The transition metal diboride compounds, ZrB_2 and TiB_2, interposed between Ni/Ge/Au Ohmic contact metallization on n‐type GaAs wafers and an overlying thick Au contact layer, have been investigated to evaluate their effectiveness in stabilizing the Ohmic contact by limiting the in‐diffusion of Au. All of the metal layers were e‐beam deposited except the ZrB_2 which was rf‐diode sputtered. The barrier layer thicknesses were 50 and 100 nm for the TiB_2 and the ZrB_2, respectively. Postdeposition alloying of the contacts was performed at 400, 425, or 450 °C. Auger electron spectroscopy depth profiling of the resultant Ohmic contacts demonstrates that the barrier layers effectively preclude penetration of Au to the Ohmic contact structure. Specific contact resistivities for such contacts are in the low 10^(−7) Ω cm^2 range; although some degradation of the contact resistivity is observed after long term annealing, the values of resistivities do not exceed 1.5×10^(−6) Ω cm^2 after 92 h at 350 °C

Finnegan, J.

Fox, D.

Kattelus, H.

Kwiatkowski, J.

Lux, R.

Nicolet, M.

Shappirio, J.

English

Caltech Authors - Main

TiB2 and ZrB2 diffusion barriers in GaAs Ohmic contact technology 
J. Shappirio, J. Finnegan, R. Lux, D. Fox, and J. Kwiatkowski 
U. S. Army Electronics Technology and Devices Laboratory, Fort Monmouth, New Jersey 07703 
H. Kattelus and M. Nicolet 
California Institute of Technology, Pasadena, California 91125 
(Received 10 May 1985; accepted 17 June 1985) 
The transition metal diboride compounds, ZrB2 and TiB2, interposed between Ni/Ge/ Au Ohmic 
contact metallization on n-type GaAs wafers and an overlying thick Au contact layer, have been 
investigated to evaluate their effectiveness in stabilizing the Ohmic contact by limiting the in-
diffusion of Au. All of the metal layers were e-beam deposited except the ZrB2 which was rf-diode 
sputtered. The barrier layer thicknesses were 50 and 100 nm for the TiB2 and the ZrB2, 
respectively. Postdeposition alloying of the contacts was performed at 400, 425, or 450 ·c. Auger 
electron spectroscopy depth profiling of the resultant Ohmic contacts demonstrates that the 
barrier layers effectively preclude penetration of Au to the Ohmic contact structure. Specific 
contact resistivities for such contacts are in the low 10-7 fl cm2 range; although some 
degradation of the contact resistivity is observed after long term annealing, the values of 
resistivities do not exceed 1.5 X 10-6 n cm2 after 92 h at 350 ·c. 
I. INTRODUCTION 
A wide variety of metallization schemes have been investi-
gated to provide reliable Ohmic contacts to III-V semicon-
ductors in general, and to n-type GaAs specifically. The 
principle of these contacts is to produce a highly doped re-
gion at the surface of the GaAs in order to achieve a low 
contact resistance between the semiconductor and the con-
tact metal. 1 For n-type GaAs this is accomplished by choos-
ing a metal composition which will alloy with the GaAs, and 
where one constituent of the metallization becomes a donor 
atom. The most widely studied and the most commonly ap-
plied Ohmic metallization consists of the eutectic Au-Ge 
alloy (88% Au) usually in combination with a Ni layer de-
signed to minimize ball-up during subsequent alloying as 
well as to assist in the decomposition of the GaAs. Alloying 
of the metallization layers is required at temperatures in the 
range 400-500 •c for times ranging from seconds to several 
minutes to obtain contact resistivities on the order of 10-6 
fl cm2 • Numerous studies (cf. Braslau2) have established that 
the resultant contact is laterally and vertically nonhomogen-
eous. In addition, the thick Au overlayer normally deposited 
above the Ni/Ge/ Au metallization in order to facilitate later 
device contacting provides a large reservoir of reactive mate-
rial that contributes both to the increasing depth of the con-
tact region by further in-migration of gold, as well as to con-
tact instability during subsequent device processing and 
operation. Total penetration depth of such alloyed contacts 
may reach 200 nm. Such a depth is unacceptably high for 
Ohmic contacts in emerging GaAs field-effect transistor 
(FET) and high electron mobility transistor (HEMT) struc-
tures which may require contact depths less than 50-100 
nm. 
ZrB2 thin films have been shown3 to possess rather 
remarkable diffusion barrier properties when interposed 
between silicon and second-level aluminum metallization 
heat treated to temperatures of 625 •c for up to 2 h; since x-
ray diffractometry indicates that as-deposited diboride films 
are amorphous, such films may function as diffusion barriers 
owing to the absence of fast diffusion paths (grain boundar-
ies) as suggested by Nicolet eta/. 4 We report here, studies on 
the use of ZrB2 and TiB2 diffusion barriers between 
Ni/Ge/ Au Ohmic contact metallization and overlying thick 
Au in order to evaluate their effectiveness in stabilizing 
Ohmic contacts by limiting the in-diffusion of gold. 
II. EXPERIMENTAL 
GaAs wafers employed for this study were 560 f-Lm thick 
n-type Si-doped (100) orientation with a carrier concentra-
tion of l-2X1018/cm3• Prior to Ni/Ge/Au Ohmic metal 
deposition, the wafers were washed with detergent, de-
greased sequentially in boiling electronic grade acetone, 
trichlorethylene, methanol, and acetone for 1 min each, 
etched in HCl to remove native oxide, and finally rinsed in 
deionized water. Ni/Ge/ Au were sequentially e-beam eva-
porated in a deposition chamber prepumped to 5 X 10-8 
Torr with Ni in contact to the GaAs and Au the final metal 
deposited. Total thickness of this metallization ranged from 
25-100 nm; Au amounted to 85 wt. %, Ge 10.4%, and Ni 
4.6% of the total metallization and the ratio of Au/Ge was 
selected to coincide with the eutectic alloy composition. Dif-
fusion barrier layers of TiB2 or Zr B2, 50-100 nm in thickness 
were next deposited. TiB2 was e-beam evaporated from a 
high purity, high density source material obtained from Ea-
gle Picher in the same chamber and in the same pumpdown 
as the Ohmic contact metals and was followed by thee-beam 
deposition of 200 nm of gold without breaking vacuum. Al-
ternatively, after Ohmic metal deposition, ZrB2 was rf diode 
sputtered onto the samples followed by a 200 nm e-beam 
gold deposition. 
Prior to metallization, samples were patterned for specific 
contact resistivity measurements by a liftoff process5 start-
ing with the deposition ofCVD Si3N4 followed by spin coat-
ing of AZ1450J positive photoresist. The resist was prebaked 
at 90 •c and a series of various sized disks exposed on a 
Kasper 3000 contact aligner. To replicate the pattern in the 
Si3N4 film, the resist was developed and the Si3N4 was etched 
2255 J. Vac. Sci. Technol. A 3 (6), Nov/Dec 1985 0734-2101/85/062255-04$01.00 @ 1985 American Vacuum Society 2255 
2256 Shapplrlo eta/. : TIB, and ZrB, diffusion barriers 
600r------r------~-----r------~----~ 
500 
\ 
u 400 \ 
w \ 
IX 
::J 
'I \ ... 
a: 300 \ IX 
w Q. \ J: 
w 200 \ ... 
' 
100 
20 30 40 50 
TIME (sec) 
FIG. 1. Temperature-time profiles used to alloy Ohmic contacts. Solid line 
represents minimum alloy cycle; dashed line represents 15 s hold at alloy 
temperature. 
in a (LFE Corp.) barrel plasma reactor using a CF4/02 gas 
mixture for 5 min at 150 W. The resist was then stripped in 
an 0 2 plasma and the wafers were recoated with AZ1450J 
and baked. A second contact mask with 100 pm circular 
openings on 200 pm center spacings was aligned concentri-
cally with the previously developed disks and exposed as 
before. The resist was then soaked in chlorobenzene for 4 
min and developed for 2. 5 min in AZMF312 developer in 
preparation for the liftoff process. For the all e-beam evapor-
ated metallizations, the resist was lifted off in acetone. For 
the sputtered ZrB2 films, the resist was soaked in acetone 
and then subjected to an oxygen plasma etch to fully remove 
the resist; this additional step was required because of dam-
age to the polymer during the rf metal deposition. 
After a preanneal nitrogen purge, both patterned and un-
patterned metallized samples were alloyed to form Ohmic 
2256 
contacts in a Penzac furnace under flowing hydrogen gas. 
Peak anneal temperatures, measured by a thermocouple 
contacting the strip heater, were 417, 438, and 455 •c for 
periods up to 2 min. Reactions leading to the formation of 
Ohmic contacts were monitored by postanneal Auger elec-
tron spectroscopy depth profiling. 
Typical annealing cycles, as shown in Fig. 1, consisted of 
an up-ramp to temperature followed by an immediate ramp 
down, or followed by a treatment at the maximum tempera-
ture for periods up to 2 min. Contact resistance was mea-
sured by a modification of the method of Cox and Strack. 6 
Resistance was measured between five pairs of circular con-
tacts ranging from 3 to 50 pm diam with contact-to-contact 
spacing of 200 pm deposited on 560 pm thick n-GaAs. For 
this geometry, where the substrate thickness and contact 
spacing are large compared to the contact radius, and the 
spacing is not large compared to the thickness, the measured 
resistance is given by7 
(1) 
1ra2Rmea.l2 = 1Tpa/4 + rc, (2) 
where a is the contact radius, p is the substrate resistivity, 
and rc is the specific contact resistivity. The termp/2a in Eq. 
( 1) is an approximation to the spreading resistance in the 
GaAs. A numerical calculation of the spreading resistance 
using the method of Berkowitz and Lux8 indicates the error 
of this approximation is 5% for the 50 pm contacts and less 
than 1 % for all others. 
From Eq. (2), a plot of half the measured resistance times 
the contact area versus diameter then gives a straight line, 
with they-intercept being the contact resistivity, as in Fig. 2. 
The major source of error is the accurate measurement of the 
diameter of the contact windows. To explore the effect of this 
error, we have artificially changed the value of all window 
diameters by ± 1 pm, or changed only the smallest window 
5 ~--r-.--~.--,---~.--~~~.~--.~--r-.--~--~.--~.---, 
,..., 
N 
E 
0 
I 
c 
::1 
....., 
N 
' .. • u 
& 
~ 
N 
IG 
t: 1 ~ ..... 
i'' 
.+' 
0' I 0 10 
,' 
.· 
,• 
' 
11.8 
.+' 
/// 
./'// 
Bk-------~~r-------~18 
I I I • I 
20 30 40 50 
DIAMETER (microns) 
J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985 
-
-
-
-
60 
FIG. 2. Typical plot of data in the form of 
Eq. (2) which is used to obtain the contact 
resistivity. The straight line is a least-
squares fit to the data. The insert is an ex-
pansion of the plot near the origin. 
2257 Shappirio et al : TiB, and ZrB, diffusion barriers 
TABLE I. Contact resistivity (.ufi cm2). 
As-deposited Peak temperature 
layer thicknesses(nm) 417'C 438'C 455'C 
25 0.1 (2) (2) 
50 0.5 (2) (2) 
75 (I) 0.2 (I) 
100 (I) 0.1 0.1 
by the same amount, leaving all others fixed. The resultant 
recomputed contact resistivities varied at most by 
± 1 X 10-7 fl cm2• In measuring the resistance, separate 
voltage and current probes were placed on the contact to 
eliminate the effect of probe resistance. 
Ill. RESULTS 
A. Specific contact resistivity 
Measurement of contact resistivity was made for four 
thicknesses of Ni/Ge/ Au metallization of 25, 50, 75, and 
100 nm, covered with 50 nm TiB2 and 200 nm Au. Three 
alloying cycles were used consisting of a ramp up to maxi-
mum temperature and an immediate ramp down. The total 
alloying cycle was approximately 50 s in duration as shown 
in Fig. 1. The results of these measurements are shown in 
Table I. 
For several combinations of alloying temperature and 
film thickness, the results did not fall on a straight line, thus 
precluding the determination of the contact resistivity. Two 
types offailures were observed: ( 1) failure of most or all of the 
3 and 6 pm contacts to form Ohmic contacts, and (2) anoma-
lously high resistance in the 3 and 6 pm contacts, well off the 
line extrapolated from the other contacts. Type (2) failures 
may be the result of incomplete removal of the resist from the 
contact openings. 
The lowest measured contact resistivities are lower than 
reported in the literature. We attribute these differences to a 
difference in measurement techniques. It has been shown 
that the transmission line model (TLM) and Kelvin tech-
1.5 
N 
e 
,A> u 
I 
..... c 
..... 
= ..... ..... 
>- ..... 
>- l.lil ..... 
.... ..... 
> 
_ .. 
.... 
>- o-Ul I .... 
Ul I 
"' ~ 
>- I 
u lii.S I a: 
>-
z I 0 
u I 
..; ~ > a: 
Iii 
Iii 50 100 
ANNEAL TIME AT 350C (hrl 
FIG. 3. Contact resistivity vs anneal time at 350 'C for contacts consisting of 
100 nm Ni/Ge/ Au capped by 50 nm TiB2 and 200 nm Au alloyed at 450 'C 
with minimum ramp times. 
J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985 
2257 
IBB 
z 
0 
.... 
..... 
a: 
Q: 
..... 
z 
w 
u 
z 
0 
u 
u 
.... 
:s:: 
0 
..... 
a: 
..... 
z 
w 
u 
Q: 
w 
D.. 
li!l 
SPUTTER TIME (min) 
FIG. 4. Auger electron spectroscopy sputter depth profile through 200 nm 
of Au over 50 nm of e-beam evaporated TiB2 aged at 500 'C for 3.5 h. 
niques as commonly used determine an effective contact re-
sistivity which for small resistivities can be greater than the 
true contact resistivity by a large factor.9 •10 For the present 
discussion of barrier layers, only the relative values of con-
tact resistivity are significant. 
B. Contact aging 
The deterioration of Ni/Ge/ Au Ohmic contacts under 
long term thermal stress has been reported by Marlow et 
al. 11 To investigate this effect a group of contact patterns 
consisting of 100 nm of Ni/Ge/ Au capped by 50 nm TiB2 
and 200 nm Au were alloyed at 450 •c with minimum ramp 
time up and down. The contacts were then annealed up to 92 
hat 350 ·c with periodic measurements of contact resistiv-
ity. The results, shown in Fig. 3, indicate a rapid deteriora-
tion of the contact resistivity after 3.5 h followed by a longer 
period of slowly increasing contact resistivity as was seen by 
Macksey. 12 The contact resistivity at 92 h did not exceed 
1.5 X 10-6 fl cm2• We attribute this performance to the ef-
fectiveness of the diboride layer in blocking in-migration of 
Au and out-migration of Ga. 
C. Barrier effectiveness 
The efficiency of the "amorphous" diboride thin film dif-
fusion barrier in preventing Au in-migration to the evolving 
Ohmic metallization can be estimated from the Auger data 
shown in Figs. 4 and 5. Figure 4 is an AES sputter depth 
profile through 200 nm of Au over 50 nm of TiB2 e-beam 
evaporated onto GaAs and heat treated for 3.5 h at 500 ·c. 
Figure 5 shows an AES profile through 200 nm Au over 100 
nm ofrf-diode sputtered ZrB2 over 100 nm total thickness of 
Ni/Ge/ Au on GaAs and alloyed at 450 ·c for 2 min. In the 
case of the TiB2 film shown in Fig. 4, the heat treatment 
conditions are far more severe than those typically employed 
in conventional GaAs Ohmic contact technology where 
temperatures are typically below 500 ·c and alloy times are 
measured in seconds; in contrast, the conditions employed 
for the ZrB2 example are generally consistent with standard 
processing. The important observation in both cases is clear, 
2258 Shappirio eta/. : TiB, and ZrB, diffusion barriers 
z 
0 
>-< 
t-
a: 
[t: 
t-
z 
w 
u 
z 
0 
u 
u 
>-< 
I: 
0 
t-
a: 
Zr 
t-
z 
w 
u 
[t: 
w 
0.. 
50 
SPUTTER TIME (minl 
FIG. 5. Auger electron spectroscopy sputter depth profile through 200 nm 
of Au over rf-diode sputtered ZrB2 over 100 nm total thickness of 
Ni/Ge/ Au on GaAs, alloyed at 450 •c for 2 min. 
namely that thin film diboride diffusion barriers are ex-
tremely effective in blocking the in-migration of Au. In the 
absence of such a blocking layer an already established me-
tallization structure achieved by some preselected alloying 
scheme could be further modified by the supply of addi-
tional, reactive gold to that interface, during either subse-
quent elevated device processing temperature, or by heat 
generated during the operation of a high power device. Fur-
thermore, the abruptness of the Au/ZrB2 and Au/TiB2 in-
terfaces which we observe suggests strongly that even thin-
ner diffusion barrier layers, perhaps as thin as 10 nm or less, 
would function effectively for this purpose. The minimum 
thickness required for such diboride barriers is presently un-
der investigation. 
IV. CONCLUSIONS 
Ni/Ge/ Au Ohmic contact metallizations at the lower 
limit of or thinner than those conventionally employed in the 
J. Vac. Sci. Technol. A, Vol. 3, No.6, Nov/Dec 1985 
2258 
industry have been alloyed on n-type GaAs and the contact 
resistivity for such contacts has been measured. The contact 
resistivity values which we obtain, in the low 10-7 !l cm2 
range, are consistent with or better than those previously 
reported in the literature, and are adequate for successful use 
in device applications. Although some degradation of con-
tact resistivity has been observed on long term annealing, 
values of resistivity do not exceed 1.5 X w-6 !l cm2• Inter-
posed diffusion barriers of e-beam evaporated TiB2 and of rf-
diode sputtered ZrB2 have been shown to limit the in-migra-
tion of gold from a top contacting layer of gold, and should 
improve the long term stability of the contacts. Effectiveness 
of such barriers has been demonstrated for thicknesses down 
to 50 nm, and the data suggest strongly that even thinner 
barriers should function with equal success. 
ACKNOWLEDGMENTS 
The cooperation of M. Wade, D. Eckart, L. Heath, D. 
Troy, and T. AuCoin is appreciated. 
'R. E. Williams, Gallium Arsenide Processing Techniques (Artech House, 
Dedham, MA, 1984), pp. 231-233. 
2N. Braslau, in Interfaces and Contacts, MRS Symposium Proceedings, 
edited by R. Ludeke and K. Rose (North Holland, New York, 1982), p. 
393. 
3J. Shappirio, J. Finnegan, R. Lux, and D. Fox, Thin Solid Films 119, 23 
(1984). 
4M.-A. Nicolet, I. Suni, and M. Finetti, Solid State Techno!. 25, 129 (1983). 
sM. Hatzakis, B. J. Canovello, and J. M. Shaw, IBM J. Res Dev. 24, 452 
(1980). 
6R. H. Cox and H. Strack Solid State Electron. 10, 1213 (196.7). 
7P. Severin, in Semiconductor Measurement Technology: Spreading Resis-
tance Symposium, NBS Special Publication, edited by J. Ehrstein (U.S. 
GPO, Washington, D.C., 1974). 
8H. Berkowitz and R. Lux, J. Electrochem. Soc. 128, 1137 (1981). 
9H. H. Berger, Solid State Electron. 15, 145 (1972). 
10M. Finneti, A. Scorzoni, and G. Soncini, IEEE Electron Device Lett. 
EDL-5, 524 (1984). 
''G. S. Marlow, M. B. Das and L. Tongson, Solid State Electron. 26, 259 
(1983). 
12H. M. Macksey, Inst. Phys. Conf. Ser. 33b, 254 (1977). 


TiB_2 and ZrB_2 diffusion barriers in GaAs Ohmic contact technology

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TiB_2 and ZrB_2 diffusion barriers in GaAs Ohmic contact technology

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