Defined etching of carbon-diamond films on silicon using an oxygen plasma with titanium masking

Diamond and Rel,ted Materials, 1 (1992) 281 284 Elsevier Science Publishers B.V., Amsterdam

281

Defined etching of carbon-diamond films on silicon using an oxygen plasma with titanium masking K. K. Chan, G. A. J. Amaratunga and T. K. S. Wong Department ~/' En~ineerin~, Cambridge University, Trumpington Street, Cambridge C B2 I PZ ( U K )

Abstract A technique for pattern transfer onto carbon-diamond films deposited by radio-frequency plasma-enhanced chemical vapour deposition is reported. Such a technique involves standard photolithography processes and reactive ion etching by oxygen and is compatible with present day microelectronic technology. The patterns transferred are well defined with very good resolution.

1. Introduction Application of diamond thin films and related materials in semiconductor devices has attracted much attention in recent years. This is due to the success of depositing such materials by various chemical vapour deposition (CVD) techniques. As in the case for other semiconductors, patterning of these materials is a very important process in device fabrication. So far the patterning of these materials has been achieved using selective deposition by defining the growth region before CVD [-1--5] or direct-write using laser ablation [6]. In this work, a technique that patterns the material after deposition is reported. It makes use of conventional photolithography processes and reactive ion etching by oxygen and has the advantage of being compatible with today's technology for device fabrication. The carbon films used in this work were deposited by radio frequency plasma enhanced chemical vapour deposition (rf PECVD) at room temperature. The surface and bulk microstructures of the films have been characterised by various techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEMI [7], scanning tunnelling microscopy (STM) and atomic force microscopy (AFM) [-8]. SEM and STM show that the films are very smooth without any pin holes. TEM confirms the films can, under certain deposition conditions, contain 10 200 nm microcrystalline diamond grains within an amorphous matrix. We refer to this type of material as carbon-diamond. Other researchers also noted similar crystal growth [9, 10] in mixed phased carbon films. Gold, aluminium and titanium were used as masks for a reactive ion etching (RIE) of carbon-diamond using oxygen because photoresists are not suitable for oxygen plasma etching. It was found that titanium was the best

masking material in terms of resistance to oxygen etching. The patterns transferred are very well defined with good resolution as characterised by optical microscopy and SEM.

2. Experimental The carbon-diamond films were deposited by 13.56 MHZ rf PECVD at room temperature. The substrate used was n-type (100) silicon with a resistivity of 1-5 f~cm. Deposition was done at a pressure of 300 mT and a DC self-bias of - 4 5 0 V for 15 min. A typical film thickness of 150 nm was obtained under such conditions. The metal mask was patterned onto the carbondiamond film by standard photolithography techniques using two approaches. Firstly, l gin thick photoresist was patterned onto the film by projection printing using a glass mask with a minimum feature size of 5 gm. Thin metal films with a thickness of 25 nm were evaporated onto carbon-diamond films. Gold and aluminium were thermally evaporated at a pressure of 10-6T. Titanium was evaporated using an electron beam evaporator at a pressure of 10 7T. Metal patterns were formed after a lift-off process in acetone. Then the metal served as a protective mask in the oxygen RIE. The experimental condilion of the RIE is described in Table 1. The rf power was chosen to etch the carbon-diamond films effectively with minimum damage to the metal mask. Such plasma etching conditions give an etching rate of 10-12 lain rain 1. The subsequent metal removal was done in 20% HC1 at 80°C for aluminium and 1:9 mixture of water and 48% HF at 32°C for titanium [11]. The gold mask was sputtered off during the RIE step and hence was not suitable for this process. Figure 1 describes the whole process in detail.

0925 9t~35'92,$05.00 i~ Elsevier Science Publishers B.V. All rights reserved

K. K. Chan et al.

282

,' DEfined etching o[carbon-diamond.films

TABLE 1. Experimental conditions for rf oxygen plasma etching Gas

Flow rate

Pressure

Power

Oxygen

1.8 sccm

300 mTorr

260 W

DC self-bias

Time 15 rain

425 V

[ silicon

I silicon

~ I

rFdep°sRl°n

II~rfdepos 'tlon

carbon-diamond

J silicon

]

photoresistcoating & patterndefinition ~ photoresist carbon-diamond

I silicon

~

metal evaporation

metal ~

I

~

mesa ovaoorotoo

carbon-diamond

silicon

. . . . ,st c o a t , .

& pattern d e f i n i t i o n

metal photoreslst

metal carbon diamond silicon

carbon diamond

silicon

~ phot.... ist Tilt-of(

~

acidetch

carbon-diamond

metal

silicon

~ ~

k

~ ~

carbon-diamond silicon

~

~ rf oxygenetching ~

carbon-diamond silicon

metal carbon diamond

phot. . . . ist

lift-off metal carbon-diamond silicon

silicon

~

~metal ...... I

~

~ rfoxygen etching metal

carbon-diamond silicon

Fig. 1. Process I: pattern transfer onto carbon-diamond with photoresist patterning prior to metal evaporation.

.~ metal removal

[ A variation of this process was also examined for pattern transfer. Titanium films were evaporated onto the carbon-diamond films before any photoresist patterning. Photoresist was then patterned onto the metal films using projection printing and the entire sample was immersed in HF to remove the exposed metal. After lifting off the photoresist in acetone, a metal mask was obtained. Then the same RIE and metal removal processes were repeated and the carbondiamond was patterned. Such a method is complementary to the one above so that this patterning technique is more flexible and can be used with positive or negative resist masks. The entire process is illustrated in Fig. 2. The etched patterns were examined using optical microscopy and SEM and found to be comparable to those obtained using the other process sequence.

carbon diamond

I s,,coo

Fig. 2. Process 2: pattern transfer onto carbon-diamond with metal evaporation prior to photoresist patterning.

3. Results and discussion Before any patterning, the carbon-diamond films were tested in the acids and found to be inert to these solutions according to thickness and refractive index measurements by ellipsometry. In the first method, the metal lift-off process was successful for the three metals used. After oxygen RIE, the gold mask did not survive the process. Aluminium was much more usiliant while titanium gave the best protection to the carbon-diamond under the mask. Figure 3 shows the etched patterns after oxygen RIE prior to aluminium removal under an optical microscope. It can be seen that partial etching

K. K. Chart et al.

D
Fig. 3. ()ptical micrograph of the AI patterns after (), etch. of aluminium occurs at the line edges and proceeds to the centre. After removing aluminium in HC1, the patterns were examined by SEM and the result is shown in Fig. 4. It can be seen that the line pattern is partially etched at the edges while the middle region is thicker. The titanium mask was still very uniform without any signiticant damage during the oxygen RIE. After the remowil of titanium, the carbon-diamond exposed was examined again under an optical microscope and found to be very well defined and uniform even for a large square pad {see Fig. 5t. More details of the etched patterns were revealed by the SEM pictures shown in Figs 6 and 7. The carbon-diamond patterns were very well detincd with good resolution. While gold and aluminium cannot survive the intense oxygen etch, much better protection is offered by the titanium mask as evidenced by the SEM examination. During the oxygen etch, various oxides such as TiO, TiO 2 and Ti203 may

Fig. 4. SEM micrograph of the etched patterns IAI maskl.

283

Fig. (~. SEM micrograph showing a large square pad [Ti mask).

Fig. 7. SEM micrograph showing line pancrns (Ti maskl.

form on the surface of titanium [12]. it has been suggested that titanium can form stable and protective oxides in an oxygen atmosphere [13]. Hence, titanium is an excellent masking metal for use in this pattern transfer process. For the second method of patterning the metal mask, very well defined patterns were transferred onto carbondiainond. No substantial difl'erence with the previous method was observed in terms of definition and resolution of carbon-diamond patterns. However, since the patterning of titanium mask relies on the isotropic HF etch, it will affect the definition of masking process as the etching of titanium in the HF solution is very rapid (12 lain min I). This effect is not too significant in this work as the minimum feature size of the pattern is 5 lain compared with 25 nm thick titanium. So long as the acid etching is done quickly, the patterning process will still be very satisfactory. This method serves as the complement to the above one so that the patterning can be done in a more convenient way without the limitations of positive or negative resist masks. Figure 8 shows the etched pattern after process 2.

4. Conclusion

Fig. 5. Oplical micrograph of lhe etched panerns ITi maski.

A technique that combines optical lithography and oxygen RIE to transfer patterns onto carbon-diamond is reported. Titanium is found as a suitable masking material in the oxygen RIE. The etched patterns are

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K. K. Chan et al. / Defined etching of carbon-diamond films

References

Fig. 8. SEM micrograph showing carbon patterns after process 2 (Ti mask).

very well defined with a high degree of integrity. Such a technique has the advantage that it is compatible with today's microfabrication technology and can be used to define microstructures in device fabrication.

Acknowledgment The authors are grateful to the technical assistance of A. Hoole.

1 T. Inoue, H. Tachibana, K. Kumagai, K. Miyata, K. Nishimura, K. Kobashi and A. Nakaue, J. Appl. Phys., 67 (12) (1990) 7329. 2 J. F. DeNatale, J. F. Flintoff and A. B. Harker, J. Appl. Phys., 68 (8) (1990) 4014. 3 K. Hirabayashi, Y. Taniguchi, O. Takamatsu, T. Ikeda, K. Ikoma and N. Iwasaki-Kurihara, Appl. Phys. Lett., 53 (19) (1988) 1815. 4 J. S. Ma, H. Kawarada, T. Yonehara, J. Suzuki, J. Wei, Y. Yokota and A. Hiraki, Appl. Phys. Lett., 55 (ll) (1989) 1071. 5 J. L. Davidson, R. Ramesham and C. Ellis, J. Electrochem. Soc.. 137 (10) (1990) 3206. 6 M. Rothschild and D. J. Ehrlich, J. Vac. Sci. Technol., B6 (1) (1988) 1. 7 G. Amaratunga, A. Putnis, K. Clay and W. Milne, Appl. Phys. Lett., 55 (August 1989) 634. 8 M. E. Welland, A. W. McKinnon, S. O'Shea and G. A. J. Amaratunga, Proc. Second European Con['erence on Diamond, Diamond-like and Related Coatings (Nice, France), 1991, (this volume). 9 Y. Namba, J. Wei, T. Mohri and E. Heidarpour, J. Vac. Sci. Technol., A7 (1) (1989) 36. 10 T. Mohri and Y. Namba, J. Appl. Phys., 55 (9) (1984) 3276. 11 L. Maissel and R. Glang, Handbook of Thin Film Technology, McGraw-Hill Book Company, New York, 1970, p. 35. 12 A. D. McQuillan and M. K. McQuillan. Titanium, Butterworths Scientific Publications, London, 1956, p. 246. 13 Wrought Titanium, Imperial Chemical Industries Ltd., U.K., 1955, p. 25.