The influence of silicon substrate crystallinity and doping on TiSi2 thin film morphology

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Applied Surface North-Holland

Science 73 (I 993) 280-284

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applied surface science

The influence of silicon substrate crystallinity and doping on TiSi, thin film morphology T.E. Karlin ‘, S. Nygren ‘,I, L. Mattsson ’ Royal Institute of Technology, Solid State Electronics.

‘, E. Andre

‘, M. Bjuggren

P.O. Box 1298, S-164 28 Kista-Stockholm, Sweden

h Institute of Optical Research, S-100 44 Stockholm, ’ CNET, B.P. 98, F-382 43 Meylan Cedex, France ’ IBM, T.J. Watson Research Center. P.O. Box 218, Yorktown Received

29 March

1993; accepted

for publication

30 June

’ and F.M. d’Heurle

U’

Sweden

Heights, NY 10.~98, USA 1993

TiSi, was formed on amorphous, polycrystalline or single crystal silicon. Undoped samples are compared with specimens where the silicon had been doped with phosphorus or arsenic prior to titanium deposition. A non-destructive optical technique. total integrated scattering (TIS) was used together with more conventional methods to evaluate the surface conditions after variou< treatments. These analyses were correlated with Rutherford backscattering spectrometry (RBS) to obtain more comprehensive information on the bulk morphologies. The dopant species play an important role for the morphology and stability of these structures. It is shown that TiSi, on undoped polysilicon is unstable at temperatures above 800°C and that an amorphous substrate offers potential advantages such as a smoother and more stable silicide.

1. Introduction

Titanium disilicide is an important compound for self-aligned silicide applications in MOS technology. Geometrical down scaling of devices calls for thinner films which leads to higher sensitivity to morphological perturbations. [ 11 The use of heavily doped n-type polysilicon gates in CMOS devices requires a shallow boron implantation into the channel region to adjust the threshold voltages. This makes the p-channel transistor a buried channel device and more susceptible to short-channel effects [2,3]. One solution to this problem is to separately tailor the workfunctions of the polysilicon gates by implanting p-type dopants for p-channel transistors and n-type dopants for n-channel transistors [4]. This leads to lower dopant concentrations in the gates. High temperature limitations of TiSi, have been reported in a number of publications [ 1,5],

’ Present address: Texas Instruments, 944, Dallas, TX 75265. USA. 0169.4332/93/$06,00

SC 1993

Elsevier

P.O. Box 655012,

Science

Publishers

MS

but the influence of the micro-structural properties of the underlying silicon and the incvitablc presence of impurities are not well known. This work focuses on the influence of silicon crystallinity and doping and titanium thickness on the morphology and stability of TiSi?. Undopcd silicon was investigated together with silicon implanted with arsenic or phosphorus since n-type dopants are known to modify the grain structure of polysilicon [6]. The morphology has been cvaluated after formation at low temperatures, phase transformation to C.54 structure at intcrmcdiatc temperatures and prolonged treatments at high temperatures.

2. Experimental 2.1. Sunlple

preparution

Titanium silicide was formed directly on single crystal silicon ((100) or (111)) wafers or on 400 nm polycrystallinc or amorphous silicon layers.

B.V. All rights reserved

281

T.E. Karlin et al. / Silicon substrate influence on TiSi,

both deposited by LPCVD on thermally grown SiO,. Some of the LPCVD samples were implanted with arsenic (4 x 1015 crnm2) or phosphorus (1 x 1016 cm-*); following which the dopants were activated by rapid thermal annealing (RTA). The wafers were divided into two groups devoted to the investigations of silicide formation and thermal stability, respectively. All wafers were cleaned and stripped of native oxide before metal deposition. Wafers used in the formation study were sputter coated with 70 nm of titanium and an additional protective TiN cap. The silicide was formed by RTA in two steps. After annealing at 700°C the nitride and unreacted titanium were removed in a selective etch and the silicide was transformed to the stable C54 phase by a second anneal at 900°C. The wafers for the thermal stability study received 20, 40 or 60 nm of titanium without any protective cap. Silicide was formed by RTA at 800°C followed by additional furnace treatments for 1 h at temperatures ranging from 700 to 1000°C in a vacuum better than 7 X lo-’ Torr. All RTA treatments were performed for less than 1 min in a nitrogen atmosphere. 2.2. Analyses Rutherford backscattering with 2.4 MeV 4Hef ions was used for bulk morphological analyses. Surface topography was measured with a profilometer (Talystep), atomic force microscope (AFM) and by total integrated scattering (TIS) [71. The Talystep 181 data obtained by a transversing diamond tip of 0.3 pm radius at a stylus load of 0.5 mg, cover surface spatial wavelengths of 0.2-20 pm. TIS was measured with a low noise optical instrument capable of measuring surface roughness down to the sub-&rgstriim range [9]. The instrument is based on a hemispherical mirror and a He-Ne laser (632.8 nm wavelength) with a spot size of 1 mm. In the diffuse reflectance mode it collects scattered light in the angular range from 2.3” to approximately 80” which corresponds to scattering from correlated surface features of 0.7-15 Frn surface spatial wavelengths. The TIS value is defined as the ratio

between the diffuse reflectance R, and the total reflectance of the surface RSpecular+ R,; for small values it is proportional to the mean square value of the height deviation from an ideally flat surface. The high values of the TIS data in this work prevent us from converting the TIS data to RMS roughness because of limitations in the scalar scattering theory for very rough surfaces [9]. We therefore restrict our data to TIS values. The silicide/ silicon interface roughness was measured with AFM after chemical removal of the silicide. Sheet resistance measurements with a 4-point probe were used together with X-ray diffractometry (XRD) to verify the transformation of the silicide from the metastable C49 phase to the stable C54 phase.

3. Results 3.1. Formation The silicide thickness on the samples in the formation study was measured by RBS after the 700°C formation anneal and selective etch (fig. 1). Total integrated scattering (TIS) data, both after the RTA at 700°C (and selective etch) and after the RTA at 900°C are shown in fig. 2. RMS roughness calculated from AFM measurements on undoped as-deposited polysilicon and amorphous silicon and on some of the silicide samples before and after the RTA at 900°C are presented in fig. 3.

L

Fig. 1. Silicide thickness measured by RBS after silicide formation by RTA at 700°C and selective etching.

2x2

20

0 Fig. 2. TIS data before

and after the transformation 900°C.

anneal

at

Both the amorphous silicon layers (undoped and arsenic doped) gave the smoothest silicides. The thickest and smoothest silicide was formed on undoped amorphous silicon (fig. 4). The silitides on polysilicon were the roughest. To some extent this is probably due to the initial roughness of the as-deposited polysilicon surface (RMS,,, = 100 A> which is rougher than the surfase of the as-deposited amorphous (RMS,,, = 6 A) or single crystal silicon. Doping the polysilicon with arsenic does not seem to significantly affect the roughness of the silicide while phosphorous doping gives a much rougher silicide. The difference in silicide roughness between the arsenic and phosphorous doped polysilicon is too large to be solely attributed to

250

200 4 z

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100 50

cI O(I)-Si

the undoped

amorphous

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the difference in silicide thickness (which is smaller than the inaccuracy of the thickness mcasurement). AFM shows that after the RTA at 700°C the roughness amplitude for the silicidc/ silicon interface is 30% to 50% higher than for the silicide surface. Both TIS and AFM show that the transformation anncal at 900°C does not significantly dcgrade the morphology (figs. 2, 3. Sa, 5b). Sheet resistance measurements and XRD verified the transformation from the metastable (‘40 phase to the stable CS4 phase. 3.2. Thermal .stahilit~

n Substrate i 700°C

150

0

Fig. 4. The AFM picture shows the smooth ailicide surface on

poly-SI

poly-Si/A\

alia-si

Fig. 3. RMS roughness calculated from AFM meaburemrnts on the as-deposited undoped polysilicon and amorphous silicon and on some of the silicide samples hefore and after the transformation anneal at 900°C.

For samples deposited with the same amount of titanium there is no significant difference in silicide thickness according to RBS, after formation at 800°C. Fig. 6 compares the temperature stability of a thin TiSi, layer (20 nm as-dcpositcd Ti) on undoped poly&stallinc, amorphous and monocrystalline silicon. The silicidc layer on undoped polysilicon is the most unstable, while the layer on undoped amorphous silicon is the most stable. The series of Talystcp scans in fig. 7, ot different samples annealed for 1 h in vacuum at different temperatures, show how the roughness of a thin TiSi, layer on undoped polysilicon ill-

creases when the temperature is raised from 800 to 900°C. After annealing at 900°C the RMS value of the height deviation measured by Talystep is as large as 90 nm, which should be compared to the 50 nm thickness of the as-formed silicide. This indicates that the structure should

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60

z 'p.

40

20

0 800

820

840

Annealing Fig. 6. Total

integrated

scattering (TIS)

series of the SO nm TiSi, on undoped

860

polysilicon

880

Temperature

900

[“Cl

from the tcmperaturc

samples showing the poor stability and the good stability

amorphous

on undoped

silicon.

be completely destroyed; indeed the corresponding RBS series in fig. 8 shows that the I h anneal at 900°C results in a surface consisting almost entirely of silicon. RBS measurements revealed that doping of the polysilicon with arsenic or phosphorus greatly enhances the stability. Doubling the silicide thickness (40 nm as-deposited Ti) improved the thermal stability considerably except for the silicide on undoped polysilicon.

4. Discussion The amorphous silicon layers give the smoothest as-formed silicide. The original smoothness (and silicide thickness) may retard

800°C

2

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840°C

z +oi -1 0.0 8 -01 Lm 2 +Ol w 00 -0 1

Fig.

5. Comparing

doped selective

polysilicon etching)

the after

AFM

pictures

formation

(a) and after

shows that the morphology

of TiSiz

by RTA

transformation

is not affected

tion anneal.

on arsenic

at 700°C at 900°C.

0

(b)

900°C

4

Fig. 7. Talystep

12

8

Horizontal

(and

by the transforma-

860°C

Talystep

16

surface profiles of 50 nm TiSi,

polysilicon annealed

in vacuum for tures.

20

Scan [pm]

I

on undoped

h at different

tempern-

3000

the silicide but through stabilising the structure.

the

polysilicon

thereby

2500

5. Conclusions

1

1 .I

1.2

1.3

Energy Fig. X. RBS profiles annealed

1.4

1.6

1.5

of SO nm TiSi L on undopcd

in vacu~~m for

1.7

1 .a

[ MeV 1

I Ii at different

polysilicon

temperaturch.

silicide agglomeration effects and thus explain the thermal stability of these structures. A thicker silicide on undopcd polycrystalline silicon does not significantly improve the stability of the structurc which indicates that silicide agglomeration per se is not the major issue in the undoped polysilicon case. RBS profiles reveal that the morphological instabilities of TiSi, on undopcd polysilicon are associated with a breakdown of the entire bilayer structure. This is probably due to silicon grain growth of the same type as has been observed with cobalt [ IO,1 I] and nickel [ 1 I] silicidcs where the driving force is the reduction in surface energy of the silicon which cannot happen within the polysilicon alone bccausc of the low mobility of silicon in silicon. The silicidc layer acts as a medium in which the mobility of silicon is high enough to allow the transformation of polysilicon from a phase with smaller grains to one with larger grains [I I]. This process makes the silicide and the silicide/ silicon interface rougher. The n-type dopants used in this work arc known to enhance silicon grain growth, by increasing the silicon self-diffusivity as a function of dopant concentration [6]. With sufficiently high concentration of n-type dopants (but low enough arsenic dose to prevent clustering) in the polysilicon the silicon atoms no longer diffuse through

/summary

The morphology of titanium disilicidc formed by a thin film reaction between pure titanium and a silicon layer depends on the morphology and doping of the silicon. The smoothest silicides were obtained with amorphous silicon. Silicide on polycrystallinc silicon was significantly rougher, particularly in the presence of phosphorus. TiSi, on top of undoped polycrystalline silicon is no longer stable at temperatures exceeding 800°C. After 1 h at 000°C in vacuum the structure is completely destroyed with the surface consisting almost entirely of silicon. The structure can be stabilised with n-type dopants or by replacing the polycrystalline silicon with an amorphous layer.

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