Texture of titanium self-aligned silicide (salicide)

Scripta Materialia, Vol. 35, No. 1, pp. 53-58, 1996 Elsevier Science Ltd Copyright 0 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/96 $12.00 + .OO

Pergamon PII S1359-6462(96)00103-O

TEXTURE OF TITANIUM SELF-ALIGNED SILICIDE (SALICIDE) Wen-Kai Wan and Shinn-Tyan

Wu

Department of Materials Science and Engineering, National Tsing-Hua University, Hsinchu,Taiwan, Republic of China (Received February I, 1995) (Revised January 3, 1996) Introduction

The conventional self-aligned silicide (salicide) in IC manufacturing [l-4] is made by depositing a thin (1 O-50 nm) titanium film on silicon wafer by physical vapor deposition (PVD) plus two rapid thermal annealing (RTA) to induce reaction between titanium and substrate. The first RTA is done at about 600°C and the second at about 800°C. Both heatings last shorter than one minute. The limited thermal budget inhibits the dopant diffusion so that p-n junction and MOS channel retain their integrity. The titanium is chosen because of low resistivity and clean silicide-substrate interface [5,6]. This is known to originate from the snow-plow effect [7-91. Furthermore the self-alignment reduces the number of lithography steps [lo] and lowers processing costs. Consequently salicide is expected to be widely applied. To make a via plug titanium nitride and aluminum alloy are deposited successively on the top of salicide to draw the electric current out of source or drain to interconnects [ 1I]. The reliability of a TiSi,-TiN-Al tri-layer is an important issue. In particular the electromigration resistance of the aluminum alloy is heavily effected by its texture[ 1211.Since the top layer nucleates on the under layer the texture of latter might influence that of the former. Therefore the texture of the aluminum alloy is expected to be correlated with that of the underlying silicide. Then, for the reliability of devices using salicide, the study of silicides’ texture seems relevant to process control in wafer fabrication. Experimental

The titanium fihns are deposited by magnetron sputtering on p-type (boron-doped), -oriented silicon wafers of four inch diameter. The resistivity is between l- 15 R-cm. The wafers are degreased in an ultrasonic bath with a solution of acetone and trichloroethylene. After rinsing with de-ionized water they are kept in methanol. Prior to sputtering the wafer is again cleaned with de-ionized water using ultrasonics. The wafer is then dipped in dilute hydrofluoric acid for ten seconds to remove the surface oxide and dried with nitrogen gas. The wafer is placed on the substrate holder immediately to avoid contamination from the ambient because the sputtering is not done in a clean room. Vacuum pumps are activated to reach 3 x IO” ton using a diffusion pump. Pure Argon (99.9995%) is flown into the chamber to raise the pressure to 5 x 10” torr. The halogen lamps on the back of substrate holder are turned on to raise the substrate temperature. In the meantime the target surface is cleaned by sputtering for thirty minutes while the shutter 53

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TEXTURE OF TITANIUM SELF-ALIGNED

SILICIDE

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shelters the substrate from contamination. A thermocouple is attached to the back of substrate. After presputtering the shutter is opened and coating starts. The rate is approximately Snm/min. The coating time is ten minutes. Then all power supplies are turned off. The substrate cools under flowing argon for two hours to about 50°C before the chamber is opened. The film thickness is determined by the method of astep to be 8Onm. The crystalline phases in the tihn are identified by X-ray diffractometer (XRD). Because the signals are weak, a step-scan method is used. It takes two hours to scan the whole range. At SOOYZor 900°C the RTA lasts 40 seconds under protective argon atmosphere. The plane-view sample is examined in a JOEL-200CX scanning transmission electron microscope (STEM) operating at 200 kV. The sheet resistance is measured by a four-point probe. Each resistivity value is the average of more than 10 readings. The error is below 10 percent. The thickness of the silicide layer is determined from cross-section SEM and depth profiling using secondary ion mass spectroscopy. Results and Discussion

The X-ray data of Fig. 1 shows that as-deposited films have a pronounced texture. The texture is sensitive to deposition temperature. Notice that the preferred orientation follows the sequence (1 0 O),(O 0 2) (1 0 1) as substrate temperature increases from 25°C to 600°C. Igasaki and Mitsuhashi [13] deposited titanium films onto glass substrates using electron beam. They found that the preferred orientation changed from (0 0 2) at 100°C to (1 0 1) at 500°C in agreement with our results. This is interesting in view of the different deposition methods and substrates used. Hibbs [ 141deposited titanium films onto steel substrates by electron beam evaporation over a range of substrate temperatures and oxygen partial pressures. They proposed that the change of preferred orientations of titanium films was the result of preferential oxidation of certain crystal planes. When a reactive gas was present during deposition, only grains with some special orientations survive as the film is growing thicker. The peaks in Fig. 1 are broad implying that the grain sizes are small. It is estimated to be about 35nm by Scherrer formula[ 151. After annealing the titanium reacts with the substrate to form the silicide phase as shown in Fig.2 and Fig.3. The silicides are mostly oriented along (3 1 1) for deposition temperature lower than 400%. It changes to (0 0 4) when 400°C is exceeded. This change also coincides with the change of orientations of as-deposited films from (0 0 2) to (1 0 1) as shown in Fig.1. An exception is shown at the top of Fig.3. RTA at 900°C of the film deposited at 600°C results in a silicide film without preferred orientation. The relative intensities of the peaks are quite similar to the powder data as listed in the JCPDS card as reproduced at the bottom of Table 1.

6OO’C

R.T 35

45

40

50

28 Figure 1. XRD patterns of Ti films. The deposition

temperatures

are indicated on the right-hand

side

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TEXTURE OF TITANIUM SELF-ALIGNED

SILICIDE

55

500%

A

L

4oo”c h 300% h

/\ R.T

R.T.

35

40

45

50

20 Figure 2. Same as Fig.1 for TiSi, by RTA at 800% for 40 sec.

Figure 3. Same as Fig. 1 for TiSi, by RTA at 900% for 40 sec.

The corresponding TEM micrograph of Fig.4 demonstrates the expected small and randomly oriented grains. In contrast, textured silicides have larger grain size. An example is shown in Fig. 5. The intensity ratios of XRD peaks derived by quasi-quantitative integrating method are shown in table.1 Clearly the silicide films possess pronounced textures. In particular the titanium film deposited at 500°C and annealed at 900°C is almost totally oriented along (0 0 4). The change of preferential orientation happens between 400°C and 500°C of deposition temperature, coinciding with the change of the titanium film texture shown in Fig. 1 At present, the mechanism behind this correlation of textures is under investigation. The resistivity of the silicide films as measured by four-point probe is shown in Fig.6 Note that if the films were deposited at temperatures below 400°C and subsequent RTA at 9OO”C,the resistivities remain below 22 p&cm, this value is close to 18 pn-cm for the pure C54 phase [16]. The low resistivity is evidence for the #absenceof major contamination of impurities or of a substantial fraction of the resistive C49 phase in the:$efihns[17,18]. The corresponding unit cell volumes calculated from Fig.2 and Fig.3 are shown in Fig.& The volume expands as deposition temperature increases. It is always smaller than the unit cell of bulk TiSi,. A major change seems to occur at temperatures between 400°C and 500°C at which also

TABLE 1 Intensity Ratio Derived from XRD Profile by Quasi-Quantitative Index (h

Depositing temperature CC)

25 300 400 so0 600 Power

Integrating Method

k 1)

temperature (“C)

800 900 800 900 800 900 800 900 800 900 data from JCPDS

(3

1 I) (004)

(0 2 2) (3 I .3)

100

*

29.8 I

2.; ‘ii

100 100 100 100 100 41 0.s 3.43 II.07 100 100

* * * ’ * 100 100 100 32.75 43

14.37 17.38 24 6S IS 03 19.56 * ’ * 24 7.3 70

I2 22 I5 72 12.66 IO 02 18.6 * * * 12 29 4s

56

TEXTURE

OF TITANIUM

Figure 4. (a) Bright-field TEM micrograph ofTiSi, from the film deposited at 600’C and RTA at 900-C for 40 sec. Small grains with random orientations are discernible.

SELF-ALIGNED

SILICIDE

Figure 4.(b) Diffraction

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pattern of Fig.4(a)

the texture changes (Fig.2 and Fig.3). At higher substrate temperature the corresponding resistivities in Fig.6 are also higher. Therefore, the resistivity as the key property correlates strongly with the silicide texture. This is in contrast to the unit cell volume which is closer to the ideal bulk value at higher deposition temperature. The unit cell data suggest that lower internal stresses and/or interstitial impurities are present when the deposition temperature is higher than 400°C. Yet the corresponding resistivities are not lower contradicting with Matthiessen’s rule[ 191. To resolve this we propose that the anisotropy of the electric conduction is related to the dependence of resistivity on silicide texture. Titanium is a highly reactive metal and impurities are always present in the deposited films. The resistivities of titanium films (Fig.7) show a tendency to increase with deposition temperature suggesting an increase of interstitial oxygen content with temperature. This agrees with the well-known fact that at higher temperature the titanium film starts to incorporate substantial amounts of oxygen [9,20]. In view of the fact that our sputtering chamber is not of ultra-high-vacuum quality, the oxygen incorporation is highly likely. In fact,

Figure 5. (a) Same as Fig. 4(a) for TiSi, from the film deposited at 5OO’C and RTA at 8OO’C for 40 set, showing a large grain of TiSi, with [0 0 l] zone axis.

Figure 5. (b) Diffraction

pattern of fig. 5(a).

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TEXTURE OF TITANIUM

SELF-ALIGNED

40

g

SILICIDE

57

4001

30

E

La

300 -

9 N. 200 C .z .

.2 .g .%IC

.s

d

IOO-

d

C

0 R.T.

300

400

500

600

Substrate Temp.(%)

RTA Temp.(%)

Figure 6. Resistivities of TiSi, films.

Figure 7. Resistivities of Ti films

the heating of th.e substrate before sputtering was found to increase the pressure in the chamber, an indication of outgassing from chamber walls. Despite the increased oxygen uptake at higher temperature the unit cell volume of salicide is closer to the ideal one. This could be the consequence of the snow-plow effect [7-91. Conclusion

It is found that the texture of salicide correlates with the texture of the titanium film deposited by magnetron sputtering. The resistivity of salicide is lower than 22pR-cm if the substrate temperature is kept below 400°C during sputter deposition of titanium. If 400°C is exceeded the electrical resistivity increases concurrently with a change of texture. The dependence of the electrical resistivity of TiSi, on crystal orientation is proposed as the cause of the influence of film texture on electrical resistivity. Resistivity measurements of single crystalline silicide could provide a check of our hypothesis. Acknowled!zments

The study has been supported by the Republic of China National Science Council under contract number NSC84-222 1-EO07-02 1.

SubsIrate Temp.(“C)

Figure 8. Unit cell volumes of TiSi, derived from XRD patterns.

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References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 1 I. 12. 13. 14. 15. 16. 17. 18. 19. 20.

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