simultaneous in situ measurement of film thickness and temperature by using multiple wavelengths...

I12 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 6, NO. 2, MAY 1993

Simultaneous In Situ Measurement of Film Thickness and Temperature by Using Multiple Wavelengths

Pyrometric Interferometry (MWPI) Friedrich G. Boebel and Heino Moller

Abstract-Film thickness and temperature are two of the most important quantities in semiconductor manufacturing. They play a fundamental role in many standard production techniques like chemical vapor deposition (CVD, LPCVD, PECVD), thermal oxidation and diffusion. They are especially important for more recently developed technologies like molecular beam epitaxy (MBE), metal organic MBE (MOMBE), metal organic CVD (MOCVD), chemical beam epitaxy (CBE), etc.

In this paper, an optical in situ method for simultaneous film thickness and temperature measurements-named Multiple Wavelengths Pyrometric Interferometry (MWP1)-is introduced, which is capable of high resolution (up to 0.1 nm for thickness and 0.025 K for temperature) and for real time data evaluation. It can be used for process control as well as in situ quality inspection without time delay or additional handling mechanisms and is suitable for monitoring single films as well as multilayer structures. MWPI is insensitive to vibration, rotation and misalignment of the wafer. Due to its optical basis it is also insensitive to hostile environments like high temperature and/or chemical reactive gases.

The physical basis of MWPI is outlined as well as the necessary steps of the evaluation procedure. The simultaneous in situ MWPI film thickness and temperature measurement during the thermal oxidation of Si is presented. The potential technical impact of MWPI on automation, reliability and long term per- formance of semiconductor manufacturing is discussed.

I. INTRODUCTION HE FUTURE production of electronic and optoelec- T tronic devices will require even better control of

semiconductor manufacturing, improved flexibility of the production line and a higher level of automation. The increasing quality demands as well as the increasing costs per wafer call for a 100% on-line quality control of each production step. To fulfill these requirements, in situ measurement technologies are most promising. They determine the wanted parameters in real time during the process, thus providing an excellent tool for a combined control of process quality and process parameters without time delay or additional handling mechanisms. Finally they are easy to integrate into a computer based control unit for automatic processes.

Manuscript received September 15, 1992; revised December 18, 1992. The authors are with the Fraunhofer Institute of Integrated Circuits, Wet-

IEEE Log Number 9207716. terkreuz 13, 8520 Erlangen, Germany.

Pyrometric Interferometry (PI) was recently introduced as a tool for high resolution, in situ, real time determination of film thickness [l]. Compared to more estab- lished methods like ellipsometry and white light interferometry, PI is insensitive to vibration, rotation and misalignment of the wafer, which allows e.g. the monitoring of rotating wafers in MBE-facilities even if they are not perfectly aligned [2]. PI employs the interference effects of the thermal radiation of the substrate at the interfaces of the growing film. These effects have been known for a long time, since they prohibit the exact determination of the substrate temperature by ordinary py- rometers. The use of interference of thermal radiation to investigate process parameters was reported first by D. J. Dumin [ l l ] , who used it for thickness monitoring by counting quarter wavelength oscillations. The achieved resolution was as low as 100 nm, which is not sufficient for most applications in electronic and opto-electronic manufacturing. In order to improve the resolution Clark et al. [3] measured the endpoint film thickness by com- paring the interference oscillations of the sample with those of a monitor wafer coated with a layer of known thickness. The main shortcomings of Clark’s approach are the restriction to end point detection only and the require- ment of a carefully prepared monitor wafer. More recently SpringThorpe et al. [4] evaluated the extrema of the interference oscillations in order to determine the av- erage constant growth rate in a GaAs/GaAlAs MBE. Un- fortunately his approach is not suitable for real time applications. Generally spoken the major disadvantages of the methods described in [3], [4] and [ 111 are based on the sole use of characteristic values rather than all data for evaluation, which results in a quite limited applicability and/or limited resolution. More severely the work in [1]- [4] and [ l 13 suffers from the restriction, that the wafer temperature has to stay constant during the process, since temperature variations influence directly the accuracy of the measurement. Tolerable fluctuations have to be typically smaller than 1°C. MWPI is designed to overcome this shortcoming.

In this paper we demonstrate the capability of MWPI for high resolution, simultaneous in situ, film thickness and temperature measurements. The physical background of MWPI is investigated in Section 111. Section IV con-

0894-6507/93$03.00 0 1993 IEEE

BOEBEL AND MOLLER: SIMULTANEOUS IN SITU MEASUREMENT 113

heated substrate detector 1

data evaluation

process chamber / detector2 chopper

Fig. 1 . Experimental configuration for a film thickness measurement by Multiple Wavelengths Pyrometric Interferometxy .

tains the evaluation and calibration procedure and the experimental results on thermal oxidation of Si are presented in Section V. The benefits to semiconductor manufacturing are discussed and an outlook to further applications like 2-dimensional in situ film thickness and temperature measurements is given (Section VI).

‘ D

‘,. A

~ -a ......... J ............

Fig. 2. Interference effects of the thermal radiation of the substrate at the growing film.

An electromagnetic wave originating from point P is divided into different rays with the electric field ampli- tudes a, pa, p2a, p3a, , where p is the product of the reflection coefficients of the substrate/film interface and of the filmhacuum interface respectively. According to Fig. 2 the optical path difference A between neighboring rays is

-

11. EXPERIMENTAL SET-UP Fig. 1 shows a typical experimental configuration for

MWPI measurements. A process chamber hosts a heated substrate, which is covered by a growing film of present thickness d( t ) and temperature T(t ) . The thermal radiation, emitted by the wafer is decoupled through a Ultra High Vacuum (UHV)-proof infrared (IR) window and divided into two detection branches, which host interference filters at different center wavelengths X1 and Xz, respectively. The bandwidth of both filters is AX.

By means of suitable IR-optics the wafer is imaged on IR-sensitive detectors. The amount of light incident to the detector is strongly influenced by the working distance Zwork, which ranged between 1 m and 2.5 m in our exper- iments. In order to avoid 1 /f noise and to exploit maxi- mum sensitivity we used an amplitude modulation tech- nique. A chopper behind the front lens modulated the thermal radiation and the detector signals were processed via lock-in amplifiers. The chopper frequency and the in- tegration time at the output of the lock-ins ranged between 150 Hz and 350 Hz and 1 s and 10 s, respectively. The lock-in outputs were digitized by a 15-bit A /D converter and transmitted to a PC for data evaluation. The thermal oxidation of silicon was carried out at substrate temperatures between 1100°C and 1180°C. The relatively high temperatures allowed for using Near Infrared (NIR)-tech- nology, i.e., the detectors were Si-photo-diodes, the center wavelengths XI = 952.4 nm and X2 = 751.2 nm, respectively, AX = 14 nm and quartz was an appropriate window material.

111. BASIC PHYSICAL THEORY The physical basis of PI are interference effects, which

occur when the thermal radiation of the substrate is re- fracted and reflected at the faces of the growing film. The situation is sketched in Fig. 2.

A = 2dd.j - sin’ CY

which results in a film thickness dependent phase shift 6. If the index of refraction of the substrate n, is larger than the index of refraction of the film nf, the phase shift is increased by ?r at the substratehlm interface resulting in 6 = (2?rA/X) + ?r. The net intensity measured at the detector ID is proportional to the square magnitude of the superposed electric fields of the individual rays, vis.:

(1)

where we used the fact that the above sum is a geometric series. Equation (1) is the well known Airy’s formula [5], which exhibits the qualitatively correct behavior for trans- parent films.

The main difficulty in deriving the quantitatively exact formulas of the measured intensities for more general cases is twofold:

1. Due to Kirchhoffs law (emissivity E equals absor- bance A), the incident medium, i.e., the substrate, is nec- essarily absorbing, which causes a coupling of the incident and the reflected field inside the substrate as well as a phase shift of the interference oscillations.

2. Most films are absorbing, at least weakly, i.e. the film does not only damp the thermal radiation of the substrate, but starts to radiate itself and the electric field suffers an additional phase shift when reflected at the interfaces of the film.

Both effects increase the conceptual complexity significantly. They describe a situation in which such basic optical concepts like transmission, reflection and absorption of the film are no longer valid. (For a detailed discussion of the problem see [7]). One may avoid these difficulties by considering the emissivity of the whole system instead

1 - - (1 - p12 + 4 p sin’ (6/2)

I14 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 6 , NO. 2 , MAY 1993

of the transmission of the film. The dependence of the detector signal on film thickness and temperature is then given by

ID(d(t) , W), A) = c * 4 fP/(L T ) ( 2 ) where C is a constant considering all geometrical and optical influences as well as the spectral response of the detector, E @ , d) the emissivity and f p I ( A , T) the spectral intensity distribution radiated by a black body at temperature T. In order to get rid of the constant C, each detector curve ID@) is usually normalized to its start value at time t = O

where To = T(0) . Sincefpl is given by Planck’s Law, we are left with finding the right expression for E in order to evaluate (3). E depends on the complex index of refraction nf = nf - ikfof the film and the optical constants of the substrate n,, k, as well as observation wavelength, observation angle, film thickness and temperature. Since the change of E caused by temperature variations is usually small, it is neglected in the following calculation.

Due to energy conservation the reflectance R, the transmission T and the absorption A of the coated wafer have to sum up to unity. Since the wafer is nontransmitting (T = 0), we gain for E by considering Kirchhoffs Law (E = A): E = 1 - R. Consequently the problem lies in calcu- lating the reflectance R of the covered substrate. It is important to realize, that the conceptual difficulties described are now circumvented, since the reflectance is calculated from the vacuum side, i.e., the incident medium is transmitting.

Due to limited space we restrict the treatment of the exact formulas to a short discussion of their results. A detailed introduction to the matrix formalism used is given by Macleod [6] and Beming [7]. The extension of the theory to the multilayer case is straight forward and given by MacLeod [6], Berning [7], and Potter [8].

For a homogeneous, isotropic and absorbing one layer system, which is observed at a = 0, E is given by

~ ( d ) = 1 - R ( d )

film and thus the expected result. For d + 0, E becomes

(1 - P ? f N - P i ) - 4 sin (Pfi) sin (PJ

1 + ( P V f P f J 2 + 2 P t f P f s cos (Pvf + Pfi)

= 1 - PZ, ( 5 )

E(d + 0) =

where 1 - p:,,, is the emissivity of the bare substrate. (The proof of (5) is elementary, but lengthy and will be omitted here). Equation (4) can be easily generalized to the case of oblique angles of observation. The corresponding calculation for R ( d , a) may be found in [7] or P I .

IV. DATA EVALUATION MWPI delivers information about film thickness d,

temperature T and the optical constants n,, k,, nf, We first consider the case that the optical parameters are known and remain constant during the process, i.e., the measured intensity is a function of d and T only. In order to evaluate d and T from the normalized data, we define a error function F(d, T), which measures the deviation between the theoretical variables d and Tand the true values of film thickness and temperature:

)2 F(d, T ) = (&!&!!d - E ( X , , d ) f P / ( X I , T ) z D ( o ? AI) E ( A I , o ) f P I ( A I , TO)

A’2) € ( A 2 3 d ) f P / ( A 2 , T) )* (6) + ( A21 € ( A 2 3 o)fP/(A2, TO)

The true values fulfill condition

F(d, T ) A 0

where d and Tare found by minimizing F . Since the min- imization of F amounts to a nonlinear, nonconvex optimization procedure, there are some numerical problems related to convergence, uniqueness of found solutions and to sensitivity towards small changes of optical parameters. They are discussed exhaustively in [9]. The present software implementation of the evaluation procedure takes less than 0.5 s per data point on a IBM-PC 80386/87’,

where pvf, ‘pup p f s , cpfs are the magnitude and the phase of the complex reflection coefficients at the interfaces between vacuum and film and film and substrate respectively. The absorption coefficient equals y = 2 7 r k f / A . The parameters pvf, pVf, pfs, pfS and y are all functions of the optical constants n,, k,, nf, kf and are given by Fresnel’s formulas (see for example [lo]). The significance of the above formula can be easily checked for limiting cases: For high yd the film gets opaque and only the radiation of the film will be measured. According to (4) ~ ( y d +

00) = (1 - p$), which is the emissivity for a infinite thick

which is sufficiently fast for most coating processes in semiconductor manufacturing.

In ( 4 ) we assumed that the emissivity of the wafer is temperature independent. Provided that the photon energy of the observation wavelength is above the bandgap of the substrate, the relative changes of E are typically of the order of 5 * lop5 per degree Celsius for a wide range of semiconductors. Consequently a temperature change of

‘This is a registered trademark of IBM Corporation

BOEBEL AND MOLLER: SIMULTANEOUS IN SITU MEASUREMENT 115

TABLE 1 OPTICAL CONSTANTS OF THE SUBSTRATE AND GROWTH CONSTANTS a AND b GAINED BY OPTIMIZING EQ. (6)

TemperatureIOC n , ( X , ) n , ( X d k , ( X , ) k,T(X,) a/nm-' b1nm-I

1 1 0 0 4.16 3.87 0.05 0.04 0.006270 1.324 1180 4.17 3.91 0.09 0.05 0.004676 0.4756

about 20°C during the process results in an relative error of 0.001, which influences the evaluation procedure only negligibly. If the temperature change is more dramatic, e.g., in RTP-facilities, Pyrometric Interferometry is not applicable in the present form.

As mentioned above the numerical procedure is rather sensitive to wrong optical constants, which have to be known at least to the fourth significant digit. Conse- quently literature values of optical parameters are of little help for adjusting a MWPI-set-up to the material combination under investigation, since optical constants are usually not recorded at process temperatures. For every material combination and for every temperature interval one needs a calibration run for gaining the appropriate set of parameters. Once the parameters are known, (6) can be used to evaluate d and T. In order to extract the necessary parameters from measured data we minimize the integral error function E over all data points:

E h , k s , nf, kf, a , b , * 1 = $ ( ~ D ( L ti) di)fpI(A, TI)'

. (7)

To relate the calculated values of E * fpI, which are thickness and temperature dependent, to the measured data, which are a function of time, one has to assume a given functional dependence of d on t . For epitaxial growth in CVD-, MBE-facilities, etc. the growth is lin- ear, i.e., d = g * t, where g is growth rate. For thermal oxidation one assumes a functional relation of the form c = ad2 + bd. Consequently the error function E for silicon oxidation depends on 6 parameters instead on 4. Since E is an integral error it averages over small fluctuations, i.e., the above procedure still works, when the gauge signal is superimposed by statistical variations of a few degrees Kelvin. However, if the temperature changes do not fol- low a statistical distribution, but are caused for example by the response function of a temperature control unit, (which is the case in a MBE, when an additional shutter is opened), one has to take into account the parameters of the response function, too. In the latter case the total number of fit parameters is increased by 2 or 3.

The use of the oxidation growth rate model to evaluate (7) does not restrict MWPI measurements to cases were this assumption is fulfilled. It is only needed to evaluate the optical constants at the corresponding process temperature. Once the parameters are known, an arbitrary functional behavior of d can be measured, i.e., for the calibration run the functional relation between d and t has to be known, for all further runs this information is no longer necessary.

i = l l D ( A , 0) o)fPI(A, TO)

Fig. 3. Measured intensities for A , = 751.2 and A, = 952.4 nm (dotted curves). Minimizing of the integral error function E leads to an excellent fit (solid curves).

If the layer under investigation is nonabsorbing (kf = 0), E (d) /E (0) will be a function of the product of the reflection coefficients and the phase shift at the substrate/ layer interface only, i.e., although the calibration procedure still works, only two of the three unknown optical parameters n,, k,, nf can be extracted from a optimization of (7). The parameters for both wavelengths and different temperatures are given in Table I. Since Si02 is nonabsorbing we had to take one of the three optical constants from literature. Due to its large bandgap the refraction index of Si02 does vary only slightly with temperature and was taken as 1.455 and 1.456 at 75 1.2 nm and 952.4 nm respectively. Taking one optical parameter from literature influences the accuracy of the film thickness measurement much less than expected. We found that literature values and experimental values differ by about 1-3 %. This means that the absolute film thickness is wrong by about the same percentage, which is a reasonable speci- fication for most process control requirements, especially if one considers that the (relative) resolution is much better than 1 nm (see Fig. 5(a)-(c)). The temperature evaluation isn't influenced by the choice of the literature value at all.

To find the absolute minimum of E is a formidable numerical task, which takes about half an hour on a IBM PC 80386/87 in the present software version. A typical example for a wet oxidation of silicon at about 1100°C is shown in Fig. 3.

V. RESULTS AND RESOLUTION We have measured the MWPI-oscillations for a SiOz

film growing on Si by thermal oxidation. The center wavelengths of filters were 751.2 nm and 952.4 nm re-

116

480.50-

4 6 0.2 4-

IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 6, NO. 2, MAY 1993

b

2015 2020 2025 Time[s]

Norm. Intensity rapid ,temperature

I 500 1000 1

a w e s

Fig. 4. Intensities at two different wavelengths at T = 1180°C. From t = 1600 s on the temperature was rapidly changed.

spectively . The corresponding signals for wet silicon oxidation are shown as a function of time in Fig. 4. From t = 1600 s on the temperature was rapidly changed by about f1O"C in order to check the stability of the evaluation procedure towards rapid temperature changes and to investigate the temperature resolution. Following the evaluation procedure of Section IV one gains film thickness d( t ) and temperature T(t ) as a function of time. Fig. 5 exhibits the excellent thickness resolution of MWPI. While in Fig. 5(a) the dependence of the film thickness for large times is clearly resolved, Fig. 5(b) shows that even film thickness changes as small as 0.1 nm can be detected. During the first minutes of oxidation (Fig. 5(c)) the growth rate is rather large, so that the limiting factor for the resolution of about 1 nm is the time constant of 1 s. Below 10 nm absolute thickness the evaluation procedure is more sensitive to noise resulting in a resolution between 1 nm and 2 nm.

The temperature curve in Fig. 6 demonstrates that MWPI is capable to determine even small temperature changes. At the beginning of the oxidation there is a temperature increase of a few degrees Celsius (indicated by the first arrow in Fig. 6). At the end of the oxidation the wafer temperature drops by about the same temperature difference (indicated by the second arrow in Fig. 6). We interpret this behavior as caused by the temperature difference between the nitrogen gas (20"C), which flows through the chamber before and after the oxidation, and the hot water vapor (96°C) flowing through the oven during the oxidation. Since the temperature control of an oxidation chamber has time constants of abut 50 s, the observed behavior results. The small oscillations in Fig. 6 are hunting regulations of the temperature control unit of the oven. As seen from the magnified area in Fig. 6 the peak-to-peak amplitude of the oscillations is about 0.25"C, while the temperature resolution can be esti- mated to be at least a factor of 10 better, i.e., 0.025"C. For a detailed discussion of resolution measures for PI see r11.

Thickness [nm] 4

5 0 0 -

ioo-

3 0 0 -

a o o -

100-

b

1000 2000 Time [ s] 0

(a)

Thickness [nm] 4

581.75. T=1180 O C

5 8 2.5 C -

Thi A

4 0 -

30 -

20.

10 .

b

80 100 120 Time[s]

(c)

Fig. 5. (a) Thickness of SiOl as a function of time during a wet silicon oxidation at 1100°C and 1180°C. The 4 behavior is clearly resolved for large times. (b) Magnified area of (a) around t = 2020 s. In the upper curve thickness changes as small as 0.1 nmm can be tracked. (c) Magnified area of (a) showing the first 50 seconds of growth.

BOEBEL AND MOLLER: SIMULTANEOUS IN SITU MEASUREMENT

Fig. 6. Temperature of the wafer surface during a wet silicon oxidation at 1180°C. The arrows indicate the beginning and the end of the water vapor flow period.

VI. EXPERIMENTAL CONSIDERATIONS AND

APPLICABILITY PI is a suitable method to determine the film thickness

and temperature for a wide range of film/substrate com- binations. The following restrictions have to be consid- ered:

a) The substrate has to be emitting (i.e., absorbing) while the film has to be at least partially transmitting at the observation wavelength.

b) The bandwidth of the interference filter AA has to be narrow enough to guarantee a sufficient coherence length lcoh of the thermal radiation, i.e., A < lcoh. For narrow band filters the relation between bandwidth and coherence length is approximately given by lcoh A2/AA, so that the coherence condition AA < A2/A = A2/2 dn is always fulfilled for very thin films.

c) Condition a) defines a certain wavelength range. If the substrate temperature is not high enough to give sufficient radiation intensity within the defined interval, MWPI will not be a suitable tool for simultaneous film thickness and temperature measurements.

d) The evaluation procedure introduced in Section IV assumes constant optical parameters. Variations in nf and kf will strongly increase the error of d and T. Consequently MWPI is of little help in processes where the optical constants changes significantly during film growth.

PI seems to be a promising tool for many tasks in the field of basic research as well as for production of electronic and optoelectronic components. Below we give a listing of potential applications and expected benefits.

1. Investigation of dynamics of growing films includ- ing experimental verification and optimization of simulation models.

2. Extraction of optical parameter and related quantities (e.g., donator concentration) under process conditions [2].

I I7

3. Elimination of drifts.

4. Automation of coating processes.

5 . In situ temperature control during film growth.

6. In situ film thickness control during film growth.

We like to emphasize the fact, that besides infrared transmission spectroscopy [12] MWPI is the only measurement methods, which is capable of in situ determination of the wafer temperature during film growth. While infrared transmission spectroscopy averages over the com- plete wafer thickness, MWPI measures the true surface temperature, which is the important process parameter. In addition the temperature resolution of MWPI is about two orders of magnitude better.

Since it doesn’t need an external light source, MWPI is insensitive to vibration, rotation and misalignment of the wafer. It is this very well suited for in situ monitoring of rotating wafers (e.g., MBE), which are usually not perfectly aligned.

The Fraunhofer Institute for Integrated Circuits is working on improving MWPI-techniques to expand the range of applications like 2-dimensional PI and 2-dimensional MWPI.

In many processes film thickness and temperature vary significantly over the wafer during the process. Replacing the detector in Fig. 1 by a solid-state detector array cam- era leads to 2-dimensional pictures of film thickness and temperature distributions. The main difficulty towards a 2-dimensional in-situ film thickness and/or temperature measurement system is the enormous amount of data, which has to be processed in real time.

ACKNOWLEDGMENT We are deeply indebted to Dr. K. P. Frohmader and

Dipl. Ing. U. Bonnes for technical assistance during the measurements.

REFERENCES

[ 11 F. G. Boebel, K. P. Frohmader, and U, Bonnes, “In situ film thickness measurements by using pyrometric interferometry,” in I Irh IEEE/IEMT Symp. , San Francisco, Sept. 16-18, 1991.

[2] H. Grothe and F. G. Boebel, “ I n situ control of Ga(A1)As MBE layer by multiple wavelength pyrometric interferometry (MWPI),” in Proc. 7th Int. MBE Conf., Schwabisch Gmund, Germany, Aug. 24-28, 1992.

[3] C. A. Clark, J. F. Roberts, and C. A. Dumbri, “Kontinuierliche Messung der Dichte heiBer Dunnschichten,” German Patent P 19 39 677.9, 1970.

[4] R. J. SpringThorpe, T. D. Humphreys, A. Majeed, and W. T. Moore, “In situ growth rate measurements during molecular beam epitaxy using an optical pyrometer,” Appl. Phys. Lett . , vol. 5 5 , no. 20, pp.

[5] M. V. Klein and T. E. Furtak, Optics, 2nd ed. New York: Wiley,

[6] H. A. Macleod, Thin-film Optical Filters, 2nd ed. Bristol: Adam

[7] P. H. Berning, “Theory and calculations of optical thin films,’’ in New York: Academic

[8] R. F. Potter, “Basic parameters for measuring optical properties,” New

2138-2140, NOV. 1989.

1986, ch. 5, p. 236.

Hilger, 1986, ch. 2.

Physics of Thin Films, vol. 1, G. Haas, Ed. Press, 1963, pp. 69-121.

in Handbook of Optical Constunts of Solids, E. D. Palik, Ed. York: Academic Press, 1985, pp. 11-34.

118 IEEE TRANSACTIONS ON SEMICONDUCTOR MANUFACTURING, VOL. 6, NO. 2. MAY 1993

I

[9] H. Moller, “2-Wellenlangen-Pyrometrische Interferometrie,” di- is responsible for the development of in situ systems for semiconductor manufacturing. Further interests are digital signal processing and syner-

101 J. D. Jackson, Classical Elecrrodynamics, 2nd ed. New York: getic computation. ploma thesis, University of Erlangen, 1991.

Wiley, 1975. [ 111 D. J. Dumin, “Measurement of film thickness using infrared inter-

ference,” Review ofScienriJic Instrumenfs, vol. 38, no. 8, pp. 1107- 1109, Aug. 1967.

[12] W. S. Lee, G. W. Yoffe, D. G. Scholm, J . S. Hams, Jr., “Accurate measurement of MBE substrate temperature,” Journal of Crysral Growrh, vol. 111, pp. 131-135, 1991.

Friedrich G. Boebel was born in Schweinfurt, Germany in 1961. He received the M.Sc. in physics at SUNY Albany, N.Y. in 1987 and the diploma degree in physics from the University of Wiirzburg, Germany in 1989. His diploma thesis dealt with magneto-resistance in very high mag- netic fields. The measurements for the thesis were done at the Institute of Solid State Physics of the Todai University at Tokyo, Japan.

In 1990 he joined the Fraunhofer Institute of In- tegrated Circuits, Erlangen, Germany, where he alternative energy soul C

Heino Moller was born in Erlangen, Germany in 1966. He received the diploma degree in electrical engineering in January 1992. His diploma thesis dealt with film temperature and thickness measurement on the basis of Pyrometric Interferome- try.

He is currently working at the Fraunhofer In- stitute of Integrated circuits, Erlangen, Germany, where he is engaged in the development of systems for optical temperature and film thickness measurement. Further interests are in the field of

:es and energy saving techniques.

---I- -

simultaneous in situ measurement of film thickness and temperature by using multiple wavelengths...

Documents