Branch of Engineering: Electronics and Instrumentation Engineering
The following discussion will concentrate on the well-established CMOS fabrication technology, which requires that both n-channel (nMOS) and p-channel (pMOS) transistors be built on the same chip substrate. To accommodate both nMOS and pMOS devices, special regions must be created in which the semiconductor type is opposite to the substrate type. These regions are called wells or tubs. A p-well is created in an n-type substrate or, alternatively, an n-well is created in a p-type substrate. In the simple n-well
CMOS fabrication technology presented here, the nMOS transistor is created in the p-type substrate, and the pMOS transistor is created in the n-well, which is built into the p-type substrate. In the twin-tub CMOS technology, additional tubs of the same type as the substrate can also be created for device optimization.
The simplified process sequence for the fabrication of CMOS integrated circuits on a p-type silicon substrate is shown in Fig. 1.1. The process starts with the creation of the n-well regions for pMOS transistors, by impurity implantation into the substrate. Then, a thick oxide is grown in the regions surrounding the nMOS and pMOS active regions. The thin gate oxide is subsequently grown on the surface through thermal oxidation. These steps are followed by the creation of n+ and p+ regions (source, drain, and channel-stop implants) and by final metallization (creation of metal interconnects).
Fig.1.1 Simplified process sequence for the fabrication of the n-well CMOS integrated circuit with a single polysilicon layer, showing only major fabrication steps.
Fabrication Process Flow: Basic Steps
Note that each processing step requires that certain areas are defined on chip by appropriate masks. Consequently, the integrated circuit may be viewed as a set of patterned layers of doped silicon, poly-silicon, metal, and insulating silicon dioxide. In general, a layer must be patterned before the next layer of material is applied on the chip. The process used to transfer a pattern to a layer on the chip is called lithography. Since each layer has its own distinct patterning requirements, the lithographic sequence must be repeated for every layer, using a different mask. To illustrate the fabrication steps involved in patterning silicon dioxide through optical lithography, let us first examine the process flow shown in Fig. 1.2. The sequence starts with the thermal oxidation of the silicon surface, by which an oxide layer of about 1 m thickness, for example, is created on the substrate (Fig. 1.2(b)). The entire oxide surface is then covered with a layer of photoresist, which is essentially a light-sensitive, acid-resistant organic polymer, initially insoluble in the developing solution (Fig.1.2(c)). If the photoresist material is exposed to ultraviolet (UV) light, the exposed areas become soluble so that they are no longer resistant to etching solvents. To selectively expose the photoresist, we have to cover some of the areas on the surface with a mask during exposure. Thus, when the structure with the mask on top is exposed to UV light, areas which are covered by the opaque features on the mask are shielded. In the areas where the UV light can pass through, on the other hand, the photoresist is exposed and becomes soluble (Fig. 1.2(d)).
Fig.1.2 Process steps required for patterning of silicon dioxide.
The type of photoresist which is initially insoluble and becomes soluble after exposure to UV light is called positive photoresist. The process sequence shown in Fig. 1.2 uses positive photoresist. There is another type of photoresist which is initially soluble and becomes insoluble (hardened) after exposure to UV light, called negative photoresist. If negative photoresist is used in the photolithography process, the areas which are not shielded from the UV light by the opaque mask features become insoluble, whereas the shielded areas can subsequently be etched away by a developing solution. Negative photoresists are more sensitive to light, but their photolithographic resolution is not as high as that of the positive photoresists. Therefore, negative photoresists are-used less commonly in the manufacturing of high-density integrated circuits. Following the UV exposure step, the unexposed portions of the photoresist can be removed by a solvent. Now, the silicon dioxide regions which are not covered by hardened photoresist can be etched away either by using a chemical solvent (HF acid) or by using a dry etch (plasma etch) process (Fig. 1.2(e)). Note that at the end of this step, we obtain an oxide window that reaches down to the silicon surface (Fig. 1.2(f)). The remaining photoresist can now be stripped from the silicon dioxide surface by using another solvent, leaving the patterned silicon dioxide feature on the surface as shown in Fig. 1.2(g).
The sequence of process steps illustrated in detail in Fig. 1.2 actually accomplishes a single pattern transfer onto the silicon dioxide surface, as shown in Fig. 1.3. The fabrication of semiconductor devices requires several such pattern transfers to be performed on silicon dioxide, polysilicon, and metal. The basic patterning process used in all fabrication steps, however, is quite similar to the one shown in Fig. 1.2. Also note that for accurate generation of high-density patterns required in sub-micron devices, electron beam (E-beam) lithography is used instead of optical lithography. In the following, the main processing steps involved in the fabrication of an n-channel MOS transistor on a p-type silicon substrate will be examined.
Fig. 1.2 Process steps required for patterning of silicon dioxide (continued).
Fig. 1.3 The result of a single lithographic patterning sequence on silicon dioxide, without showing the intermediate steps. Compare the un-patterned structure (top) and the patterned structure (bottom) with Fig. 1.2(b) and Fig. 1.2(g), respectively.
Fabrication of the nMOS Transistor
The process starts with the oxidation of the silicon substrate (Fig. 1.4(a)), in which a relatively thick silicon dioxide layer, also called field oxide, is created on the surface (Fig. 1.4(b)). Then, the field oxide is selectively etched to expose the silicon surface on which the MOS transistor will be created (Fig. 1.4(c)). Following this step, the surface is covered with a thin, high-quality oxide layer, which will eventually form the gate oxide of the MOS transistor (of fig 1.4(d)). On the top of thin oxide layer, a layer of polysilicon (polycrystalline silicon) is deposited (Fig. 1.4(e)). Polysilicon is used both as gate electrode material for MOS transistors and also as an interconnect medium in silicon integrated circuits. Un-doped polysilicon has relatively high resistivity. The-resistivity of polysilicon can be reduced, however, by doping it with impurity atoms. After deposition the polysilicon layer is patterned and etched to form the interconnection and the MOS transistor gates (Fig. 1.4(f)). The thin gate oxide not covered by polysilicon is also etched away, which exposes the bare silicon surface on which the source and drain junctions are to be formed (Fig. 1.4(g)). The entire silicon surface is the n-doped with a high concentration of impurities, either through diffusion or ion implantation (in this case with donor atoms to produce n-type doping). Fig. 1.4(h) shows that the doping penetrates the exposed areas on the silicon surface, ultimately creating two n-type regions (source and drain junctions) in the p-type substrate. The impurity doping also penetrates the polysilicon on the surface, reducing its resistivity.
Fig. 1.4 Process flow for the fabrication of an n-type MOSFET on p-type silicon.
Note that the polysilicon gate, which is patterned before doping, actually defines the precise location, of the channel region and, hence, the location of the source and the drain regions. Since this procedure allows very precise positioning of the two regions relative to the gate, it is also called the self-aligned process.
Fig. 1.4 Process flow for the fabrication of an n-type MOSFET on p-type silicon (continued).
Once the source and drain regions are completed, the entire surface is again covered with an insulating layer of silicon dioxide (Fig. 1.4(i)). The insulating oxide layer is then patterned in order to provide contact windows for the drain and source junctions (Fig. 1.4(j)). The surface is covered with evaporated aluminium which will form the interconnection (Fig. 1.4(k)). Finally, the metal layer is patterned and etched, completing the interconnection of the MOS transistors on the surface (Fig. 1.4(l)). Usually, a second (and third) layer of metallic interconnect can also be added on top of this structure by creating another insulating oxide layer, cutting contact (via) holes, depositing, and patterning the metal.
Device Isolation Techniques
The MOS transistors that comprise an integrated circuit must be electrically isolated from each other during fabrication. Isolation is required to prevent unwanted conduction paths between the devices, to avoid creation of inversion layers outside the channel regions of transistors, and to reduce leakage currents. To achieve a sufficient level of electrical isolation between neighbouring transistors on a chip surface, the devices are typically created in dedicated regions called active areas, where each active area is surrounded by a relatively thick oxide barrier called the field oxide. One possible technique to create isolated
active areas on silicon surface is first to grow a thick field oxide over the entire surface of the chip, and then to selectively etch the oxide in certain regions, to define the active areas. This fabrication technique, called etched field-oxide isolation, is already illustrated in Fig. 1.4(b) and Fig. 1.4(c). Here, the field oxide is selectively etched away to expose the silicon surface on which the MOS/h transistor will be created. Although the technique is relatively straightforward, it also has 29 some drawbacks. The most significant disadvantage is that the thickness of the field oxide leads to rather large oxide steps at the boundaries between active areas and isolation Fabrication (field) regions. When polysilicon and metal layers are
Fig. 1.4 Process flow for the fabrication of an n-type MOSFET on p-type silicon (continued).
deposited over such boundaries in of MOSFETs subsequent process steps, the sheer height difference at the boundary can cause cracking of deposited layers, leading to chip failure. To prevent this, most manufacturers prefer isolation techniques that partially recess the field oxide into the silicon surface, resulting in a more planar surface topology.
Local Oxidation of Silicon (LOCOS)
The local oxidation of silicon (LOCOS) technique is based on the principle of selectively growing the field oxide in certain regions, instead of selectively etching away the active areas after oxide growth. Selective oxide growth is achieved by shielding the active areas with silicon nitride (Si3N4) during oxidation, which effectively inhibits oxide growth. The basic steps of the LOCOS process are illustrated in Fig. 1.5. First, a thin pad oxide (also called stress-relief oxide) is grown on the silicon surface, followed by the deposition and patterning of a silicon nitride layer to mask (i.e., to define) the active areas (Fig. 1.5(a)). The thin pad oxide underneath the silicon nitride layer is used to protect the silicon surface from stress caused by nitride during the subsequent process steps. The exposed areas of the silicon surface, which will eventually form the isolation regions, are doped with a p-type impurity to create the channel-stop implants that surround the transistors (Fig. 1.5(b)). Next, a thick field oxide is grown in the areas not covered with silicon nitride, as shown in Fig. 1.5(c). Notice that the field oxide is partially recessed into the surface since the thermal oxidation process also consumes some of the silicon. Also, the field oxide forms a lateral extension under the nitride layer, called the bird's beak region. This lateral encroachment is mainly responsible for a reduction of the active area. The silicon nitride layer and the thin pad oxide layer are etched in the final step (Fig. 1.5(d)), resulting in active areas surrounded by the partially recessed field oxide. The LOCOS process is a popular technique used for achieving field oxide isolation with a more planar surface topology. Several additional measures have also been developed over the years to control the lateral bird's beak encroachment, since this encroachment ultimately limits device scaling and device density in VLSI circuits.
Fig. 1.5 Basic steps of the LOCOS process to create oxide isolation around active areas.
The CMOS n-Well Process
Having examined the basic process steps for pattern transfer through lithography and having gone through the fabrication procedure of a single n-type MOS transistor, we can now return to the generalized fabrication sequence of n-well CMOS integrated circuits, as shown in Fig. 1.1. In the following figures, some of the important process steps involved in the fabrication of a CMOS inverter will be shown by a top view of the lithographic masks and a cross-sectional view of the relevant areas. The n-well CMOS process starts with a moderately doped (with impurity concentration typically less than 1015 cm-3) p-type silicon substrate. Then, an initial oxide layer is grown on the entire surface. The first lithographic mask defines the n-well region. Donor atoms, usually phosphorus, are implanted through this window in the oxide.
Layout Design Rules
The physical mask layout of any circuit to be manufactured using a particular process must conform to a set of geometric constraints or rules, which are generally called layout design rules. These rules usually specify the minimum allowable line widths for physical objects on-chip such as metal and polysilicon interconnects or diffusion areas, minimum feature dimensions, and minimum allowable separations between two such features. If a metal line width is made too small, for example, it is possible for the line to break during the fabrication process or afterwards, resulting in an open circuit. If two lines are placed too close to each other in the layout, they may form an unwanted short circuit by merging during or after the fabrication process. The main objective of design rules is to achieve, for any circuit to be manufactured with a particular process, a high overall yield and reliability while using the smallest possible silicon area. Note that there is usually a trade-off between higher yield, which is obtained through conservative geometries, and better area efficiency, which is obtained through aggressive, high-density placement of various features on the chip. The layout design rules which are specified for a particular fabrication process normally represent a reasonable optimum point in terms of yield and density. It must be emphasized, however, that the design rules do not represent strict boundaries which separate "correct" designs from "incorrect" ones. A layout which violates some of the specified design rules may still result in an operational circuit with reasonable yield, whereas another layout observing all specified design rules may result in a circuit which is not functional and/or has very low yield. To summarize, we can say, in general, that observing the layout design rules significantly increases the probability of fabricating a successful product with high yield.
The design rules are usually described in two ways:
i. Micron rules, in which the layout constraints such as minimum feature sizes and minimum allowable feature separations are stated in terms of absolute dimensions in micrometres, or,
ii. Lambda rules, which specify the layout constraints in terms of a single parameter (λ) and thus allow linear, proportional scaling of all geometrical constraints.
Lambda-based layout design rules were originally devised to simplify the industry-standard micron-based design rules and to allow scaling capability for various processes. It must be emphasized, however, that most of the submicron CMOS process design rules do not lend themselves to straightforward linear scaling. The use of lambda-based design rules must therefore be handled with caution in submicron geometries. In the following, we present a sample set of the lambda-based layout design rules devised for the MOSIS (MOS Implementation System) CMOS process and illustrate the implications of these rules on a section of a simple layout which includes two transistors (Fig. 1.6). The complete set of MOSIS CMOS scalable design rules are also illustrated in colour in Plate 6 and Plate 7.
MOSIS Layout Design Rules (sample set)
Rule number Description λ-Rule
Active area rules
R1 Minimum active area width 3λ
R2 Minimum active area spacing 3λ
R3 Minimum poly width 2λ
R4 Minimum poly spacing 2λ
R5 Minimum gate extension of poly over active 2λ
R6 Minimum poly-active edge spacing 1λ
(poly outside active area)
R7 Minimum poly-active edge spacing 3λ
(poly inside active area)
R8 Minimum metal width 3λ
R9 Minimum metal spacing 3λ
R10 Poly contact size 2λ
R11 Minimum poly contact spacing 2λ
R12 Minimum poly contact to poly edge spacing 1X
R13 Minimum poly contact to metal edge spacing 1λ
R14 Minimum poly contact to active edge spacing 3λ
R15 Active contact size 2λ
R16 Minimum active contact spacing 2λ
(on the same active region)
R17 Minimum active contact to active edge spacing 1λ
R18 Minimum active contact to metal edge spacing 1λ
R19 Minimum active contact to poly edge spacing 3λ
R20 Minimum active contact spacing 6λ
(on different active regions)
Fig. 1.6 Illustration of some of the typical MOSIS layout design rules.
MOSFET Layout Design