In this article we are going to discuss briefly the main steps in the fabrication of the microprocessor. The microprocessor is considered a chip or an integrated circuit with a very high density of logic. Any chip contains lots of logic gates and each logic gate, in turn, is composed of lots of transistors that are arranged in some way to perform the desired functionality from the logic gate. So, before we get into the details of the fabrication of chips, we should briefly introduce transistors and their history.
Transistors can be thought as electrically connected switches, with a control terminal and other two terminals that are connected or disconnected depending on the voltage applied to the control terminal.
Transistors developed from single contact transistors to Bipolar Junction Transistors(require small current applied to the base to switch much more current between the terminals) , Metal Oxide Semiconductor Field Effect Transistor (MOSFETs) (come in two flavours nMOS and pMOS using n-type and p-type dopants ) .
First logic gates were described in 1963 using MOSFETs. These gates used both nMOS and pMOS earning the complementary Metal Oxide Semiconductor (CMOS) .The circuits used discrete transistors but consumed only nanowatts of power, six orders of magnitude less than their bipolar counterparts.
pMOS provided poor performance. nMOS dominated because its transistors occupied less area and it had a less expensive fabrication process. Intel dominated nMOS industry in 1970 using its first 256 SRAM, 4004 4-bit microprocessor.
while nMOS process was less expensive than CMOS, it consumed power while idle. power consumption became a major problem in the 1980s as hundreds of thousands of transistors were integrated on a single die. since then, CMOS replaced nMOS in all digital logic applications.
The level of integration has been classified as: small scale (SSI) (less than 10 logic gates, half dozen of transistors for each gate), medium scale (MSI) (up to 1000 gates) , large scale (LSI)(such as 8-bit microprocessors , have up to 10,000 gates) , very large scale (VLSI) and ultra large scale integration (ULSI).
How a processor is made?
The process of processor fabrication is mainly the same as of any CMOS process, taking into consideration the different level of integration of course. So, the following steps apply to most current IC fabrication processes:
1-getting silicon from sand (Silicon Dioxide)
2-fully purifying silicon
3-silicon wafer fabrication: silicon enters melting phase. a mono-crystal ingot is produced.
4-ingot is sliced into individual silicon discs , called wafers. Inside a wafer, there are many small blocks or cells, these individual cells are called dies or chips. A die is a small piece of silicon material upon which a certain circuit is fabricated. Typically, integrated circuits are produced in large batches on a single wafer. The resultant wafer is then cut into pieces, each containing one copy of the desired integrated circuit.
slicing a silicon ingot into wafers
a wafer with dies
5-Photolithography: in this step , the wafer surface is cleaned, heated to drive off any moisture, covered with a photoresist using spin coating then the photoresist is exposed to a pattern of Ultra-Violet light. The exposure causes a chemical change that allows some of the photoresist to be removed by a special solution called “developer”. Then in etching, a chemical agent removes the uppermost layer of the substrate in the areas that are not covered by the photoresist. The process is explained graphically in the following figure:
Photolithography main steps
6-doping (ion implantation) : adding impurties to silicon wafer to control the way silicon in some areas conduct electricity.
7-electroplating: wafers are put into a copper sulphate solution
8-copper ions settle as a thin layer on the wafer surface. excess material is polished off leaving a very thin layer of copper. Multiple metal layers are created to interconnect between various transistors.
9-chips go through the first functionality test
10-dies that have correct response to the previous tests are put to the next stage
That was a brief introduction to motivate reading about the topic of VLSI , you can get into more details searching in VLSI and chip design in general.
In a coming article, we’ll be talking about building the processor from computer architecture perspective (architecture & microarchitecture). Any inquiries are very welcome.