Backside deposition

One difficult problem in early blanket tungsten processing was that of backside deposition. In many cases there is no need for an adhesion layer at the back side of the wafer. Especially in the case of an oxide backside, tungsten will flake from the backside if the tungsten deposition is not suppressed in this area. Needless to say, flaking is totally unacceptable from a particle contamination point of view. Several approaches are available today to the backside deposition problem of which three will be discussed. 

Genus clamp method The backside of the wafer is shielded by a clamp in combination with a backside purge (see figure 7.11). The method is very efficient and is proven in production. 

Applied Materials method In this approach backside deposition does occur, however, the tungsten at the backside is stripped back using a plasma etch. The etch is done such that the tungsten at the frontside remains untouched. 

Novellus method Novellus uses vacuum clamping of the wafer center while purging the edge of the wafer with an inert gas (see figure 7.12). 

All these methods have their advantages and disadvantages. Nevertheless it can be said that the problem of back side tungsten flaking can be solved by one of these methods and is no longer a serious drawback for blanket tungsten processing. 

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Figure 7.11. Cross-section of chuck with backside deposition prevention using a clamp and a back side inert gas purge (Genus 8720 and 6020 systems).

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. 

Deposition of a metal film on the backside by evaporation or CVD techniques and subsequent annealing. 

Platinum chemically deposited on a Nafion membrane was used as a platinum SPE (Solid Polymer Electrolyte) electrode. The electrochemical measurements were performed using the half cell shown in Fig. 2-2. The cell body is made from Teflon (PTFE). The cell is divided into two compartments one for backside gas supply one for the electrolyte. SPE electrodes are placed between them with the deposited side facing the gas compartment. A gold foil with a hole was placed behind the SPE electrode... 

Another CMOS-process modification included the deposition of a nitride layer on the wafer backside. The backside nitride is identical with the CMOS passivation. All wafers are already delivered with this backside nitride by the CMOS foundry aus-triamicrosystems. 

Backlapplng is done to remove deposited materials from the backside of the wafer. The wafers are wax mounted to a lapper plate and wet lapped with a colloidal silica slurry. 

After the cleaning procedure, deposit an A1 layer with a thickness of about 300 nm by, e.g., an electron-beam evaporation process anneal the contact at 420°C for 15 min to create an Ohmic backside contact. 

Cong and co-workers [54] have prepared a ternary library of transition metals by sputter deposition on a quartz wafer. The catalyst samples were supplied with reactants through a concentric tube that also delivered the product gas flow to a sensor for spectroscopic analysis (see Chapter 3). The catalysts could be activated by a C02 heating laser from the backside of the wafer. 

Substrates The substrates in microelectronics are mainly Si wafers. For mobile applications, silicon-on-insulator (SOI) wafers increasingly replace bulk Si wafers and for very specific high-frequency applications, III-V compound semiconductors (e.g., GaAs) are used. The majority of substrates in microfabrication are Si wafers, but metal, glass, and ceramic substrates are also common. Particularly when using glass, quartz, and ceramic wafers in CMP processes, it has to be taken into account that they are brittle and easy to break. The situation is worse when the material is also under stress induced by deposited layers. For applications where the backside of the wafer has to be structured (e.g., in bulk micromachining), double-side polished substrates are employed. 

It is known that nickel can serve as a catalyst for the subsequent CNTs growth. Ni nanoclusters were deposited electrochemically on the exposed silicon at the bottom of etched tracks. For this purpose the nanoporous samples have been inserted into an electrochemical cell with the nanoporous oxide facing solution of 0.5 mol/1 H3BO3 0.5 mol/l NiS04, and the backside being contacted as the cathode with application of-1.2 V DC [5]. 

An example of a two-electrode PECS that does not need a membrane is polycrystaUine Cd(Se,Te) in an alkaline Sn VSn electrolyte (G. Hodes, unpublished results). Menezes et al. (1977) reported that acidic Sn VSn" electrolyte stabilised CdTe photoelectrodes to some extent. In experiments using polycrystalline Cd(Se,Te) on an exposed (on the back side) Ti substrate, this electrolyte was found to stabilise the photoanode somewhat, but not very efficiently. However, use of alkaline Sn VSn" resulted in considerably improved stability (although still inferior to polysulphide). On illumination, metallic Sn deposited on the exposed backside (the Ti substrate) of the photoelectrode. In the dark, this Sn discharged (current flow in the... 

Nickel Schottky contacts with a diameter of 300 am and a thickness of 1500 A were deposited on the porous SiC by magnetron deposition followed by photolithography patterning. A blanket ohmic contact was formed by Ni deposition on the backside and rapid thermal annealing at 1000 °C was done prior to anodization. The schematic cross-section of the formed structure is shown in Figure 2.21. Note, nickel contacts were deposited on a porous substrate with the skin layer which is characterized by low porosity and pore diameters of <20 nm. Thus, the effect of contact... 

In order to improve the sensor response time, a thin hydrogel layer was directly deposited onto the backside of the bending plate covered with a 220 nm thick PECVD silicon nitride film and with a 17 run thick adhesion promoter layer (Fig. 2c). The final thickness of the dried and then cross-linked hydrogel layer was 4... 50 pm. 

PVA and PAAc polymers obtained from Aldrich Chemical Co. were dissolved separately in distilled water under stirring at 80°C (PVA 15 wt% and PAAc 7.5 wt%). For hydrogel formation, the solutions are then mixed in such a manner that 80 wt% were PVA and 20 wt% PAAc. This mixture is stirred for 1 h at 60°C to manufacture a homogeneous solution (Arndt et al. 1999). The films of PVA/PAAc blends were deposited onto the Si wafer (Fig. 2a) or onto the backside of the silicon bending plate (Fig. 2c). A solution of a-amino propyltriethoxysilane was used as adhesion promoter. Finally, the dried hydrogel films were isothermaUy annealed in an oven at 130°Cfor20min. 

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