Tham khảo tài liệu 'microsensors, mems and smart devices - gardner varadhan and awadelkarim part 2', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | ELECTRONIC MATERIALS AND THEIR DEPOSITION 13 b Figure continued CVD is used extensively in depositing S1O2 SÌ3N4 and polysilicon. CVD S1O2 does not replace thermally grown S1O2 that has superior electrical and mechanical properties as compared with CVD oxide. However CVD oxides are instead used to complement thermal oxides and in many cases to form oxide layers that become much thicker in relatively short times than do thermal oxides. S1O2 can be CVD-deposited by several methods. It can be deposited by reacting silane and oxygen at 300 to 500 C in an LPCVD reactor wherein _ 500 C _ S1H4 O2 S1O2 2H2 It can also be LPCVD-deposited by decomposing tetraethylorthosilicate Si OC2H5 4-The compound abbreviated as TEOS is vaporised from a liquid source. Alternatively 14 ELECTRONIC MATERIALS AND PROCESSING dichlorosilane can be used as follows 900 C SÌCI2H2 2H2O-------- SiO2 4- 2H2 2HC1 A property that relates to CVD is known as step coverage. Step coverage relates the surface topography of the deposited film to the various steps on the semiconductor substrate. Figure a shows an ideal or conformal film deposition in which the film thickness is uniform along all surfaces of the step whereas Figure b shows a nonconformal film for a discussion of the physical causes of uniform or nonuniform thickness of deposited films see Fung et al. 1985 . Table compares different S1O2 films deposited by different methods and contrasts them with thermally grown oxides. Similarly SÌ3N4 can be LPCVD-deposited by an intermediate-temperature process or a low-temperature PECVD process. In the LPCVD process which is the more common process dichlorosilane and ammonia react according to the reaction __ _ 800 C _ _ 3SiCl2H2 4NH3 -------- SÌ3N4 -I- 6HC1 -I- 6H2 Figure a Conformal . ideal b nonconformal deposition of a film Table Properties of deposited and thermally grown oxide films Sze 1985 Property Composition Step coverage Density p g cm3 .
