8.1 Bipolar integrated circuits
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Junction isolated bipolar circuit: In the first developed bipolar integrated circuits, the transistors are isolated from each other by reverse biased p-n junctions. An n type epitaxial layer is grown on top of a p-type substrate (for fabricating npn transistors). A deep p+ diffusion in selected areas penetrates the epi-layer, and forms islands of n regions which are used as the collectors. Subsequent diffusions will form the base and emitter regions.
The isolation between collectors of adjacent transistors is through reverse biased junction diodes. A parasitic depletion layer capacitance and leakage current exists due to the isolating junction. The substrate doping should be light to minimize the parasitic capacitance. (5cm-20cm) The resistivity of the epitaxial layer (0.1cm-10cm) affects the collector resistance, collector-base junction capacitance. The isolation diffusion must be of high concentration to minimize separation between islands.
Process sequence for junction - isolated bipolar circuits with buried layers
(a) after selective diffusion of the n +- buried layer;
(b) after epitaxial layer decomposition and growth of protective oxide;
(c) after isolation diffusion;
(d) after drain collector diffusion;
(e) after base diffusion
The base diffusion consists of two steps: pre-deposition and drive-in. The emitter diffusion consists of only one step, because the emitter junction depth is low. A n+ diffusion is made for collector contacts to avoid possible surface inversion or Schottky barrier contact with aluminum.
8.2.1The MOS capacitor is an useful structure in characterizing the performance of MOS transistors.
Ideal MOS – electric field in MOS equal to zero if the applied voltage VGS is zero;
In practice, a flat-band voltage VFB has to be applied at the metal to maintain zero electric field in Si, due to (a) work function difference between metal and Si, and (b) charges at the oxide-semiconductor interface. Three possible conditions:
Accumulation – majority carriers accumulated near the Si surface, VGS<VFB in p-type substrates
Depletion – majority carriers depleted away from the surface, VGS>VFB in p-type substrates
Inversion – Si is inverted near the surface, i.e.large concentrations of electrons present for p-type substrates when VGS>>VFB
When the gate voltage is above the threshold voltage, Si surface is inverted with high concentration of electrons. The n+ source and drain regions are thus connected and a channel is formed. The gate bias can alter the electron concentration in the channel and thus control the drain current.
In the linear region, drain current nearly increases linearly with drain voltage because a channel exists continuously between source and drain. In the saturation region where VDS>Vdsat, drain current increases slightly with drain voltage. Two reasons: (a) pinch-off of channel near drain in long channel devices, (b) limiting of carrier drift velocity due to hot electron effect.
We first determine the flatband voltage of MOS capacitor:
where ms=m-siis themetal-semiconductor work function difference, Nssis the interface state density (per unit surface area); note the minus sign in the second term because of charge induced on the gate is opposite in sign to interface charges
Let the Si surface potential be swhen the gate voltage is equal to the threshold voltage. The surface is in strong inversion. The voltage across the gate and Si surface is equal to VT-VFB- s . Therefore the charge on the gate (which is equal and opposite to the charge in the Si) is equal to:
where NA is the substrate doping concentration, W is the depletion width & s is the surface potential given by:
Adjustment of threshold voltage by ion implantation of a charge sheet Qim (per unit area) at the Si surface:
The flatband voltage VFB is modified as:
Qim can be +ve or –ve depending on whether donors or acceptors.
Let the voltage at the channel with respect to the source be V(x) at a point of distance x from the source. If Qn(x) is the channel charge (per unit surface area), Gauss law gives:
where ox and d are the permittivity and thickness of oxide.
Considering the drift current only, the channel current is given by:
where W is width of channel, is the carrier mobility
In long channel MOSFET, saturation occurs when the channel charge is zero at the drain when VDS is increased. Solving Qn(L)=0 gives the saturation drain voltage
The saturation drain current at VDatis given by:
In saturation region, ID increases slightly with drain voltage
The first commercial MOS IC was thick oxide, metal gate p-channel. The gate oxide is between 500 to 1000A, and the thick field oxide is about 1.5m to maintain a high field threshold voltage.
The disadvantages of the thick oxide process are: i) a large field oxide step exists at the source, drain regions, which may cause problems of step coverage in the metallisaton, ii) the diffusion and metal gate are defined by separate masks. In order to allow for tolerances in the photolithographic process, an overlap between the gate and source, drain usually exists. This results in higher capacitance between gate and drain.
silicon gate process:
Process sequence for self-aligned polysilicon gate NMOS circuits:
(a) after etching of upper oxide to expose all regions that will not be transistors;
(b) after selective growth of the thick-field oxide and the gate oxide;
(c) after deposition and patterning of the polysilicon layer;
(d) after source-and-drain diffusion;
(e) completed structure.
For a n-channel MOS process, a p-type Si substrate is first thermally oxidized, and then coated with CVD silicon nitride and photoresist. The silicon nitride in the isolation regions is then etched using dry etching. The etched regions of the oxide exposes the Si area which will subsequently be oxidized using the local oxidation technique (to serve as the isolation region between active transistors.) Before this oxide is formed, a boron channel-stop is implanted into the isolation regions. The resist is then removed, and the field oxide is grown. This oxidation also acts to drive in the channel-stop implant. The nitride/pad oxide is stripped and a thin gate oxide is formed. A threshold-adjust implant follows.
The polysilicon gate is deposited and doped with n-type impurities. After the polysilicon gate is patterned, the source/drain regions are implanted with arsenic or phosphorus. This implant is self-aligned to both the gate polysilicon and the field oxide. This is done by choosing the implant energy so that the dopant easily penetrates the thin gate oxide left in the source\drain regions but will not penetrate either the polysilicon gate or the field oxide. This self-aligned feature is an important aspect which allows minimum possible overlap between the gate and source\drain regions.
The final part of the processing serves to form metal connection between the devices and provide protection for the devices. A CVD oxide (glass) doped with phosphorus is deposited on the wafer. The phosphorus included in the glass serves two purposes:
The last layer deposited on the wafer is a capping layer of plasma deposited silicon nitride or oxide used to protect the wafer from scratching or contamination's. Windows are then etched in this capping layer to make external wire connections.
i) Isolation between active devices is usually achieved by local oxidation process. Thick field oxide and channel-stop implant in the isolation regions are used to prevent surface inversion.
ii) A shallow implant (with dopants close to the interface) in the gate region is often used to adjust the threshold voltage. (In n-channel MOS, a boron implant raises the threshold voltage, and a phosphorus implant reduces the threshold voltage.)
iii) A deep implant in the gate region can alter the effective impurity concentration in the channel. For example, deep boron implant in p-type substrate is often applied to reduce the punch through susceptibility of short channel MOS devices.
iv) Heavily doped n-type polysilicon is most widely used as the gate material in NMOS.The work function of n+ polysilicon is ideal for n-channel device since it yields a threshold voltage of 0.7V for suitable values of channel doping and oxide thickness.
The major disadvantage to using polysilicon gate is the higher gate resistance (10/sq) causing RC delay of signals. Using refractory metals and silicides can reduce the sheet resistance of gate interconnect to about 1 /sq, but may cause adhesion problems between the gate material and gate oxide. One approach is to use a refractory metal silicide on top of the polysilicon gate (called polycide). Common silicides used include MoSi2, WSi2, TaSi2, TiSi2.
v) The n+ source/drain region of NMOS device should have as low resistance as possible. Shallow junction is also required to allow for small dimensions in VLSI. Arsenic is used as the dopant for source/drain since it has a s high solubility and low diffusion rate.
Aluminum deposited on silicon is unstable even at moderate temperatures. At 450C, dissolution of silicon by diffusion into Al occurs, leading to pit formation on the Si surface. The pit growth can lead to contact and junction failure. Contact-failure also exists due to the precipitation of dissolved Si from aluminum on cooling. The silicon precipitates may cause an undesirable increase in the contact resistance, especially in Al-n-Si contacts.
The p-well process involves implanting or diffusing p-type dopants into an n-substrate at a concentration high enough to over-compensate the n-substrate and ensure proper p-channel device characteristics. However excessive p-well doping produces adverse effects in the n-channel device, such as increased back-gate bias effects, reduction in mobility, and increased source/drain to p-well capacitance.
The twin-well process allows two separate wells to be implanted into very lightly doped silicon. This allows the doping profile in each well to be tailored independently so that neither type of device will suffer from excessive doping effects.
BiCMOS integrated circuit:
CMOS finds wide applications in VLSI circuits due to its excellent noise margin, low drive current and vitually zero standby power consumption. However there are increasing demands to fabricate bipolar transistors on CMOS chips, particularly for drivers and high performance analog circuits.
In principle, the same number of steps in CMOS fabrication can be used to incorporate the base and emitter regions of the bipolar transistor. However it assumes that the emitter and base regions are compatible with the requirements of the CMOS source and drain. In reality, for CMOS which already has a given set of design rules for source and drain implants, and channel implants (for controlling the threshold voltage), at least one additional step is required in order to incorporate a bipolar transistor on the same chip.
9.1 Overview of the IC design process
The goal of the IC designer is to design an integrated circuit that meets a given set of specification with minimum labor in the shortest time. The conventional approach to circuit design often involves much iteration at the breadboard level. Due to the complexity in VLSI, powerful computer-aided-design tools are required to perform schematic capture, simulation and mask layout in the design process.
i) in the bottom-up approach, the designer starts at the transistor or gate level and designs subcircuits of increasing complexity, which are then interconnected to realize the required functionality;
ii) in the top-downapproach, the designer repeatedly decomposes the system-level specification into groups, and subgroups of simpler tasks. The lowest-level tasks are ultimately implemented in silicon, either with standard circuits or with low-level circuits designed to meet the required specification. In the extreme case, the top-down approach results in a silicon compiler, in which all blocks are automatically designed with a computer. The top-down approach is especially useful in large scale digital circuits which are regular and repetitive in structure. In practice, combinations of top-down and bottom-up concepts are used in varying degrees.
Gate arrays are integrated circuits containing large numbers of digital gates or transistor cells, which can be interconnected in different ways to implement various logic functions.
Wafers containing these gate arrays have been processed up to all steps except the metallisation layers. Using computer-aided-design tools, only the metallisaton layer patterns are required to be generated from the circuit specification, and the fabrication time can be very short. Gate arrays can have several metallisation layers to facilitate interconnection.
Standard cell design relies on the extraction of mask layouts from a set of pre-designed logic/circuit cells. To facilitate placement and routing, the standard cells are designed to have equal height but variable widths, so that the final IC layout will have a regular pattern with rows of cells and interconnect routing running between the rows.
Design rule is a set of regulations which define the acceptable dimensions and electrical characteristics achievable in a fabrication process. The IC designer must observe these rules in the creation of circuits and mask geometry layout.
Design rules usually consist of three parts:
i) geometric rules governing the features, patterns and relationship between mask layers as determined by the process technology.
ii) mandatory requirements including alignment marks, scribe channel definition, bonding pads.
iii) electrical parameters, e.g. transistor gain, threshold voltage, capacitance values.
i) the minimum width of a layer (e.g. metal, diffusion, poly)
ii) the minimum space in a layer, e.g. the minimum space that can be reliably etched in metal, poly layers, or that is electrically feasible in diffusion layer
iii) the minimum overlap between two layers, e.g. between the contact and diffusion layers, an overlap is necessary due to misalignment, enlarging of contact size and shrinking of diffusion area.
iv) the minimum separation between two different layers, e.g. due to misalignment, shrinkage or enlargement of features during etching.
Mead and Conway (1980) introduced the concept of based design rule by normalizing all geometric design rules by a basic parameter , i.e. all mask pattern dimensions are expressed as multiples of . The unit is set by mask alignment tolerance, overstretching or under-etching, photoresist tolerance etc. The advantage of using based design rule is that the mask layout can be simply scaled down proportionally if the minimum feature of the fabrication process is reduced.
10.1 NMOS logic circuits
(a)Basic inverter: consists of a pull-down transistor M1 and a pull-up transistor M2. M2 can be either a depletion transistor (in depletion-load inverter) or an enhancement transistor (in enhancement-load inverter). Following diags. show the circuit of the inverters and the transfer characteristics. In the enhancement load inverter, the output voltage in the high state is only VDD-VTN, which is less than that (VDD) in the depletion load inverter. Therefore the depletion load inverter is preferred due to the larger output voltage swing and higher noise immunity.
Inverter device sizing: Consider the depletion-load inverter. Because the output high voltage VH=VDD, it is desirable to design the switching voltage at VDD/2. With proper design, both transistors are saturated, and the output voltage is equal to VDD/2 when the input is at VDD/2. Equating the drain currents of M1 & M2,
where VTN,VTD are the threshold voltages of the enhancement and depletion devices. In a typical 5V supply NMOS, VTN is about 1 V, VTD is about -3V. Solving, we obtain
Minimum sizing rule:
Minimum Sizing rule:
Series resistance of M1 & M2 is equivalent to the pull-down transistor in the inverter.
CMOS IC has low power consumption (power is only consumed during switching of the gates), and high noise immunity. The CMOS fabrication process is more complicated than NMOS. It consumes more Si area as compared with NMOS. The power consumption in CMOS is proportional to the clock frequency in digital circuits.
CMOS is the present standard logic family used in VLSI digital ICs.
The CMOS inverter is a ratioless logic circuit, meaning that the steady state output voltage levels are independent of the ratio of the pull up/pull down transistor sizes. The voltage swing is almost between VDD and zero. The primary effect of the sizes of the pull-up and pull-down transistors is on the equivalent resistance of the transistors in the conducting state.
Thus, sizing can be used to provide approximately equal capability to source or sink load current. In contrast, NMOS inverters provide asymmetric output drive because of the difference in pull-up and pull-down resistance needed to achieve useful logic levels. The symmetric drive capability of CMOS allows comparable transition times for output voltages irrespective of the direction of transition.
The transductacne parameter K'N of an n-channel transistor is about 2 to 3 times that of the p-channel transistor K'P. (This is because K’ is proportional to majority carrier mobility in the channel, and electron mobility is higher than hole mobility.) The equivalent resistance for the n-channel and p-channel transistors are given by:
Simple models for the transistors consist of an ideal switch in series with the equivalent resistances. The model is useful for predicting current drive capability and propagation delays. For symmetric output drive, RN=RP, and the device sizing should be:
It means p-channel transistor has a width of 2.5 times that of the n-channel transistor for equal channel length.
The pull-down resistance can be either RN1, RN2 or their parallel combination depending on the input conditions. Usually the devices are sized so that the worst drive capability is as good as that of the reference inverter. If M1 M2 are of minimum size, then the pull-up transistors would both have W/L=5 to maintain the worst-case drive capability.
For the NAND gate, the series path to ground uses n-channel transistors, and the parallel path to VDD uses p-channel transistors. Based on minimum-size transistors and a 2.5:1 transconductance advantage of n-channel over p-channel transistors, the series resistance of two n-channel transistors to ground is nearly matched by a single p-channel pull-up to VDD. This near-symmetry makes the NAND gate the preferred CMOS logic form.
Because a MOS transistor is an excellent switch, it is possible to connect the transistor in series with a logic signal so that the signal is either passed or inhibited. This structure is called a transmission gate.
NMOS transmission gate