Polysilicon Residues

Thin polysilicon is transparent to visible light. Therefore, it is not always easy to see the poly-Si residues. With experience, one can detect the slight color 

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A large body of research exists on the deposition and processing of structural polysilicon [8-10]. The key parameter for films made of structural polysilicon is residual stress and therefore the flatness of the films. The deposition of polysilicon from silane (SiH4) gas in a horizontal furnace has been extensively used. The addition of phosphorus as a dopant for the polysilicon has been used to control resistivity and stress. Including phosphorus dopant in the sacrificial oxide layer under the structural polysilicon layer and/or as ions implanted after deposition of the polysilicon is the most common methods used. 

The effective mass for the micro-resonator is easily determined from the geometry, given fabrication process information and the density of the structure material which is polysilicon in this case. Uncertainties in polysilicon s elastic material properties and manufacturing-induced residual stress make it difficult to predict the spring constant, though the measured resonant Irequency data can be... 

To provide a measure of residual stress (Tm, which is necessary to estimate pull-in voltage, the fabrication process usually involves deposition of a film of the device plate material, commonly polysilicon, onto a solid Si substrate. This structure can then be observed by means of wafer curvature techniques, for example, to infer a value of residual stress in the film layer. Among the system parameters that influence functional characteristics, the residual stress is the most difiicult to assess properly. On the other hand, control of its value provides an additional means for controlling device characteristics. 

Figure 1.6 Deformation of a released structure due to stress-gradients at MCNC circa 1996. The residual stress is tensile on top of the polysilicon, causing the released structure to bend away from the substrate after release. In general, the polysilicon from MCNC was quite flat, but in this case an anneal of the sacrificial doped oxide (PSG) was left out, resulting in residual stress-gradients. (Reprinted with permission of Kristofer S.J. Pister, Berkeley Sensor and Actuator Center, UC Berkeley.)...

In this process, as shown step-by-step in cross section in Figure 1.7, the surface of a 150 mm substrate (n-type, 1-2 Qcm) is heavily doped with phosphorus to a resistance of 10 Q/D to avoid charge buildup at the substrate-nitride interface when high voltages are applied between the substrate and the subsequent conducting layers. The surface is protected by a blanket low-pressure chemical vapor deposition (LPCVD) of a 0.6 pm thick insulating silicon nitride (90 MPa residual tensile stress). A 0.5 pm thick layer of LPCVD polysilicon (n-type, 30Q/D, 25 MPa residual compressive stress) PolyO is... 

After patterning the Oxide2 layer with anchors and vias, the second layer of released structural polysilicon is deposited, annealed, and patterned following the procedures used for Polyl. Poly2 is 1.5pm thick and has a compressive residual stress of —10 MPa and a resistance of 20 O/D. A common use for this structural layer is to provide constraints for features defined in Polyl, such as a hub that constrains a rotor in an electrostatic motor [9], [10] or a staple that constrains a pin in a hinge [11], [12]. It is also possible to make a... 

To obtain flat mirrors, with a radius of curvature more than 250 mm, the processing conditions for the polysilicon and mirror metallization had to be controlled to minimize residual stresses and stress-gradients. The thickness of the polysilicon mirror membrane was 3.5 pm, a thickness that can be obtained by stacking Polyl (2 pm) and Poly2 (1.5 pm) in the PolyMUMPS process. 

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]. 

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