Twin-well process

The twin-well process is typically the first step in CMOS wafer fabrication and is used to define the active regions of the nMOS and pMOS transistors. A twin well consists of a p-well and an n-well, with each well requiring some number of steps to fabricate. The twin-well process thus consists of two processes n-well formation and p-well formation. 

Following the phosphorus ion implantation, the resist layer is stripped off in an oxygen-plasma etcher, and subsequently cleaned hy a wet chemical process to remove residual resists and polymers created hy the plasma process. The silicon dioxide on the wafer surface is also removed during this step. 

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The use of high-energy ion implantation makes alternative well formation schemes possible. In the so-called twin well process, two separate wells for n- and p-channel transistors are formed in a lightly doped substrate. The twin well structure, as illustrated in Fig. 14.9, has the advantages of independent adjustment and optimization of n-channel or p-channel devices. Devices fabricated using a twin well processes are independent of the original substrate type. More important, twin well structures reduce the resistance across the base and the emitter of both npn and pnp bipolar transistors (RS and RW in Fig. 14.2.3). Therefore, the latchup is greatly reduced. 

Following the spacer formation, medium-dose implants are made to penetrate the silicon slightly beyond the LDD junction depth, but not as deep as the original twin-well implants (see Section 16.2.1). The spacer oxide from the previous step serves to protect the channel from the dopant atoms during the implant process. TTiere are two S/D implant processes, namely, n S/D implant and S/D implant. 

CMOS ICs utilize both NMOS and PMOS devices. Starting with a p-substrate, the NMOS would be fabricated on the p-substrate and the PMOS in an -weU, and vice versa. With the addition of an n-well, p-well or twin tub process, CMOS fabrication is similar to that for NMOS, although more complex. Table 8.4 (Fabricius, 1990) shows the Mead-Conway scalable CMOS design rules. The dimensions are given in multiples of k and the rules are specified by the MOS Implementation System (MOSIS) of the... 

These processes are considerably more complex in actual CMOS fabrication. First, the lower layers of a CMOS structure typically have a twin-tub design which includes both PMOS and NMOS devices adjacent to each other (see Fig. 3b). After step 1, a mask is opened such that a wide area is implanted to form the -well, followed by a similar procedure to create they>-well. Isolation between active areas is commonly provided by local oxidation of silicon (LOCOS), which creates a thick field oxide. A narrow strip of lightly doped drain (LDD) is formed under the edges of the gate to prevent hot-carrier induced instabilities. Passivation sidewalls are used as etch resists. A complete sequence of fabrication from wafer to packaged unit is shown in Figure 10. 

The powder produced in the prior operation is combined with the second active ingredient (B2), as well as several other excipients in a twin-shell blender and mixed for several min. For reasons previously discussed, mix time is of interest, and thus it is listed as a critical process step. 

In polymer processing, compaction is an important and necessary step in order to reduce the interparticle, unoccupied spaces and thus eliminate air. It is essential for melting in both single-screw extruders as well as for twin-rotor processors, as we shall see in Chapters 5 and 10. In twin-rotor devices, such as Co-TSEs, for example, the large and repeated deformation of compacted particulates by the kneading elements, which induces large plastic deformation of particulates, is the dominant melting mechanism. 

The computational capabilities of the Funatsu et al. modeling are listed in Table 10.10 with reference to the publication reporting their computational work. Some of the following are evident from the Tables 10.9 and 10.10. All common twin-rotor polymer processing equipment and screw-, rotor-, or kneading-element types, as well as element sequences have been treated ... 

Water and steam are nearly always present in most chemical processes. Their physical properties are generally well understood. Process perils are sometimes related to the infamous deeds of water and steam. The thrust of OSHAs Process Safety Management Law of 1992 focused on the processes that handled a list of 140 highly hazardous chemicals as well as processes that contained an over 10,000 pounds (4,540 kg) of flammable liquids or gases. Even after those chemicals are handled with higher levels of mechanical integrity and additional scrutiny, we will still need to keep our eye on water and its sometimes evil twin, steam. 

The twin-screw injection molding extruder is an injection molding machine that is capable of both blending/compounding and extrusion in one step. Because it is a one step process, the fibers never go through the entire extrusion process as well as the pelletization that limits the fiber size, but are blended into the molten plastic before injection. The screw part of this machine is based on a non-intermeshing, counterrotating twin-screw extruder (Chapter 5). One of the screws in this machine is capable of axial movement and has a non-return valve on the end. This action enables the screw to inject and mold parts. 

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