Models for Analog Design. Models for Digital Design. Static Logic Gates. Clocked Circuits. Dynamic Logic Gates. Memory Circuits. Sensing Using AE Modulation. Operational Amplifiers I. Dynamic Analog Circuits. This is followed In ariei Hug he Arc command, moving and clicking the mouse in the desired i i ulei til the curie, and leluinmg and clicking the mouse at point A. AX Figure PI.
Frequently umM i omm. What does the depth level mean? Show that, by pressing i on the keyboard or top right corner of the drawing screen , the cells are also drawn as outlines. Use potyl in your examples. How do you Get a poly or path object? This approach will build a solid foundation ini understanding the performance limitations and parasitics the pn junctions, i HjuiettMices, and resistances inherent in a CMOS circuit of the CMOS process.
Our discussion will center around a p-type n. Often an epitaxial layer is grown on the wafer. We will not make a distinction hr! Some processes use a p-well or both n- and p-wells sometimes called twin tub processes. The n-well acts as the substrate or body of the p-channel transistors.
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Another important consideration is that the n-well and the p-substrate form a dUnlc Fig. In CMOS circuits, the substrate is usually tied to the lowest voltage in Hu i m uit to keep this diode from forward biasing. Ideally, zero current flows through ihi' substrate connection. Many processes don't use the epi layer. I lptiii 1 I lllii ii. Consider the following sequence of events that apply, in a fundamental way, to any layer we need to pattern. We start out with a clean, bare wafer as shown in Fig. The distance given by the line A to B will be used as a reference in Figs.
Figures 2. Figure 2. However, semiconductor processes must have tightly controlled conditions to precisely set the thickness and purity of the oxide. The oxide resulting from the reaction with steam is called a wet oxide, while the reaction with 0 2 is a dry oxide. Both oxides are called thermal oxides due to the increased temperature used during oxide growth. The growth rate increases with temperature.
The main benefit of the wet oxide is fast growing time. The main drawback of the wet oxide is the hydrogen byproduct. In general terms, the oxide grown using the wet techniques is not as pure as the dry oxide. The dry oxide, as we can conclude, generally takes a considerably longer time to grow.
Both methods of growing oxide are found in CMOS processes. An important observation we should make when looking at Fig. This is illustrated in Fig. The next step of the CMOS patterning process is to deposit a photosensitive resist layer across the wafer see Fig. Keep in mind that the dimensions of the layers, that is, oxide, resist, and the wafer, are not drawn to scale. The thickness of a wafer is typically im, while the thickness of a grown oxide or a deposited resist may be only a few jim or even less.
After the resist is baked, the mask derived from the layout program, Figs.
In practice, a single mask called a reticle, with openings several times larger than the final illuminated area on the wafer, is used to project the pattern and is stepped across the wafer with a machine called a stepper to generate the patterns needed to create multiple copies of a single chip. The light passing through the opening in the reticle is photographically reduced to illuminate the correct size area on the wafer. I lu photoresist is developed Fig 2 Jh , removing the areas that were ilhiri itnniiH j I his process is cut led a positive resist process because the area that was III tiled whs removed A negative resist process removes the areas of resist that wrM not exposed to the light.
Using both types of resist allows the process designer to t til down on the number of masks needed to define a CMOS process. Since creating the tun. This is also important in large manufacturing plants where fewer steps are t limited with lover cost. The next step in the patterning process is to remove the exposed oxide areas i big, 2 m. Notice that the etchant etches under the resist, causing the opening in the oxide to be larger than what was specified by the mask.
Some manufacturers Intentionally bloat make larger or shrink make smaller the masks as specified by the layout program. Referring to our generic fuillerning discussion given in Fig. This is followed by exposing the resist to Light through a mask Figs. I he mask used here can be generated with the LASI program.
The next step in lubricating the n-well is to expose the wafer to donor atoms. The resist will block the diffusion of the atoms, while the openings will allow the donor atoms to penetrate into i hr wafer. This is shown in Fig.
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After a certain amount of time, dependent on the depth of the n-well desired, the diffusion source is removed Fig. Notice that the n well "outdiffuses" under the resist; that is, the final n-well size is not the same as the mask size.
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Again, the foundry where the chips are fabricated may bloat or shrink the mask to compensate for this lateral diffusion. The final step in making the n-well is the removal of the resist Fig. The following example illustrates how to lay out an n-well of size 10 pm square. Example 2.
Sketch the e mss -sectional view of the layout. Assume we are using the CN20 setups given in the Jasi chapter and using the layer table select Layr from the drawing screen commands and the layer "nwel. The bottom of the display should show that the object is a box ami that the grids are I pm. The resulting display is shown in the top of Fig. Notice that the drawn size, both width and length, of the n-well differs from the actual size because of the lateral diffusion. Can we make the distance between the n-wells 1 pm? As we might expect, there are minimum spacing and size requirements for all layers in a CMOS process.
Process engineers, who design the integrated circuit process, specify the design rules. A complete listing of the CN20 design rules can be found in Appendix A. The minimum width of any n-well is 3 pm, while the minimum spacing between different n-wells is 9 pm. As the layout becomes complicated, the need for a program that ensures that the design rules are not violated is needed.
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Enter the name of the cell see below to be checked and type cn Since we only know two design rules at this point, that is, from Fig. If there is an error a bit map of the layout will be generated and can be viewed with the Map command and the error will be reported in a report file which can be read using the Read command.
The resistance of a material is a function of the materials resistivity, p, and the materials dimensions. For example, the slab of material in Fig. In semiconductor processing, all of the fabricated thicknesses, such as the u well, are constant. We only have control over W and L. Also notice that the Wand L Iff what we see from the top view, that is, the layout view.
We can rewrite Eq.
AS1 has a resistor calculator that will help in the calculation of nonrectangular i cm stances. Often, to minimize space, icsistors are laid out in a serpentine pattern. Ml sections in Fig 2. Avoiding corners in a resistor is the generally preferred method ul layout in analog circuit design where the ratio of two resistors is important. The field implantation is sometimes called the "channel stop implant". Therefore, it is important to understand how to model a diode for hand calculations and in SPICE simulations. The SPICE circuit simulation program assumes that the value ni I f supplied in the model statement was measured for a device with a reference area of 1 If an area factor of 2 is supplied for a diode, then I s is doubled in Eq.
Formation of a pn junction results in a depleted region at the p-n interface Fig.