high density plasma sources such as ICP and ECR

4. W. N. G. Hitchon, “Plasma processes for semiconductor fabrication”, (Cambridge Univ. Press, Cambridge, 1999)- Theoretical approach to the plasma etching and plasma deposition process

5. R. J. Shul and S. J. Pearton, “Handbook of advanced plasma processing techniques”, (Springer, Heidelberg, 2000).- Helpful textbook for the researcher in the field of compound semiconductor process

6. http://newton.hanyang.ac.kr/plasma/- Dedicated to the plasma physics for graduate student in physics

http://newton.hanyang.ac.kr/plasma/

Contents


1. Introduction2. What is plasma??3. Reaction processes in plasma4. Mechanism of plasma etching5. Dry etch reactor6. Process requirement of dry etching7. In-situ diagnostic method of plasma etch8. Device damage from plasma9. Case study

9.1 Silicon etch9.2 Metal etch9.3 GaAs and InP etch9.4 GaN and related material etch

7. 1. Introduction


Etch removal of unwanted area during the fabrication of semiconductor

Etching is the most important step in the fabrication of semiconductor devices along with a lithography technique.

InGaN mesa for LED etched by ICPInP via-hole etched by RIE GaAs laser-facet etched by ICP


Dry etching by using plasma.Anisotropic feature profile – Fig. 2 (c) and (d)High aspect ratio etching – Fig. 1

Wet etching by using wet chemical solution.Isotropic feature profile – Fig. 2 (a)Low aspect ratio etching

44

Figure 1 Figure 2

Advantage of plasma etching


Etching can be anisotropicLess consumption of chemicals; cost, environment impactClean process (vacuum)Compatible with automationPrecise pattern transfer

Deep silicon etching for sensor application

Formation of sheath region


The fast-moving electrons hit the wall before the ions do and some stick to the wall.

The wall charges up negatively and this negative charge pushes other electrons away at the same time as attracting positive ions.

The field near the wall holds the electrons away from the wall and accelerates the positive ions toward the wall.

High energy ion bombardment

cf) Generally, the voltage drop couldn’t be measured. In practice process engineers usually monitor the dc potential (relative to ground) of the electrode instead, which is called “dc-bias”.


Processes in the sheath region

7. 4. Mechanism of plasma etching


(a) Physical sputtering - purely physical process by energetic ion bombardment

(b) Chemical etching - purely chemical process by forming volatile by-product through chemical reaction between substrate and active radicals in plasma

(c) Accelerated ion-enhanced etching -chemical etching + physical etching: removal of volatile product is accelerated by energetic ion bombardment

(d) Sidewall-protected (inhibitor driven) ion-enhanced etching – deposition of etch-resistant layer with ion bombardment isotropic etching

7. 4. 1. Etch mechanism

7. 4. 2. Sequential steps in etching


① Formation of active etchant by electron collisions

② Transport of active etchant to the wafer surface

③ Adsorption of etchant to wafer surface④ Reaction of etchant and wafer to form

etch-product⑤ Desorption of etch-product form the

wafer surface⑥ Acceleration of desorption of etch-

product by ion bombardment⑦ Transport of etch-product to the bulk

plasma ⑧ Redissociation of etch-product in the

plasma or pumped out⑨ Redeposited on the reactor wall or

pumped out

cf) If any of these steps fails to occur,the overall etch cycle ceases and the step failed is a rate-limiting step

7. 4. 3. Radicals in plasma


Radicals are generated through dissociation and ionizationex) e + O2 O+ + O* + 2e,

e + CF4 CF3+ + F* + 2e

Radicals are much more abundant than ions in plasma because;(1) They are generated at a higher rate due to;

- lower threshold energy and ionization is often dissociative(2) Radicals survive longer than ions

Although the concentration of radicals is much larger than that of positive ions, the reactive fluxes incident on the surfaces can be comparable, since ions are moving faster because they have large energy obtained from the electric field in the sheath.

7. 4. 4. Volatility and evaporation


Volatility of etch-products is a key distinction between plasma etching and sputtering.In general, desorption is a rate-limiting steps in the plasma etching Highly volatile by-product formation is important.Evaporation rate of material (a) of molecular weight Ma is proportional to its vapor pressure, pa, (refer to Chap. II)

The evaporation rate is increased with increasing temperature. However, plasma etching generally done at room temperature.

formation of volatile product at RT is most important.

RTHaaa

RTHaaa

aa

eRT

MC

eCppRT

M

∆−−

∆−−

⎥⎦⎤

⎢⎣⎡=∴

=⎥⎦⎤

⎢⎣⎡=

21

21

2

, 2

παµ

παµ

Boiling point of etch product (Si and metal)


Etch product Boiling point (℃) Comment

Si(SiO2, Si3N4)

SiH4 -111.6 Gas at RTSiF4 -95.7 Gas at RTSi2H6 -15 Gas at RT

SiHCl3 31.7SiCl4 56.7

Si2OCl6 135.5Si2Cl6 147

Metal(Ag, Al, Ti, Au, Co, Cr, Cu, Ni,

Pb, Pt, Ta, W, Zn)

AgCl 1550AlCl3 182.7 SublimationTiCl4 136.45TiF4 284 Sublimation

Au2Cl3 - Non volatileAu2Br3 - Non volatileCoCl2 1050

CrO2Cl2 117Cr(CO)6 151CuCl2 655 Non volatileCuBr2 900 Non volatile

Ni(CO)4 -25PbCl2 954PtF6 69.1TaF5 229.5WF6 17 Sublimation

(CH3)2Zn 46ZnCl2 756 Non volatile

Boiling point of etch product (III-V semiconductor)


Etch product Boiling point (℃) Comment

III-V semiconductor(GaAs, InP, GaN)

Ga2H6 -63 Gas at RTGaCl3 201.3GaCl2 535GaF3 ~ 1000GaBr3 279GaI3 < 345

(CH3)3Ga 55.7(C2H5)3In -32 Gas at RT(CH3)3In 88

InCl3 418 SublimationInBr3 371 SublimationAsH3 -54.8 Gas at RTAsF5 -52.9 Gas at RTAsF3 63AsCl3 130.4AsBr3 221PF3 -101 Gas at RTPH3 -88 Gas at RTPF5 -75 Gas at RTPCl5 62NCl3 < 71NF3 -129 Gas at RTNI3 - ExplodeNH3 -33 Gas at RTN2 -196 Gas at RT

(CH3)3N -33 Gas at RT


Examples that show extremely low etch rate.Etch Al in fluorine-based gas : AlF3 is not volatileEtch Ni in chlorine-based gas : NiCl2 is not volatileEtch Al2O3 in Cl2 plasma:Al2O3 + Cl2 AlCl3 + O2

(Uphill thermodynamically, but etched with UV laser irradiation)Etch SiO2 in Cl2 plasma: Uphill thermodynamically, but etched with energetic ion bombardment.

Typical gases used for plasma etching


Feed gas Mechanism Selective to

n type SiCl2 Chemical

SiO2Cl2/C2F6 Ion-inhibitorSiCl4

SiCl2

Ion-energetic SiO2CCl4/O2

SiCl4/O2

Al

Sl2/SiCl4

Ion inhibitor/energetic

SiO2, some resist, Si3N4

Cl2/CCl4

Cl2/CHCl3

Cl2/BCl3

III-V semiconductor

Cl2 Chemical

SiO2, resist

Cl2/BCl3

Ion-inhibitorCl2/CH4

Cl2/CCl4

CCl4/ O2

SiCl4/O2

III-V semiconductorWithout Al

Cl2/O2 Chemical Al-containing alloy, SiO2CF2Cl2 Ion-inhibitor

7. 5. Dry etch method and reactor type


7.5.1. Dry etch methodPlasma method(a) Plasma etching (PE)(b) Reactivel ion etching (RIE) (c) High density plasma etching: Electron cyclotron resonance

etching (ECR) and inductively coupled plasma etching (ICP)

Ion beam method(a) Ion beam etching (IBE)(b) Reactive ion beam etching (RIBE)(c) Chemically-assisted ion beam etching (CAIBE)

Detailed characteristics of dry etching technique


Parameter PE RIE MERIE ICP ECR IBE(sputter)

frequency 13.56MHz 13.56MHz 13.56MHz 13.56MHz 2.45GHz -Pressure

(torr) 0.1 ~ 10 0.01 ~ 0.1 0.01 0.001~ 0.01

0.001~ 0.01 0.001 ~ 0.1

Te (eV) ~ 8 ~ 8 ~ 5 ~ 4 ~ 4Plasma density ~ 3e8 ㎝-3 ~ 1e10㎝-3 ~5e10㎝-3 ~5e11㎝-3 ~5e11㎝-3 -

Wafer location

Grounded electrode

Powered electrode

Powered electrode

Powered electrode

Powered electrode

Powered electrode

Ion voltage 25 ~ 100 V

250 ~500 V

400 ~1000 V

0 ~ 1000 V

0 ~ 1000 V

500 ~ 2000 V

Ion energy NotControllable

NotControllable

Not controllable

Controllable Controllable Controllable

Chemical reaction Yes Yes Yes Yes Yes No

Physical reaction No Yes Yes Yes Yes Yes

Selectivity Excellent Good Good Good Good Poor

Anisotropy poor Good Good Good Good Excellent

Comparison of dry etching technique


advantage disadvantage

RIE economical slow etch rate, plasma damage

CAIBE Relatively fast etch rate low versatility

ECR fast etch rate high price, low scalability

ICP fast etch rate, low plasma damage

7. 5. 2. Reactor types of dry etch


(a) Plasma etching (PE) and Reactive ion etch (RIE)


Plasma etching (PE)Same reactor geometry as PECVD systemLow ion bombardment energy due to the low sheath voltage drop sample was loaded on the grounded electrode (anode)Mainly chemical reactions and negligible physical etchingIsotropic etch profileAt relatively high pressure: 0.1 ~ 10 Torr

Reactive ion etch (RIE)Combination of chemical activity of reactive radicals with physical effects due to high sheath drop sample was loaded on the powered electrode (cathode)Ion bombardment strongly enhances the chemical processAnisotropic etch profile due to ion bombardmentLower operation pressure of 0.01 ~ 0.1 Torr

(b) Magnetically enhanced reactive ion etching (MERIE)


Reduce the plasma loss on the chamber wall using magnetic field by electromagnet bucketElectron collisional efficiency increase by interaction of E and B fieldSubstrate rotation for uniformity increase

(c) Electron Cyclotron Resonance etching


z

Magnet

Plasmageneration

Quartz window

k B

ER

y

x

v

-

)( BVEeF ×+−=Circularly polarized wave

Electron cyclotron motion

ECR: One of the high density plasma source (5 x 1011㎝-3)


ECR: Plasma generation by combining microwave(2.45 GHz) and the magnetic field by additional magnet.Plasma generation mechanism

Microwave (2.45 GHz) is introduced into reaction chamber through quartz windowMagnetic field is generated in the reaction chamber by magnet (permanent or electro magnetic)Electrons rotate around the magnetic line of force with the electron cyclotron angular frequency of ωc:

When the electric field E of microwave is perpendicular to the magnetic field and the circular wave of magnetic field satisfies ω = ωc, electrons are continuously accelerated by the electric filed of the microwave, obtaining high energy, and then ionizing the gas molecules by collisions.If microwave of 2.45 GHz are used, the ECR takes place at the magnetic field flux density of 875 G.

ec m

eB=ω

(d) Inductively coupled plasma


13.56 MHz currents pass through ICP coil

RF magnetic field formation along z axis

Induction of vortex electric field

Electrons oscillation

Increase of electron collision efficiency

More effective plasma generation than conventional RIE high radical density

Electrostatic shield configuration eliminates capacitive coupling

Independent ion energy control by table power

Z

Types of Inductively coupled plasma


Cylindrical type ICPContamination-free geometry

Planar type ICP Contamination of wafer by sputtering of window material.


Laser interferometer on chamber-topOptical emission spectroscopy through

sidewall windowElectrostatic shield btw quartz and coil ICP/PECVD cluster tool in K-JIST

Inductively Coupled Plasma in K-JIST

(e) Ion beam-based reactor


IBE – inert gas ion (Ar+) formation in external RF ion source and extracted to the reaction chamber by acceleration electrode (grid).RIBE – reactive gas besides inert gas ions are extracted from the external source to the reaction chamber. Etch rate is increased by the additional chemical reactionCAIBE – inert gas ion (Ar+) are extracted from the external source and the reactive gas are independently supplied to the wafer surface through shower-ring just above the wafer.

bibliography - gistmaster/lecture/tft2009/c5-2(41-plasma... · plasma etching k - jistk - jist...

Documents