CMOS fabrication and process technology

Before Priya lets anyone draw a transistor, she spends a morning with the team on the process itself. A designer who knows only the schematic symbols treats the chip as an abstract circuit; a designer who knows how the layers are made understands why the design rules exist, which devices match well, where parasitic diodes and transistors hide, and why a resistor drawn in one layer varies by twenty percent while a capacitor drawn in another varies by ten. This lesson follows a wafer through a simplified CMOS process, introduces the layers that the CX-1 layout will use, and designs the first passive devices on the chip.

From silicon wafer to transistor

A CMOS process starts with a lightly doped p-type silicon wafer, polished flat, about 0.7 mm thick and 200 mm in diameter for a 180 nm process. Every structure on the chip is built in or on its top few micrometers; the rest of the wafer is mechanical support. The process is a long sequence of steps, often several hundred, but they repeat a small number of operations. Photolithography transfers a pattern from a mask to a light-sensitive resist on the wafer. Etching removes material where the resist is open. Ion implantation fires dopant atoms into the silicon where the resist is open, and annealing repairs the damage and activates the dopants. Deposition adds layers of oxide, polysilicon or metal, and chemical-mechanical polishing (CMP) planarizes the surface so the next layer can be patterned accurately.

Each mask defines one layer, and the order of the layers follows the order of fabrication. A simplified 180 nm CMOS sequence runs as follows.

The cross-section shows the two transistor types side by side. The NMOS transistor sits in the p-type substrate or a p-well; its source and drain are heavily doped n+ regions, and its channel forms at the silicon surface under the gate when the gate is driven positive. The PMOS transistor sits in an n-well; its source and drain are p+ regions. Because the well and the substrate form a pn junction, the n-well must always be biased at or above the substrate so that the junction stays reverse-biased. In a standard process the substrate is tied to ground and every n-well to the positive supply or to the source of its PMOS devices.

The self-aligned gate is the trick that made CMOS scale. The polysilicon gate is patterned first, and the source and drain implants that follow use the gate itself as a mask. The channel length is therefore set by the width of the poly line, which lithography controls tightly, rather than by the alignment between two masks. The nominal 180 nm of the process name is the minimum drawn gate length; the effective electrical channel is somewhat shorter because the source and drain diffuse slightly under the gate edges.

Layers, design rules and lambda

Design rules translate the limits of the process into geometry. Every layer has a minimum width and a minimum spacing; layers that must overlap have minimum enclosures, such as how far active area must extend past a contact or how far poly must extend past active area to make sure the gate cuts all the way across the channel. There are also density rules, which require each layer to cover a minimum and maximum fraction of every region of the chip so that chemical-mechanical polishing leaves a flat surface, and antenna rules, which limit how much metal can be connected to a gate during fabrication, before the diffusion that will protect it is wired in. A long metal line attached only to a gate collects charge from the plasma etch and can damage the thin oxide, so the layout either breaks the line with a jumper to a higher metal or adds a small protection diode.

Older textbooks express rules in units of λ, half the minimum feature size, so that a design could be scaled from one process generation to the next by changing λ alone. Modern rule sets are written in absolute dimensions and contain far too many special cases for λ scaling to work, but λ rules remain a good way to learn. In λ units a minimum transistor needs a gate 2λ long, active area extending 3λ past the gate on each side for contacts and a contact 2λ square, which makes a minimum transistor about 8λ by 6λ. In the illustrative CX-1 process λ is 90 nm.

Two kinds of transistor

The process offers core transistors with a 4 nm gate oxide, rated for 1.8 V, and input/output transistors with a thicker oxide of about 7 nm, rated for 3.3 V. Core transistors are faster and denser; I/O transistors are slower but tolerate the higher voltage, and they are used at the pads, in level shifters and, often, in analog circuits that need more signal swing than 1.8 V allows. The CX-1 loop oscillator drivers run from 3.3 V to put more signal into the loop, while the counters and the serial interface run at 1.8 V to save power. The oxide capacitance per unit area, Cox, follows from the oxide thickness and sets both the transistor's current-carrying ability and the capacitance it presents to the circuit that drives its gate.

Resistors on a chip

Integrated resistors are drawn as strips of a resistive layer, and their value is set by the layer's sheet resistance R_s, the resistance of one square of the layer, multiplied by the number of squares: R = R_s L/W. Because resistance depends only on the ratio L/W for a given layer, a 1 µm by 1 µm square and a 10 µm by 10 µm square have the same resistance. The contact heads at each end add a fixed amount, and the effective width is slightly less than the drawn width because the edges etch back, which is one reason precise resistors are drawn wider than the minimum.

The CX-1 bias generator needs a 400 kΩ resistor. Drawn in silicided poly it would be absurdly long, so Priya chooses the process's high-resistance poly option, in which a silicide-block mask keeps the silicide off the resistor body. She also notes the resistor's spread and temperature coefficient, because they will set how much the bias currents vary.

# Passive devices on the course's illustrative 180 nm process
eps_ox = 3.9 * 8.854e-12      # F/m
tox = 4.0e-9                  # gate oxide thickness, m
cox = eps_ox / tox            # F/m^2
print(f"gate oxide Cox = {cox * 1e3:.2f} fF/um^2")

# A 400 kilohm bias resistor in high-resistance poly
rs_hipo = 1000.0              # ohms per square
w = 1.0                       # drawn width, um
r_contact = 2 * 50.0          # two contact heads, ohms
target = 400e3
squares = (target - r_contact) / rs_hipo
length = squares * w
print(f"high-res poly: {squares:.1f} squares, length {length:.0f} um at W = {w} um")
# fold it into a serpentine of 40 um legs at a 2 um pitch
legs = int(length // 40) + 1
print(f"  folded into {legs} legs of 40 um: about {legs * 2.0:.0f} um x 40 um")
# the same resistor in silicided poly (8 ohms/sq) would be absurd
print(f"  in silicided poly it would need {target / 8.0 / 1000:.0f} mm of track")

# Process spread and temperature on that resistor
for corner, f in (("high (+20 %)", 1.2), ("typical", 1.0), ("low (-20 %)", 0.8)):
    print(f"  {corner:<13} R = {target * f / 1e3:.0f} kohm")
tc = -1.0e-3                  # per kelvin, high-res poly is typically negative
for t in (-40, 27, 125):
    print(f"  at {t:>4} C: {target * (1 + tc * (t - 27)) / 1e3:.0f} kohm")

# A 2 pF MIM capacitor and a 2 pF gate-oxide (MOS) capacitor
c_mim = 2.0e-3                # F/m^2 (2 fF/um^2)
side_mim = (2e-12 / c_mim) ** 0.5 * 1e6
side_mos = (2e-12 / cox) ** 0.5 * 1e6
print(f"2 pF MIM: {side_mim:.1f} um square; 2 pF MOS: {side_mos:.1f} um square")

gate oxide Cox = 8.63 fF/um^2
high-res poly: 399.9 squares, length 400 um at W = 1.0 um
  folded into 10 legs of 40 um: about 20 um x 40 um
  in silicided poly it would need 50 mm of track
  high (+20 %)  R = 480 kohm
  typical       R = 400 kohm
  low (-20 %)   R = 320 kohm
  at  -40 C: 427 kohm
  at   27 C: 400 kohm
  at  125 C: 361 kohm
2 pF MIM: 31.6 um square; 2 pF MOS: 15.2 um square

Four hundred squares at 1 µm wide makes a 400 µm strip, which Priya folds into ten legs of 40 µm to give a block about 20 µm by 40 µm. The absolute value will vary by about ±20 % from wafer to wafer, and by about ten percent over the temperature range because high-resistance poly has a negative temperature coefficient. Neither is acceptable for an absolute specification, which is why the CX-1 design never relies on the absolute value of a resistor where it can rely on a ratio. Two resistors drawn from the same layer, side by side, with the same width and orientation, match to a fraction of a percent even when their absolute values are twenty percent off. Lessons 7 and 8 use that property again and again.

Capacitors on a chip

Capacitors matter in almost every analog block on CX-1: the op-amp's compensation capacitor, the sampling capacitors of the data converter and the filter capacitors of the reference. A 180 nm process offers several kinds. The MOS capacitor uses a transistor's gate oxide and has the highest capacitance per area, about 8.6 fF/µm², but its value depends strongly on the voltage across it, because the channel under the gate forms or disappears with the bias. The metal-insulator-metal (MIM) capacitor is an optional module that adds a thin dielectric between two special metal plates near the top of the stack, at about 1–2 fF/µm², with very low voltage coefficient and good matching. Metal-oxide-metal (MOM) capacitors use the lateral fringing between interdigitated fingers of ordinary metal layers and need no extra masks; they are widely used in data converters on processes without a MIM module.

The script compares a 2 pF MIM capacitor, 32 µm on a side, with a 2 pF MOS capacitor of 15 µm on a side. Priya chooses MIM for the data converter, where linearity and matching matter, and uses MOS capacitors only for supply decoupling, where nonlinearity does not matter and density does.

Parasitic devices and latch-up

The layers of a CMOS process form devices that nobody drew. Every source and drain is a diode to its well or substrate. Every wire is a capacitor to the substrate and to its neighbors. Most important, the n+ source of an NMOS transistor, the p substrate and the n-well form a lateral NPN transistor, and the p+ source of a PMOS transistor, the n-well and the substrate form a vertical PNP transistor. Together they make a four-layer pnpn structure, a thyristor, connected directly between the supply and ground.

Normally both parasitic transistors are off, because the well and substrate are tied to their supplies. But if a current pulse, from a voltage spike on a pin, a fast supply transient or radiation, raises the local substrate potential near an NMOS source or lowers the local well potential near a PMOS source, one parasitic transistor can turn on, its collector current turns on the other, and the pair latches into a low-resistance path from supply to ground. The chip draws a large current until the power is removed and may be destroyed. This is latch-up, and preventing it is a layout discipline. Substrate and well contacts are placed frequently, every few tens of micrometers, so that the resistances that would let the local potential rise are small; guard rings of contacts surround circuits connected to pins; and the spacing between NMOS and PMOS devices near the pads is increased. The foundry's design rules include latch-up rules, and lesson 12 adds the latch-up and electrostatic discharge checks to the tape-out list.

Variation: where the numbers come from

No two transistors on a wafer are identical, and the designer needs to know how they differ. Variation comes in two kinds. Global, or inter-die, variation shifts every device on a die together: the gate oxide is slightly thicker on one wafer, or a threshold implant slightly heavier. Foundries capture it with process corners, model sets that represent the slow and fast extremes of NMOS and PMOS behavior; the names TT, SS, FF, SF and FS give the NMOS speed first and the PMOS speed second. Local, or intra-die, variation makes neighboring devices differ: random fluctuations in the number and position of dopant atoms under each gate, roughness at the gate edges, and gradients across the die. Local variation is captured by mismatch models, usually expressed by Pelgrom's law, which says that the standard deviation of the difference between two identical devices falls as one over the square root of their area. Lesson 7 develops that law and the layout techniques that exploit it.

The distinction drives the whole design philosophy of analog integrated circuits. Global variation is large but common to all devices, so designs that depend on ratios and matching tolerate it. Local variation is small but random, so designs that need precise matching use large devices and careful layout. The CX-1 resistor's ±20 % is global; its 0.3 % match to its neighbor is local.