Polysilicon films

Metal and polysilicon films are formed by a chemical-vapor deposition process using organometallic gases that react at the surface of the IC structure. Various metal silicide films may also be deposited in this manner by reaction with the surface of the silicon wafer to form metal silicides. Glass and pol3uner films are deposited or spin cast or both, as are photoresist films (those of a photosensitive material). This process is accomplished by applying a liquid polymer onto a rapidly rotating wafer. The exact method used varies from manufacturer to manufacturer and usually remains proprietary. 

Figure 13 Polysilicon film deposited by LPCVD, after Murarka.19...

An interesting effect was observed by varying the pressure between 80 and 400 mTorr. At pressures of 180 mTorr and above, the deposition rate jumped from 600 to 2000 A/min. At the same time, resistivity rose as high as 4000 pi2-cm. The variation with pressure is shown in Figure 12. Apparently, the stoichiometry changed dramatically at pressures of 180 mTorr and higher. In fact, there is very little Ta in the film created at 400 mTorr (about 13%), so this is mostly a polysilicon film. 

The major advantage to the use of PECVD for deposition of polysilicon films is the possibility of depositing them at low temperatures. The conventional LPCVD process requires T 650°C for times on the order of a half hour. PECVD of polycrystalline films can be carried out with a wide variety of gases (SiH4, Ar, He, H2 and doped or undoped) at temperatures ranging from 200°... 

It is noted that the polishing of the polysilicon film will employ the well-known technology of polishing monocrystalline silicon substrates. This gives an opportunity to examine the effect of polycrystallinity and thus of grain boundaries and grain orientation in CMP of Si. Doped vs. undoped polysilicon will also shed... 

In silicon based MEMS processing, common CVD films include polysilicon, silicon oxide, and silicon nitride. For polysilicon films (usually the structural layer), an LPCVD pyrolysis method is generally used with silane (SiH4) as the source gas [see Eq. (1)]. To obtain a imiform film across the wafer, the process is carried out at low pressure to ensure that the deposition is surface reaction controlled and not diffusion limited. Typical process temperatures are in the range of 580-650°C, and pressures between 0.1 and 0.4Torr. 

K. Nunan et ah, Developing a manufacturable process for the deposition of thick polysilicon films for micro machined devices, Proc. 11th Annual IEEE/ SEMI Advanced Semiconductor Manufacturing Conference and Workshop (ASMC), September 2000. 

M. Furtsch, M. Offenberg, A. Vila, A. Cornet, J.R. Morante, Texture and stress profile in thick polysilicon films suitable for fabrication of microstructures, Thin Solid Films 296, 177, 1997. 

Fig. 5.4.7 D ifferential temperature coefficient of the gauge factor of boron-doped polysilicon films as a function of dopant concentration Na for different annealing temperatures [24]... 

The term polysilicon arises from the structure of these silicon layers, which are essentially polycrystalline in contrast to single-crystalline silicon substrates, due to the growth of these films on amorphous starting layers (in silicon surface micromachining, a silicon oxide layer is usually used as both a seed layer and a sacrificial layer). Polysilicon is widely used in sensor technology it can be used as part of a membrane layer, as an electrical connector, or as a part of a thermopile structure. In this contribution, we focus on its most important function - as the functional layer in surface-micromachined structures. In surface micromachining basically two approaches for producing polysilicon films are used ... 

The stability of the properties of polysilicon films to processing is not ideal. The structure and intrinsic stress of LPCVD-poly films created by different organizations but with the same deposition recipe were different [21]. The properties of LPCVD-poly can differ even on wafers from the same deposition run, due to small temperature fluctuations in the deposition furnace [25]. However, since several LPCVD-poly surface-micromachined sensors are in fact produced in high volumes for the automotive industry (e.g., by Analog Devices, Infineon, and Motorola), these literature data may not represent the current state of the art. 

Surface-micromachined moving structures are usually made up of polysilicon layers and are basically produced by two different techniques, to produce thinner or thicker films. In the meantime, sensors based on polysilicon are widely used and represent state-of-the-art technologies. The fracture strength of polysilicon films is generally sufficient for the loads that appear in an automotive environment. 

Fig. 37. The notching effect. When the polysilicon film clears, the oxide charges up to the point of deflecting oncoming ions which then etch the sidewall. Electrons can t reach the bottom of the trench to neutralize the positive charge.

X. Xu, C. P. Grigoropoulos, and R. E. Russo, Transient Temperature During Pulsed Excimer Laser Heating of Thin Polysilicon Films Obtained by Optical Reflectivity Measurement, ASMEJ. of Heat Transfer, 117, pp. 17-24,1995. 

Adams, A.C. (1988) Dielectric and polysilicon film deposition, Chapter 6 in VLSI Technology 2nd edition, edited by S.M. Sze, McGraw-Hill, New York. Gives an overview of the use of ceramic insulating layers in semiconductor device fabrication. 

In this approach, the bulk silicon substrate is not etched away, but only the sacrificial layer between the silicon substrate and the polysilicon microstractures is removed by lateral underetching using the selective wet etchant of HE or BOB. Such etching solutions do not have any etching effect on the polysilicon film. 

Fig. 3.78 Microstracture of the 2 pm thick polysilicon films a cross-sectional TEM image, b Lomer-Cottrell lock, and c microtwins [16]. With kind permission of Professor Ritchie...

Chasiotis I, Knauss WG (2003) The mechanical strength of polysilicon films part 1. The influence of fabrication governed surface conditions. J Mech Phys Solids 51 1533-1550 Chattopadhyay S, Li X, Bohn PW (2002) In-plane control of morphology and tunable photoluminescence in porous silicon produced by metal-assisted electroless chemical etching. J Appl Phys 91 6134-6140... 

Silicon (base wafer or polysilicon film) Nitric Acid 50 Water 20 Hydrofluoric Acid 3 Room... 

Another source of topography on the mirror surface resulted from residual stress-gradients in the polysilicon film after it was released, as described previously in Section 4.5 on obtaining flatness in optical MEMS devices. Here it was found that a post-fabrication ion bombardment step could be used to increase the radius of curvature due to residual stress gradients, as shown in Figures 8.8 and 8.9 [19]. 

Poly (semiconductor processing) Slang term for a polysilicon film. 

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