Polysilicon wire

The short penetration depth of UV/blue photons is the reason that frontside CCD detectors have very poor QE at the blue end of the spectrum. The frontside of a CCD is the side upon which the polysilicon wires that control charge collection and transfer are deposited. These wires are 0.25 to 0.5 /xm thick and will absorb all UV/blue photons before these photons reach the photosensitive volume of the CCD. For good UV/blue sensitivity, a silicon detector must allow the direct penetration of photons into the photosensitive volume. This is achieved by turning the CCD over and thinning the backside until the photosensitive region (the epitaxial layer) is exposed to incoming radiation. 

To produce a very thick n-channel device, the resistivity of the silicon must be made relatively high, about 5,000 to 10,000 H-cm, as opposed to the 20-100 H-cm material used in standard n-channel CCDs. Higher resistivity is required for greater penetration depth of the fields produced by the frontside polysilicon wires (penetration depth is proportional to the square root of the resistivity). These thick high resistivity CCDs have been developed for detection of soft x-rays with space satellites and can be procured from E2V and MIT/LL. 

Figure 15. Photovoltaic detector potential well. The example in this figure is the p-n junction of a n-channel CCD. The x-y-z axes match the orientation shown in Fig. 5. The charge generated in the 3-D volume of a pixel is swept toward a 2-D layer, which is the buried channel that is 0.25-0.5 pm from the front surface of the detector. The z-direction potential is created by the p-n junction combined with the voltages on the polysilicon wires deposited on the frontside of the CCD (not shown in this figure).

Polycrystalline silicon. Although the contacts between polycrystalline silicon and the filament are inherently perfect, the high resistance in combination with the high positive TCR of the poly-Si limits the operation temperature of the structure. Increasing the input power (and temperature) the resistance of the polysilicon wires on the suspension beams may dominate, resulting in malfunction of the device. 

Figure 5.2 Optical images of a polysilicon wire (a) before the functionalization and (b) after the functionalization with fluorescent molecule [14].

Figure 8.14 Cross section of a single deformable mirror pixel using a buried polysilicon wiring layer to address the actuator electrode. (Used with permission from S.A. Cornelissen, P.A. Bierden, and T.G. Bifano, Development of a 4096 element MEMS continuous membrane deformable mirror for high contrast astronomical imaging, Proc. SPIE 6306, p. 630606-1 [2006].)...

The true challenge lies in developing a wiring system which can be functionalized as easily as the polysilicon electrodes in the handful of operating devices in existence today, and connected to terminals, so that, even although the actual computations might be carried out on the molecular scale, the system can still be accessed by macroscopic components. It is no use having a molecular computer if one needs to employ an STM in order to type ... 

The epi-polysilicon functional layer is patterned and then etched by the trench etch process described in Section 5.3.4.2. The trench etching forms cantilever beams that act as comb fingers in the acceleration sensor, as well as the insulating trench required for electrical separation of the epipolysilicon wiring and bond pad structures (Fig. 5.3.11 e). Straight, unnotched sidewalls are strictly required, especially for all design elements that form springs or capacitive comb structures. 

Example One type of transistor is a p-n sandwich with two separate wires source and emitter) connected to the n-side. It acts as a switch in the following way When a positive charge is applied to a polysilicon contact base wire) positioned between the source and emitter wires, a current flows from the source to the emitter. Other examples include diodes and integrated circuits. 

Structural elements, polysilicon for resistors and wires, and nitride for electrical insulation. The nickel sidewalls can be further coated with a thin gold layer to improve the electrical conductivity for improved electrical contact in relays. Structural elements can be released by either using sacrificial oxide or undercutting with trenches formed in a bulk KOH etch. An example of a microrelay formed in the MetalMUMPS process is shown in Figure 1.15. A cross section showing all of the layers used to fabricate the relay is shown in Figure 1.16. 

We have used an approximate value of 50W/mK for the thermal conductivity of doped polysilicon at room temperature [2]. For the four arms the total thermal conductance would be 15 pW/K. The thermal conductance would increase if we include the metal wires that go to the temperature-sensitive element on the suspended platform. The thermal capacitance of the platform would be... 

An additional layer of unreleased polysilicon, PolyOa, has been added for wiring below the PolyOb layer that forms the counter-electrode, as shown in Figure 8.14. 

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