Infrared transmission topography for whole-wafer gallium arsenide materials characterization

Solid-State Electronics Vol. 35, No. 3, pp. 319-323, 1992

0038-1101/92 $5.00+ 0.00 Pergamon Press pie

Printed in Great Britain

INFRARED TRANSMISSION TOPOGRAPHY FOR WHOLE-WAFER GALLIUM ARSENIDE MATERIALS CHARACTERIZATION M. G. MIER, 1 D. C. LOOK, 2 D. C. WALTERS 2 and D. L. BEASLEYl ISolid State Electronics Directorate (WL/ELR), Wright Laboratory, Wright-Patterson Air Force Base, OH 45433-6543, U.S.A. 2University Research Center, Wright State University, Dayton, OH 45435, U.S.A. (Received 20 August 1991; in revisedform 30 September 1991)

Abstract--Infrared transmission topography is shown to be useful for evaluating GaAs wafers. Wholewafer, half-millimeter resolution plots of EL2 density and dislocation density are shown to correlate with plots of saturation current in MESFET devices at an early stage of fabrication.

INTRODUCTION

Infrared transmission is well-established as a technique for determining various parameters of electronic materials. If the material is partially transparent at the i.r. measuring wavelength and the parameter of interest has an absorption or scattering cross-section at this wavelength, it is possible to determine quantitative values of this parameter by appropriate sample preparation and calibration techniques. In double-sided polished semi-insulating (SI) GaAs substrates, i.r. transmission has been used to determine neutral and total EL2 concentration, net acceptor concentration and free electron concentration[I--4]. Dislocation density (etch pit density), has been determined by i.r. transmission after delineating the dislocations by an appropriate dislocation etch[5,6]. The spatial variation of the measured parameter is potentially of fundamental importance in determining the ultimate manufacturing yield of electronic devices and monolithic circuits fabricated on a semiconductor wafer. Spatial correlation of device or circuit parameters with materials parameters such as EL2 and dislocation density is an important component of the monolithic microwave and millimeterwave integrated circuit (MIMIC) program sponsored by DARPA. While manufacturing process immaturity tends to obscure such correlations, sufficient information is now available to begin to demonstrate such correlations. In this paper, we will discuss the apparatus and data treatment for the measurement of the spatial variation of the i.r. transmission and the application of this technique in semiconductor wafer evaluation. APPARATUS

The i.r. transmission measurements are based on an apparatus and technique developed for the

measurement of neutral EL2 density in semi-insulating GaAs wafers[4]. A half-millimeter square aperature defines the light from a tungsten-halogen source. The light is filtered using a polished Si wafer to eliminate wavelengths shorter than 1 pm. The light is focused through a quarter-meter monochrometer onto a 3 cm focal length i.r. glass lens which demagnifies the light to a half-millimeter square spot. This beam passes through a motor-driven chopper to the sample which is mounted on a stage controlled by orthogonal d.c. motor-driven positioners. The positioners are capable of linear translation speeds between 0.8 and 8 mm s -1 controlled by an 80286 personal computer. The transmitted signal is detected by a 3 mm dia. thermoelectrically cooled Ge detector and a lock-in amplifier. A block diagram of the system is shown in Fig. 1. The personal computer records both the sample position and transmitted amplitude at each position in an array and in a disk file. The spatial resolution of the measurement is determined by the i.r. beam spot size used in the transmission measurement[5,6]. Using the half-millimeter square beam, a total of 16,597 locations are recorded for a 3 in. wafer. Lower resolution can be used if spatial variations are demonstrated to be slowly varying in magnitude and position. These dense data are displayed for analysis by sorting the transmission data into 14 color-coded bins. The data may then be graphically plotted using the on-wafer positions stored with each transmission value. Color plots are most effective for demonstrating the spatial variation. Histograms are also available for showing the magnitude and distribution of the parameter being measured. Transmission data for EL2 measurement are acquired at wavelengths of 1.1 and 1.2#m. These data are corrected location-by-location for reflections at front and back surfaces and, using the

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Wafer holder 1/4-meter I monochromator

I Detector

I

I

=fer

x-z Jr ve

readout

I Monochromator I control

/

I

puter I

Fig. I. Block diagram of the full-wafer i.r. transmission system. measured wafer thickness, the absorption coefficient is calculated. The absorption coefficients are related to the EL2 concentration by the formula: O~ = N E L 2 [ f ~

n -I-

(l --f)ap],

(1)

where f is the fraction of neutral EL2 and a, and ap are the electron and hole photoionization cross-sections, respectively. Since a, and ap are accurately known as function of wavelength[7] it is possible to determine both f and NEE2 by making measurements at two different wavelengths (two equations in two unknowns). In fact, it turns out that a, = ap at 1.19 # m wavelength, so that the total NEE2

c =

1 a

610

235

115

255

0 Ixl

1.40E416

mm r e s o l u t i o n

0

1.00E + 0 5 lxl

(2)

Clipped = 118/4,557

230

1.15E+16

-

T ~ / : - T~/2'

where a is the average etch pit area, and TO and TE are the transmissions which would result from totally unetched or totally etched surfaces, respectively. The parameter a can be measured with a

470

0 9.00E+15

T 1/2- T 1/2

PD = ' - - - In -

Clipped : 2 0 / 3 8 6 3

Clipped : 9 4 / 3 4 9 0

g,

can be determined at this wavelength alone. However, it is also sometimes useful to determine f, since f is related to the acceptor concentration NA by (I --f)NEL2 = NA + n ~ N A (n is small in SI GaAs). Furthermore, f can be related to the free electron concentration n by the EL2 Fermi function, since the EL2 energy and degeneracy are known. The values of N A and n can be used as consistency checks since the former is usually close to the carbon concentration, which can be determined by i.r. absorption at 582cm -1, and the latter is easily found by a Hall measurement. The dislocation density PD is determined from the measurement transmission T at each location after the whole wafer has been etched in molten KOH. The etching process creates pits at places where dislocations intersect the surface, and the pits scatter the light (1.45#m wavelength, in our case), thus reducing the transmission. The conversion algorithm[5,6], which accounts for etch pit overlap and multiple light reflections at the surfaces, is:

2.00E+05

0 0.23

0.25

0.27

mm r e s o l u t i o n

Neutrol EL2 density

Dislocation density

(a)

(b)

Ungated unrecessedIDSS

(c)

Fig. 2. Spatial correlation of neutral EL2 and dislocation density with ungated, unreeessed Ia,, of processed MESFETs: (a) spatial variation of neutral EL2; (b) spatial variation of dislocation density; and (c) spatial variation of ungated, unrecessed Ion.

Neutrol E L 2 density

Neutrol EL; > density

Neutrol EL:> density

Dislocotion

DJslocotion

Dislocotion

density

(a)

density

(b)

density

(c)

Fig. 3. Variability of decoupling EL2 from the dislocation density in annealed LEC crystals: (a) unanncaled LEC; (b) partially decoupled annealed LEC; and (c) fully-decoupled annealed LEC. Clipped = 6 6 / 3 9 7 7

Clipped = 4 6 / 3 9 2 0 240

:550

1:>0

175

0 6.00 E+15

1.25E +16 lxl

rnm resolution

Neutrol EL2 density

(a)

1.70E + 16

0 3.OOE +15

5.00E ÷ 1,5 lxl

7.00 E +15

mm resolution

Neutral E L 2 density

(b)

Fie. 4. Difference in EL2 densitv and snatial variation between LEC and VGF grown GaAs: (a) LEC

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calibrated microscope, and often T0~0.54 and TE*0. However, it is more accurate to manually count the etch pits at three locations, and then determine a, To and T~ from the three resulting equations. It is even better to manually count the pits at several more locations, and then carry out eqn (2). APPLICATIONS

Semi-insulating GaAs wafers are the basis for manufacture of monolithic GaAs integrated circuits using metal-semiconductor field effect transistor (MESFET) technology based on ion implantation. Figure 2 shows the spatial variation of neutral EL2 (Fig. 2a), and the dislocation density (Fig. 2b), in an unannealed semi-insulating wafer grown by the liquid encapsulation Czochralski (LEC) technique. For unannealed material, the EL2 and dislocation densities are spatially correlated. In Fig. 2c, the results of the source-drain current measurements (Idss) on unrecessed, ungated, ion-implanted MESFETs fabri-

Clipped

cated on an adjacent wafer are shown. The Id, values show the same spatial variation as that observed for EL2 and dislocation density indicating the effect of one or both of these parameters on device fabrication. Most GaAs wafer manufacturers have instituted whole-boule annealing of the semi-insulating boules to minimize boule strain and homogenize the EL2 density from the dislocation pattern. The success of such techniques can be assessed by the use of i.r. transmission topography as illustrated in Fig. 3. The varied success of this process can be easily seen from these data. The four-fold symmetry of the dislocation density is preserved while the EL2 density is more or less effectively decoupled and homogenized by the anneal. Figure 3b illustrates partial decoupling while almost complete decoupling is shown in Fig. 3c. Device correlation studies of these materials are presently in progress. This technique is also useful to assess the relative differences of semi-insulating material prepared by different bulk growth techniques. Figures 4 and 5

Clipped = 2 0 1 / 1 4 5 7 7

= 45/3945

270

123C

135

615

0

0 0

4.50 E + 04 lxl

rnm

Dislocation

resolution

density

(a)

9.00 E + 04

0

5.00 E + 03 0.5x0.5

mm

1.00 E + 0 4

resolution

Dislocotion density

(b)

Fig. 5. Difference in dislocation density and spatial variation between LEC and VGF grown GaAs: (a) LEC grown; and (b) VGF grown.

Infrared transmission topography for GaAs materials

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R132A, 0 degree

R133C, 0 degree

R148A,2 degrees

R148C, 2: degrees

R148 D, 0 degree

R149A,O degree

Fig. 6. Thickness variation of MOCVD-grown GaAs on Si revealed by transmission i.r. topography. illustrate the difference between conventional LEC GaAs and vertical gradient freeze (VGF) GaAs. The wafer plots of the dislocation density and EL2 are clear evidence of the different thermal geometries under which the crystals are grown. The decreased magnitudes of both EL2 and the dislocation density in the VGF GaAs relative to the LEC GaAs are shown in the histograms which accompany the wafer plots. Another use of i.r. transmission topography is illustrated in Fig. 6. These topographs show the ratio of transmission at 1.1 and 1.45/~m wavelengths for 6 GaAs on Si wafers prepared by metal-organic chemical vapor deposition (MOCVD). This ratio can be shown to be dependent on the thickness of the GaAs films and the spatial variation shown is the thickness variation of the epitaxial GaAs layer. This variation is probably a result of the gas flow patterns in the MOCVD system and as a result, such data can be used to optimize growth conditions for these types of structures. CONCLUSIONS Infrared transmission topography has been shown to be a powerful tool in the analysis of the magnitude

and spatial variation of important parameters of GaAs materials. These results can be used to improve the crystal growth processes for GaAs crystals as well as provide material parameter data for correlation with device fabrication results.

Acknowledgements--The authors would like to thank D. Elsaesser, S. Dudley and J. Sewell for assistance in computer programming. The authors also wish to thank Dr R. E. WaUine for helpful discussions and Ms P. Woosley for preparation of the manuscript and illustrations.

REFERENCES

1. G. M. Martin, J. P. Farges, G. Jacob, J. P. Hallais and G. Poiblaud, J. appl. Phys. 51, 2840 (1980). 2. S. K. Brierly and D. S. Lehr, Appl. Phys. Lett. 55, 2426 (1989). 3. M. Suemitsu, M. Nishijima and N. Miyamoto, J. appl. Phys. 69, 7240 (1991). 4. P. Dobrilla and J. S. Blakemore, J. appl. Phys. 60, 169 (1986). 5. J. S. Sewell, S. C. Dudley, M. G. Mier, D. C. Look and D. C. Waiters, J. Electron. Mater. 18, 191 (1989). 6. D. C. Look, D. C. Waiters, J. S. Sewell, S. C. Dudley, M. G. Mier and J. R. Sizelove, J. appL Phys. 65, 1375 (1989). 7. P. Silverberg, P. Omling and L. Samuelson, Appl. Phys. Lett. 52, 1689 (1988).