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Page 1: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

Boron contamination and antimony segregation at the interface of directly bondedsilicon wafersF. P. Widdershoven, J. Haisma, and J. P. M. Naus Citation: Journal of Applied Physics 68, 6253 (1990); doi: 10.1063/1.346866 View online: http://dx.doi.org/10.1063/1.346866 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/68/12?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Direct imaging of boron segregation to extended defects in silicon Appl. Phys. Lett. 97, 242104 (2010); 10.1063/1.3526376 Effect of Au contamination on the electrical characteristics of a “model” small-angle grain boundary in n-typedirect silicon bonded wafer J. Appl. Phys. 108, 053719 (2010); 10.1063/1.3471817 A model of interface defect formation in silicon wafer bonding Appl. Phys. Lett. 94, 101914 (2009); 10.1063/1.3100780 Directional diffusion and void formation at a Si (001) bonded wafer interface J. Appl. Phys. 92, 1945 (2002); 10.1063/1.1491590 Oxygen diffusion in heavily antimony-, arsenic-, and boron-doped Czochralski silicon wafers Appl. Phys. Lett. 74, 3648 (1999); 10.1063/1.123210

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Page 2: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

F. P. Widdershoven, J. Haisma, and J. P. M. Naus Philips Research Laboratories Eindhoven, P. 0. Box 80.000, 5600J A Eindhoven, The Netherlands

(Received 9 Apri11990; accepted for publication 14 August 1990)

The characterization of the interface between directly bonded n-type silicon wafers by capacitance-voltage (C- V) profiling, secondary-ion mass spectrometry (SIMS), and transmission electron microscopy (TEM) is reported. At the interface a boron diffusion peak is found that can be explained by initial boron contamination of the wafer surfaces before bonding. Furthermore, the presence of an amorphous SiOo.s4 layer at the bonding interface is revealed by TEM and SIMS. Antimony is shown to diffuse through this layer and to segregate at the interface. Diffusion and segregation phenomena are simulated with the process simulation package SUPREM-3 and C- V profiling is simulated with the device simulation package CURRY. Features in the C- V profiling results are explained in terms of the observed boron contamination. The behavior of the resistivity near the interface of directly bonded silicon wafers, reported in literature, is also explained by boron contamination.

I. INTRODUCTION

Direct bonding of silicon wafers is a promising technol­ogy for silicon-on-insulator applications as well as for bipo­lar power devices. \-7 Two clean wafers, whether or not oxi­dized, are brought together and bonded, most likely by hydrogen bonds between water molecules adsorbed on the wafer surfaces. Next, the bonded wafer pair is annealed to turn the relatively weak hydrogen bonds into strong chemi­cal bonds. After thinning one of the wafers to a suitable thickness, devices can be processed.

For application of this technology in bipolar devices, the electrical behavior of the bonding interface is of importance. This paper reports the electrical and structural characteriza­tion of the interface region of bonded n-type silicon wafers, without an intentional insulating Si02 layer between the wa­fers. Capacitance-voltage (C- V) profiling, secondary-ion mass spectrometry (SIMS), and transmission electron mi­croscopy (TEM) are used to study the bonded wafers. Mea­surements are compared with process and device simula­tions of such a bonded wafer pair. Finally, phenomena emerging from the measurements are explained in terms of an initial boron contamination of the wafer surfaces, present before bonding, and of antimony segregation at the bonding interface during annealing. The findings are discussed and the boron contamination is employed to explain the behavior of the resistivity near the interface of directly bonded p-type wafers6 and n-type wafers,7 as reported in literature.

II. SAMPLE PREPARATION

Two different n-type Si( 111) wafers, 4 in. in diameter, are used as the starting material. One consists of 50-0 cm, phosphorus-doped float-zone (FZ) material, the other one of 0.017-0 cm, antimony-doped Czochralski (CZ) materi­al. The surface of the CZ wafers has a misorientation of 3· towards the nearest (110) plane.

The wafers are polished and cleaned to have flat sur-

faces, free of contamination. The cleaning procedure con­sists of a modified RCA clean followed by a soft polish step.·l Finally the wafers are immersed in HNO, (65%) of 90 ·C for 20 min to grow a hydrophilic thin chemical oxide on the surfaces. After rinsing in super-Q water and drying, the FZ wafer is bonded to the CZ wafer, with their primary flats aligned parallel. To turn this relatively weak bond into a strong chemical bond the wafer pair is annealed for 3 h at a temperature between 1100 and 1125·C in N,. Next, the thickness of the FZ wafer is red uced to 1 00 ,u~ by grinding and polishing, after which the wafer pair is cut into several pieces for further characterization.

The electrically active doping profile of some pieces is measured with a mercury probe. After careful cleaning, the pieces are further thinned by etching in a mixture of SO ml HNO, (65%),10 ml HN03 (100%),20 ml CH1COOH (100%), and 16 ml HF (50%). This thinning pro~edure is interrupted several times to measure the doping concentra­tion with a mercury probe. After a subsequent cleaning step, thinning is continued. This sequence is repeated until the interface between the bonded wafers becomes visible in the doping concentration profile.

III. MEASUREMENTS

Electrically active doping profiles are obtained from ca­pacitance-voltage (C- V) curves, measured with a mercury probe. The mercury spot has a diameter of 600,um. Ohmic contacts are made by rubbing a gallium/indium eutectic on the back of the sample. This eutectic can easily be removed in HCI before cleaning and further thinning. Measurements are done at a frequency of I MHz and an oscillator voltage of 30 m V rms. The reverse bias of the mel~'U"''j '2.c'o.o\\\\'1 0)00=

is varied between 0 and 100 V. In the translation of the c-v curves to doping profiles, the depletion approximation is em­ployed. The donor concentration profiles obtained with this approximation are therefore apparent ones, and may deviate

6253 J. Appl. Phys. 68 (12), 15 December 1990 0021-8979/90/246253-06$03.00 © 1990 American Institute of Physics 6253

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Page 3: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

from the real ones where the use of the depletion approxima­tion is not justified.

The donor concentration of the phosphorus doped part is found to be 8 X lOll cm .l throughout the whole thickness of the layer. Only near the bonding interface is a deviation detected. The solid line in Fig. 1 shows an apparent concen­tration profile of a sample that has been thinned to a thick­ness of approximately 10 /Lm, measured from the bonding interface. Starting from a bulk concentration of 8.0 X 1013 cm " the profile shows a dip, followed by a steep increase.

Apart from the dip, this profile can be explained by dif­fusion of antimony from the CZ wafer, through the inter­face, into the FZ wafer during the high-temperature anneal. Obviously, the thin chemical oxides on the bonded surfaces do not evolve into a diffusion barrier for antimony during the anneal step, as has often been observed for arsenic diffusion in polysilicon emitters. R

Now the phosphorus from the FZ wafer is also expected to diffuse into the CZ wafer. This would lead to a small decrease of the donor concentration,just before the steep rise due to the diffused antimony, because phosphorus has a higher diffusion constant than antimony. This phosphorus diffusion, however, cannot explain the sudden onset of the deep dip, as it would lead to a more gradual and modest dip of only a few percent in donor concentration.

To discover the origin of this dip, we plotted the profile in the region of the steep increase with a higher depth resolu­tion, as shown in Fig. 2 (solid line). N ow a kink in the profile can be observed at the transition of the antimony diffusion profile into the dip. Such a kink was reproduced in all sam­ples and cannot be explained by diffusion of phosphorus and antimony only. A thin p-type layer in between the uniform background phosphorus doping and the antimony diffusion

1017

~ 10\6 I

E U

C 0

. .-' 10\5 ..., 0 L ..., C ill U

loti

1 C 0

~: u '\\"/ /

" . . ~,' i 10

13 .~ .. L. i 0.0 3.0 6.0 9.0 12.0

Oepth [f-l m)

FIG. I. Apparent donor concentration profiles from C- V profile measure· ments (solid line) and CURRY simulations (dashed line). both showing the dip due to the p-type layer. The position of the bonding interface is marked with an arrow.

6254 J. Appl. Phys., Vol. 68, No. 12, 15 December 1990

1017

'" 1016 I

E U

C 0

. .-' 10

15 ..J 0 L

..J C ill U

10li C 0

u

1013

B.7 B.9 9.1 9.3 9.5

Oepth (f.jm)

FIG. 2. Apparent donor concentration profiles from C- V profile measure· ments (solid line) and CURRY simulations (dashed line). both showing the kink and the tail of the antimony diffusion profile.

tail, however, can explain both the dip and the kink, as will be shown further on.

The suspected source of the acceptors causing this p­type layer is boron contamination of the wafer surfaces, pres­ent before bonding. This phenomenon is weIl known from molecular-beam epitaxy (MBE) of silicon, 9~ II where it gives rise to the so-called boron peak at the interface of the epi­layer and the substrate. This boron contamination is intro­

duced during wet chemical cleaning procedures which pro­duce hydrophilic surfaces. Although the source of the boron is not yet known, its presence on hydrophilic silicon surfaces is reported by several groups independently, and seems to be a common problem in silicon technology. Because the clean­ing procedure employed here is much like that for MBE wafers, the chemical oxide initiaIly present on the wafers to be bonded is also suspected of having such a boron conta­mination. During the high-temperature anneal this boron will diffuse into the wafers. Because boron has a higher diffu­sion constant than antimony it will diffuse deeper into the FZ wafer, overdoping both the phosphorus and the antimo­ny, provided there is enough initial boron present in the chemical oxides.

To reveal any boron present in the interface region, the FZ part of a sample was thinned to approximately 3.3/Lm. The boron and antimony profiles of this sample, measured simultaneously by SIMS with 1O.5-keV primary 0/ ions, are shown in Fig. 3 (solid lines). The presence of boron is clearly demonstrated by the Gaussian-like diffusion profile. The total boron dose amounts to 7.3 X 1011 cm 2, which is of the same order of magnitude as normally found in MBE­grown silicon layers.

Besides this, the measured antimony profile clearly demonstrates the diffusion of antimony through the inter­face. At the interface an increased concentration due to

Widdershoven. Haisma. and Naus 6254

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Page 4: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

1020

1019

'" I

1 OIB E l)

c 1017

0 . .J ...., 0

1016

L ...., C Q)

1015

U C 0

u 10

11

""";/, .. -j-- ....... ,{ 1 0

13

0.0 2.0 4.0 6.0 Depth (~m)

FIG. 3. SIMS profiles of antimony and boron (solid lines) and SUPREM-3

simulations of antimony, boron, and phosphorus profiles (dashed lines). The position of the bonding interface is marked with an arrow.

segregated antimony is measured. This segregation is related to the segregation of arsenic at the poly/mono interface in n­

p-n bipolar transistors with polysilicon emitters. H

From the width of the antimony segregation peak it can be concluded that the SIMS depth resolution, although poor, is much better than the width of the boron profile. This means that boron is already measured in the FZ wafer before the bonding interface is reached by the SIMS sputter pro­cess. Therefore, matrix effects in the boron measurement (e.g., an increased ionization efficiency of boron near the interface as a result of excess oxygen) are excluded. This, furthermore, is supported by the fact that the measured bo­ron profile is smooth and continuous across the interface.

The SIMS detection limits for antimony and boron are 2.3 X 1015 cm - J and 1.5 X 1014 cm - -', respectively. The bo­ron profile clearly intersects the extrapolated antimony pro­file at a concentration of 2.0X 1015 cm - 3. This value is 25 times as high as the phosphorus background doping level, proving the existence of the suspected p-type layer.

The crystalline structure of the bonding interface was studied by transmission electron microscopy (TEM). Fig­ure 4 shows a high-resolution TEM cross section of the inter­face region, which reveals the presence of a 4.4-nm-thick amorphous layer at the interface. Lower magnifications showed this layer to be continuous. No disintegration of the layer by the formation of oxide spheroids 12 has been detect­ed. From the diffusion of antimony through the interface, however, it can be concluded that the amorphous layer does not behave as a stoichiometric Si02 layer. If this were the case, the layer would act as a diffusion barrier, because of the high segregation coefficient of antimony at the SilSi02 in­terface (-10.0) and the low diffusion constant of antimony in Si02 ( - 2 X 10" Ib cm2 s - I) at the anneal temperature. Here the segregation coefficient is defined as the equilibrium

6255 J. Appl. Phys., Vol. 68, No. 12, 15 December 1990

FIG. 4. High-resolution TEM cross section of the interface of the bonded silicon wafers, revealing the presence of a 4.4-nm·thick amorphous interfa. ciallayer.

ratio of the antimony concentration in Si to that in Si02 •

Because of the absence of oxide spheroids and the pres­ence of antimony segregation, the interfacial layer is expect­ed to be an amorphous SiO x layer, with x < 2. To check this hypothesis, the total oxygen dose present in the interfacial layer was measured by SIMS with 14.S-keV primary Cs +

ions. With this high ion energy the interfacial layer is mixed with the surrounding silicon before it is sputtered off, leading to a dilution of the oxygen concentration in the silicon ma­trix. This makes the calibration of the oxygen dose more accurate. The loss of depth resolution, caused by the mixing process, is not a serious drawback because the FZ part of the thinned sample was slightly wedge shaped as a result of the thinning procedure. Therefore, the depth resolution was al­ready far from sufficient to resolve the depth distribution of the oxygen explicitly. The measured oxygen profile consists of a sOD-nm-wide (full width at half maximum) peak with a peak concentration of l. 7 X 1O~() cm J on a background of 6.0X WiN cm 3. This background ccnC'!.~t",>'b.t";"~...,:,.,, ~"<>t "<'0

the SIMS detection limit and is about one order of magni­tude above the expected oxygen concentration in the CZ wa­fer. Therefore, interference with the oxygen present in the CZ wafer is excluded, and the measured oxygen is attributed

Widdershoven, Haisma, and Naus 6255

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Page 5: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

n 1

E

1020

U 1019

C o .~

..-J o L

..-J

~ 101B U C

8 /,:::'-'

3.1

_.1 ... ----1J]Lk _~ __ ---' 3.2 3.3 3.4 Depth (~ml

3.5

FIG. 5. SIMS profile (solid line) and SUPREM-3 simulation (dashed line) of the antimony segregation peak at the bonding interfa·~". The positIons of the Si( FZ)/SiO

ll <_ interface and of the SiO,,,"/Si(Cl) interface are marked

with an arrow.

to the interfacial layer. After subtraction of the background, I' 2 . the total oxygen dose amounts to 9.0 X 10 . cm . AssumIng

the oxygen to be distributed uniformly over the width of the interfacial layer, an oxygen concentration of2.0 X 1022 cm .1

in the interfacial layer is deduced. To determine the compo­sition of the interfacial layer, the average volume of one SiO x

molecular unit as a function of the parameter x is approxi­mated by interpolation between that of Si and that of Si02 with the expression

V(x) = (1 - ;) V(O) + ; V(2)

= xC (; I, (1)

where Co is the oxygen concentration in the interfacial lay­er. From this expression x can be solved:

2V(0)Co x = .. c. ..... ~~_~_~ __ (2) 2 + [V(O) - V(2)]Co

With V(O) = 2.0X 10 2.1 cm" V(2) = 4.6X 10 23 cm" and Co = 2.0 X 1022 em " the value x = 0.54 is obtained.

The interfacial layer is thought to be prevented from crystallizing by the presence of the oxygen and by the surface misorientation (3°) of the CZ wafer. The oxygen deficiency probably stems from a lack of oxygen in the initial chemical oxides. ~ Epitaxial realignment, as is observed for polysilicon grains,I2 is not possible because both sides of the interface are monocrystalline and have a mutual surface misorienta­tion of 3°.

IV. COMPUTER SIMULATIONS

To gain more insight into the diffusion profiles, we sim­ulated the anneal step with the process simulation package

6256 J. Appl. Phys .. Vol. 68. No. 12.15 December 1990

SVPREM-3. D The interfacial layer was treated as a 4.4-nm­thick insulating layer of unknown composition. In this layer the diffusivities of boron and phosphorus were set to 1. 8 X 10 \0 cm 2 s I, and that of antimony to 1.2 X 10 12

cm 2 s I. The interface transport coefficients were set to 6.0X 10 .1 em s" I for all impurities. At these high values of the diffusivities and interface transport coefficients the inter­facial layer is almost transparent to all simulated impurities in the temperature range of 1100-1125 dc. Segregation of antimony was simulated by setting the segregation coeffi­cient at the SiiSiOo ,4 interface to 0.016. A segregation coef­ficient of 1.0 was used for both boron and phosphorus. For boron this value is supported by the SIMS measurements, which did not show any evidence of boron segregation. For phosphorus this value is arbitrary, but it turns out to have no practical significance for the net donor/acceptor profiles, because near the interface phosphorus is overdoped com­pletely by either antimony or boron. The initial values of the phosphorus, boron, and antimony concentrations were set to 8.0X10 11 cm \1.66XIO IK cm \and2.17XIO IK cm .lin the FZ wafer, the interfacial layer, and the CZ wafer, respec­tively.

Figure 3 (dashed lines) shows the simulated profiles. The excellent fit with the SIMS profiles of boron and antimo­ny was achieved by varying the antimony segregation coeffi­cient and the anneal temperature. The profiles were found to be very sensitive to minor changes of the temperature. The latter, however, could not be measured with the desired ac­curacy in the annealing furnace, and therefore had to be treated as a fitting parameter. The resulting value of 1115 °C is well between the expected limits (1100-1125 °C) and, to­gether with the antimony segregation coefficient, is the only value that gives a good fit of both the antimony and the boron profiles simultaneously. The small skewness near the top ofthe SIMS boron profile, caused by the influence of the antimony profile on the diffusion of boron, is particularly well reproduced by the SVPREM-3 simulations.

The segregation coefficient of 0.016 leads to a calculated antimony concentration of 4.4 X 10 14 em' in the interfacial

l' layer. The segregated dose thus amounts to 1.94 X 10 ' em 2. In the SIMS antimony profile there is a segregation peak at the interface. Due to the high sputter rate, the sput­ter-induced surface roughness, and the etch-induced wedge shape of the FZ part of the thinned sample, the SIMS depth resolution is far from sufficient to reveal the true shape of the antimony profile in the interface region. A magnification of the measured profile is shown in Fig. 5 (solid line), together with the simulated one (dashed line). The integrated dose of the measured peak is between 1.5 and 3.4X 10" em 2. The first value applies to an integration over the two measure­ment points nearest to the interface, the second one to an integration over the four nearest measurement points. So the simulated dose fits well in the experimental range. This, fur­thermore, proves the negligible influence of matrix effects 12

on the SIMS profile, because the sample is almost saturated with oxygen due to the high energy of the primary 0/ ions.

With the simulated impurity profiles as input, a C- V curve of a Schottky diode at a frequency of 1 MHz is simulat­ed with the general purpose device simulation package CUR-

Widdershoven. Haisma. and Naus 6256

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Page 6: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

Ry.14 For this purpose only the impurity profiles in the FZ wafer are needed because the depletion layer never reaches the interface at realistic reverse-bias voltages. The thickness of the simulated layer is 9.9 11m. On top of this layer a Schottky contact with a barrier height of 0.70 eV is defined. At the interface of the FZ wafer and the interfacial layer an ohmic contact is defined.

From the simulated c- V curves the apparent donor con­centration profile is extracted with the same algorithm as used for the measured C- V curves. Figure I (dashed line) shows the result. Like the measured profile, the simulated one has a dip in the boron-dominated region. This dip, how­ever, is narrower and deeper. This difference can be ex­plained by the etch-induced wedge shape of the sample which leads to a wider and shallower dip. The influence of such a wedge shape is more prominent in the C- V concentra­tion profile than in the SIMS profiles, because of the larger diameter of the Schottky contact (600 11m) compared to that of the SIMS analysis spot (10 11m).

The simulation also shows the kink present in the appar­ent donor concentration profile, as can be seen in Fig. 2 (dashed line). The small deviations from the measured pro­file again can be explained by the wedge shape of the sample and furthermore by small errors in the boron and antimony profiles (measured by SIMS) relative to the phosphorus background doping level (measured by C-Vprofiling). Sim­ulations of a wedge-shaped sample indeed confirm the influ­ence on the width and depth of the dip, but no attempt was made to get a perfect fit.

v. DISCUSSION

The SIMS measurements clearly demonstrate the pres­ence of boron at the interface. This may lead to unexpected electrical behavior of the interface in device structures. Here it gives rise to a dip and a kink in the apparent donor concen­tration profile. In directly bonded p-type wafers it will lead to an unexpected decrease of the resistivity near the inter­face. This phenomenon has already been observed in Ref. 6, where the authors do not give an explanation. In Ref. 7 an increase of the spreading resistance near the interface of di­rectly bonded, lightly doped (10-50 n em) n-type Si wafers is reported and explained in terms of a potential barrier due to charged interface states. The presence of diffused boron near the interface would be an alternative explanation.

The dip and kink occurring in the apparent donor con­centration profile are artifacts of the use of the depletion approximation in the C- V profiling algorithm. The p-type layer turns out to be depleted completely by the diffusion potentials of the p-n junctions at either side. So no free holes are present and the p-type layer only acts as a potential bar­rier for electrons. The onset of the dip occurs when the deple­tion layer of the Schottky diode and that of the phosphorus­boron p-n junction approach each other within a few Debye lengths. The depletion approximation then breaks down. However, there are still enough free electrons present in the potential well between the merging depletion layers to have a region where the local electron current dominates the local displacement current. The width of this region, however, will decrease very rapidly with increasing reverse bias. This,

6257 J. Appl. Phys., Vol. 68, No. 12, 15 December 1990

in turn, gives rise to a fast decrease of the capacitance which, in the depletion approximation, is interpreted as a decrease in the donor concentration. At a certain reverse bias the width of the electron-current-dominated region becomes zero and the displacement current takes over. The fast de­crease of the capacitance then stops suddenly. This leads to the steep increase of the apparent donor concentration, which ends up at the kink. At still higher reverse-bias vol­tages the capacitance will be determined by the antimony diffusion profile. Because the antimony profile has already been partly depleted by the diffusion potential of the boron­antimony p-n junction, the concentration at the kink is high­er than the concentration at the metallurgical p-n junction. After the kink, the depletion approximation becomes valid again and the measured donor concentration profile equals the antimony diffusion profile.

An explanation of the dip by charge trapping at the bonding interface is excluded here, because the depletion layer edge never reaches the interface. Even at the highest reverse-bias voltage of 100 V the distance to the interface is still 380 nm. This is 24 times the upper limit of the Debye length in the neutral region between the depletion layer edge and the interface (16 nm at the depletion layer edge for 100-V reverse bias). Therefore, the influence of the bonding in­terface on the C- V measurement is completely screened by the neutral region, and any effect of charge trapping on the C- V measurement is excluded. Furthermore, it is most un­likely that charge trapping can explain the presence of the dip and the kink at the same time, anyway.

Despite the presence of a 4.4-nm-thick continuous amorphous SiOo54 layer at the interface of the bonded wa­fers, the interface does not behave as a diffusion barrier to antimony, as it would in the case of a stoichiometric Si0

2

layer of the same thickness. The difference in chemical na­ture between the SiOo54 layer and stoichiometric Sial is also demonstrated by the low value of the antimony segregation coefficient (0.016) of the SiiSiOo54 interface, needed to fit the simulations to the SIMS measurements, in contrast to that of the Si/Si02 interface ( - 10.0).

VI. CONCLUSIONS

Boron contamination at the interface of bonded n-type silicon wafers is deduced from C- V profiling results and is confirmed by SIMS measurements. The boron is thought to be present in thin initial chemical oxides on top of the wafers to be bonded. It gives rise to a p-type layer present near the interface of bonded n-type wafers and may lead to unexpect­ed electrical behavior of devices processed in bonded silicon wafers. This is demonstrated here by a dip and a kink in the apparent donor concentration profile, determined by C- V profiling. The decrease of the spreading resistance near the interface of directly bonded p-type silicon wafers, reported in Ref. 6, and the increase near the interface of directly bonded n-type silicon wafers, reported in Ref. 7, are other examples that can be explained by initial boron contamination oJ tne wafer surfaces. In silicon MBE growth, this boron conta­mination is already known from the boron peak at the inter­face of the epilayer and the substrate.

Antimony is shown to diffuse through the bonding in-

Widdershoven, Haisma, and Naus 6257

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Page 7: Boron contamination and antimony segregation at the interface of directly bonded silicon wafers

terface and to segregate there. The presence of a 4.4-nm­thick amorphous interfacial SiOn q layer is revealed by TEM and SIMS. The diffusion and segregation phenomena, detected by SIMS. are simulated satisfactorily by the process simulation package SUPREM-3. in which the interfacial layer is given an initial boron contamination. The fit to the SIMS profile results in an antimony segregation coefficient of 0.016 at the SilSiO" 54 interface. Simulation of the C- V pro­tiling results by the device simulation package CURRY, with the SUPREM-3 results as input, confirms the presence of a dip and a kink in the apparent donor concentration profile. Both phenomena can be explained by the breakdown of the deple­tion approximation in the interpretation of the C- V curves.

ACKNOWLEDGMENTS

The authors wish to thank T. M. Michielsen for the bonding and polishing of the wafers, P. C. Zalm and C. J. Vriezema for the SIMS measurements, and C. W. T. Bulle­Lieuwma for the TEM work.

6258 J. Appl. Phys .. Vol. 68, No. 12. 15 December 1990

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Widdershoven, Haisma, and Naus 6258

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128.193.164.203 On: Sun, 21 Dec 2014 02:44:10