2.2 The CMOS Process

Figure 2.6 outlines the steps to create an integrated circuit. The starting material is silicon, Si, refined from quartzite (with less than 1 impurity in 10 ¹⁰ silicon atoms). We draw a single-crystal silicon boule (or ingot) from a crucible containing a melt at approximately 1500 °C (the melting point of silicon at 1 atm. pressure is 1414 °C). This method is known as Czochralski growth. Acceptor ( p -type) or donor ( n -type) dopants may be introduced into the melt to alter the type of silicon grown.

The boule is sawn to form thin circular wafers (6, 8, or 12 inches in diameter, and typically 600 m m thick), and a flat is ground (the primary flat), perpendicular to the <110> crystal axis—as a “this edge down” indication. The boule is drawn so that the wafer surface is either in the (111) or (100) crystal planes. A smaller secondary flat indicates the wafer crystalline orientation and doping type. A typical submicron CMOS processes uses p -type (100) wafers with a resistivity of approximately 10 W cm—this type of wafer has two flats, 90° apart. Wafers are made by chemical companies and sold to the IC manufacturers. A blank 8-inch wafer costs about $100.

To begin IC fabrication we place a batch of wafers (a wafer lot ) on a boat and grow a layer (typically a few thousand angstroms) of silicon dioxide , SiO ₂ , using a furnace. Silicon is used in the semiconductor industry not so much for the properties of silicon, but because of the physical, chemical, and electrical properties of its native oxide, SiO ₂ . An IC fabrication process contains a series of masking steps (that in turn contain other steps) to create the layers that define the transistors and metal interconnect.

FIGURE 2.6 IC fabrication. Grow crystalline silicon (1); make a wafer (2–3); grow a silicon dioxide (oxide) layer in a furnace (4); apply liquid photoresist (resist) (5); mask exposure (6); a cross-section through a wafer showing the developed resist (7); etch the oxide layer (8); ion implantation (9–10); strip the resist (11); strip the oxide (12). Steps similar to 4–12 are repeated for each layer (typically 12–20 times for a CMOS process).

Each masking step starts by spinning a thin layer (approximately 1 m m) of liquid photoresist ( resist ) onto each wafer. The wafers are baked at about 100 °C to remove the solvent and harden the resist before being exposed to ultraviolet (UV) light (typically less than 200 nm wavelength) through a mask . The UV light alters the structure of the resist, allowing it to be removed by developing. The exposed oxide may then be etched (removed). Dry plasma etching etches in the vertical direction much faster than it does horizontally (an anisotropic etch). Wet etch techniques are usually isotropic . The resist functions as a mask during the etch step and transfers the desired pattern to the oxide layer.

Dopant ions are then introduced into the exposed silicon areas. Figure 2.6 illustrates the use of ion implantation . An ion implanter is a cross between a TV and a mass spectrometer and fires dopant ions into the silicon wafer. Ions can only penetrate materials to a depth (the range , normally a few microns) that depends on the closely controlled implant energy (measured in keV—usually between 10 and 100 keV; an electron volt, 1 eV, is 1.6 ¥ 10 ^–19 J). By using layers of resist, oxide, and polysilicon we can prevent dopant ions from reaching the silicon surface and thus block the silicon from receiving an implant . We control the doping level by counting the number of ions we implant (by integrating the ion-beam current). The implant dose is measured in atoms/cm ² (typical doses are from 10 ¹³ to 10 ¹⁵ cm ^–2 ). As an alternative to ion implantation we may instead strip the resist and introduce dopants by diffusion from a gaseous source in a furnace.

Once we have completed the transistor diffusion layers we can deposit layers of other materials. Layers of polycrystalline silicon (polysilicon or poly ), SiO ₂ , and silicon nitride (Si ₃ N ₄ ), for example, may be deposited using chemical vapor deposition ( CVD ). Metal layers can be deposited using sputtering . All these layers are patterned using masks and similar photolithography steps to those shown in Figure 2.6.

Table 2.2 shows the mask layers (and their relation to the drawn layers) for a submicron, silicon-gate, three-level metal, self-aligned, CMOS process . A process in which the effective gate length is less than 1 m m is referred to as a submicron process . Gate lengths below 0.35 m m are considered in the deep-submicron regime.

Figure 2.7 shows the layers that we draw to define the masks for the logic cell of Figure 1.3. Potential confusion arises because we like to keep layout simple but maintain a “what you see is what you get” (WYSIWYG) approach. This means that the drawn layers do not correspond directly to the masks in all cases.

(a) nwell	(b) pwell	(c) ndiff	(d) pdiff
(e) poly	(f) contact	(g) m1	(h) via
(i) m2	(j) cell	(k) phantom
FIGURE 2.7 The standard cell shown in Figure 1.3. (a)–(i) The drawn layers that define the masks. The active mask is the union of the ndiff and pdiff drawn layers. The n -diffusion implant and p -diffusion implant masks are bloated versions of the ndiff and pdiff drawn layers. (j) The complete cell layout. (k) The phantom cell layout. Often an ASIC vendor hides the details of the internal cell construction. The phantom cell is used for layout by the customer and then “instantiated” by the ASIC vendor after layout is complete. This layout uses grayscale stipple patterns to distinguish between layers.

We can construct wells in a CMOS process in several ways. In an n-well process , the substrate is p -type (the wafer itself) and we use an n -well mask to build the n -well. We do not need a p -well mask because there are no p -wells in an n -well process—the n -channel transistors all sit in the substrate (the wafer)—but we often draw the p -well layer as though it existed. In a p-well process we use a p -well mask to make the p -wells and the n -wells are the substrate. In a twin-tub (or twin-well ) process, we create individual wells for both types of transistors, and neither well is the substrate (which may be either n -type or p -type). There are even triple-well processes used to achieve even more control over the transistor performance. Whatever process that we use we must connect all the n -wells to the most positive potential on the chip, normally VDD, and all the p -wells to VSS; otherwise we may forward bias the bulk to source/drain pn -junctions. The bulk connections for CMOS transistors are not usually drawn in digital circuit schematics, but these substrate contacts ( well contacts or tub ties ) are very important. After we make the well(s), we grow a layer (approximately 1500 Å) of Si ₃ N ₄ over the wafer. The active mask (CAA) leaves this nitride layer only in the active areas that will later become transistors or substrate contacts. Thus  

the ∨ symbol represents OR (union) of the two drawn layers, ndiff and pdiff. Everything outside the active areas is known as the field region, or just field .

Next we implant the substrate to prevent unwanted transistors from forming in the field region—this is the field implant or channel-stop implant . The nitride over the active areas acts as an implant mask and we may use another field-implant mask at this step also. Following this we grow a thick (approximately 5000 Å) layer of SiO ₂ , the field oxide ( FOX ). The FOX will not grow over the nitride areas. When we strip the nitride we are left with FOX in the areas we do not want to dope the silicon. Following this we deposit, dope, mask, and etch the poly gate material, CPG (mask) = poly (drawn). Next we create the doped regions that form the sources, drains, and substrate contacts using ion implantation. The poly gate functions like masking tape in these steps. One implant (using phosphorous or arsenic ions) forms the n -type source/drain for the n -channel transistors and n -type substrate contacts (CSN). A second implant (using boron ions) forms the p -type source–drain for the p -channel transistors and p -type substrate contacts (CSP). These implants are masked as follows  

where “grow” means that we expand or bloat the drawn ndiff and drawn pdiff layers slightly (usually by a few l ).

During implantation the dopant ions are blocked by the resist pattern defined by the CSN and CSP masks. The CSN mask thus prevents the n -type regions being implanted with p -type dopants (and vice versa for the CSP mask). As we shall see, the CSN and CSP masks are not intended to define the edges of the n -type and p -type regions. Instead these two masks function more like newspaper that prevents paint from spraying everywhere. The dopant ions are also blocked from reaching the silicon surface by the poly gates and this aligns the edge of the source and drain regions to the edges of the gates (we call this a self-aligned process ). In addition, the implants are blocked by the FOX and this defines the outside edges of the source, drain, and substrate contact regions.

The only areas of the silicon surface that are doped n -type are  

where the ∧ symbol represents AND (the intersection of two layers); and the ÿ symbol represents NOT.

Similarly, the only regions that are doped p -type are  

If the CSN and CSP masks do not overlap, it is possible to save a mask by using one implant mask (CSN or CSP) for the other type (CSP or CSN). We can do this by using a positive resist (the pattern of resist remaining after developing is the same as the dark areas on the mask) for one implant step and a negative resist (vice versa) for the other step. However, because of the poor resolution of negative resist and because of difficulties in generating the implant masks automatically from the drawn diffusions (especially when opposite diffusion types are drawn close to each other or touching), it is now common to draw both implant masks as well as the two diffusion layers.

It is important to remember that, even though poly is above diffusion, the polysilicon is deposited first and acts like masking tape. It is rather like airbrushing a stripe—you use masking tape and spray everywhere without worrying about making straight lines. The edges of the pattern will align to the edge of the tape. Here the analogy ends because the poly is left in place. Thus,  

In the ASIC industry the names nplus, n +, and n -diffusion (as well as the p -type equivalents) are used in various ways. These names may refer to either the drawn diffusion layer (that we call ndiff), the mask (CSN), or the doped region on the silicon (the intersection of the active and implant mask that we call n -diffusion)—very confusing.

The source and drain are often formed from two separate implants. The first is a light implant close to the edge of the gate, the second a heavier implant that forms the rest of the source or drain region. The separate diffusions reduce the electric field near the drain end of the channel. Tailoring the device characteristics in this fashion is known as drain engineering and a process including these steps is referred to as an LDD process , for lightly doped drain ; the first light implant is known as an LDD diffusion or LDD implant.

FIGURE 2.8 Drawn layers and an example set of black-and-white stipple patterns for a CMOS process. On top are the patterns as they appear in layout. Underneath are the magnified 8-by-8 pixel patterns. If we are trying to simplify layout we may use solid black or white for contact and vias. If we have contacts and vias placed on top of one another we may use stipple patterns or other means to help distinguish between them. Each stipple pattern is transparent, so that black shows through from underneath when layers are superimposed. There are no standards for these patterns.

Figure 2.8 shows a stipple-pattern matrix for a CMOS process. When we draw layout you can see through the layers—all the stipple patterns are OR’ed together. Figure 2.9 shows the transistor layers as they appear in layout (drawn using the patterns from Figure 2.8) and as they appear on the silicon. Figure 2.10 shows the same thing for the interconnect layers.

FIGURE 2.10 The interconnect layers. (a) Metal layers as drawn in layout. (b) The corresponding structure (as it might appear in a scanning-electron micrograph). The insulating layers between the metal layers are not shown. Contact is made to the underlying silicon through a platinum barrier layer. Each via consists of a tungsten plug. Each metal layer consists of a titanium–tungsten and aluminum–copper sandwich. Most deep submicron CMOS processes use metal structures similar to this. The scale, rounding, and irregularity of the features are realistic.

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