Chip manufacturing is the most complex process in the world today. This is a complex process completed by many top companies. This article strives to summarize this process and give a comprehensive and general description of this complex process.
There are many semiconductor manufacturing processes, and it is said that there are hundreds or even thousands of steps. This is not an exaggeration. A factory with a billion-dollar investment may only do a small part of the process. For such a complex process, this article will be divided into five major categories for explanation: wafer manufacturing, photolithography and etching, ion implantation, thin film deposition, and packaging and testing.
1. Semiconductor manufacturing process - wafer manufacturing
Wafer manufacturing can be divided into the following 5 main processes:
(1) Crystal Pulling
◈ Doped polysilicon is melted at 1400 degrees
◈ Inject high-purity argon inert gas
◈ Place the single crystal silicon "seed" into the melt and slowly rotate it when "pulled out".
◈ The diameter of the single crystal ingot is determined by the temperature and extraction speed
(2) Wafer slicing uses a precision "saw" to cut the silicon ingot into individual wafers.
(3) Wafer lapping, etching
◈ The sliced wafers are mechanically ground using a rotary grinder and alumina slurry to make the wafer surface flat and parallel and reduce mechanical defects.
◈ The wafers are then etched in a nitrided acid/acetic acid solution to remove microscopic cracks or surface damage, followed by a series of high-purity RO/DI water baths.
(4) Wafer polishing and cleaning
◈ Next, the wafers are polished in a series of chemical and mechanical polishing processes called CMP (Chemical Mechanical Polish). ◈ The polishing process typically includes two to three polishing steps using increasingly fine slurries and intermediate cleaning using RO/DI water. ◈ A final clean is performed using SC1 solution (ammonia, hydrogen peroxide, and RO/DI water) to remove organic impurities and particles. Then, HF is used to remove native oxides and metal impurities, and finally SC2 solution allows ultra-clean new native oxides to grow on the surface. (5) Wafer epitaxial processing
◈ Epitaxial growth (EPI) is used to grow a layer of single-crystalline silicon from vapor onto a single-crystalline silicon substrate at high temperatures.
◈ The process of growing a single-crystalline silicon layer from the vapor phase is called vapor phase epitaxy (VPE).
SiCl4 + 2H2 ↔ Si + 4HCl
SiCl4 (silicon tetrachloride)
The reaction is reversible, i.e. if HCl is added, silicon will be etched out from the surface of the wafer.
Another reaction to generate Si is irreversible: SiH4 → Si + 2H2 (silane)
◈ The purpose of EPI growth is to form layers with different (usually lower) concentrations of electrically active dopants on the substrate. For example, an N-type layer on a P-type wafer.
◈ About 3% of the wafer thickness.
◈ No contamination to subsequent transistor structures.
2. Semiconductor manufacturing process - Photolithography The photolithography machine, which has been mentioned a lot in recent years, is just one of many process equipment. Even photolithography has many process steps and equipment.
(1) Photoresist coating
Photoresist is a photosensitive material. A small amount of photoresist liquid is added to the wafer. The wafer is rotated at a speed of 1000 to 5000 RPM, spreading the photoresist into a uniform coating of 2 to 200um thick. There are two types of photoresists: negative and positive. Positive: Exposure to light can break down the complex molecular structure, making it easy to dissolve. Negative: Exposure makes the molecular structure more complex and more difficult to dissolve. The steps involved in each photolithography step are as follows; ◈ Clean the wafer ◈ Deposit barrier layer SiO2, Si3N4, metal ◈ Apply photoresist ◈ Soft bake ◈ Align mask ◈ Graphic exposure ◈ Development ◈ Bake ◈ Etch ◈ Remove photoresist (2) Pattern Preparation Pattern Preparation IC designers use CAD software to design the pattern of each layer. The pattern is then transferred to an optically transparent quartz substrate (template) with the pattern using a laser pattern generator or electron beam.
(3) Pattern transfer (exposure) Here, a photolithography machine is used to project and copy the pattern from the template onto the chip layer.
(4) Development and Baking ◈ After exposure, the wafer is developed in an acid or alkaline solution to remove the exposed areas of the photoresist. ◈ Once the exposed photoresist is removed, the wafer is "baked" at a low temperature to harden the remaining photoresist.
3. Semiconductor Manufacturing Processes - Etching and Ion Implantation (1) Wet and Dry Etching ◈ Chemical etching is performed on a large wet platform. ◈ Different types of acid, base, and caustic solutions are used to remove selected areas of different materials. ◈ BOE, or buffered oxide etchant, is made from hydrofluoric acid buffered with ammonium fluoride and is used to remove silicon dioxide without etching the underlying silicon or polysilicon layer. ◈ Phosphoric acid is used to etch silicon nitride layers. ◈ Nitric acid is used to etch metals. ◈ Photoresist is removed with sulfuric acid. ◈ For dry etching, the wafer is placed in an etching chamber and etched by plasma. ◈ Personnel safety is a primary concern. ◈ Many fabs use automated equipment to perform the etching process. (2) Resist Stripping
The photoresist is then completely stripped from the wafer, leaving an oxide pattern on the wafer.
(3) Ion implantation
◈ Ion implantation changes the electrical properties of precise areas within existing layers on the wafer.
◈ Ion implanters use high-current accelerator tubes and steering and focusing magnets to bombard the wafer surface with ions of specific dopants.
◈ The oxide acts as a barrier while the doping chemicals are deposited on the surface and diffuse into the surface.
◈ The silicon surface is heated to 900°C for annealing, and the implanted dopant ions diffuse further into the silicon wafer.
4. Semiconductor Manufacturing Process - Thin Film Deposition
There are many ways and contents of thin film deposition, which are explained one by one below: (1) Silicon Oxide
When silicon exists in oxygen, SiO2 will grow thermally. Oxygen comes from oxygen or water vapor. The ambient temperature is required to be 900 ~ 1200℃. The chemical reaction that occurs is
Si + O2 → SiO2
Si +2H2O -> SiO2 + 2H2
The surface of the silicon wafer after selective oxidation is shown in the figure below:
Both oxygen and water diffuse through the existing SiO2 and combine with Si to form additional SiO2. Water (vapor) diffuses more easily than oxygen, so the vapor grows much faster.
Oxide is used to provide an insulating and passivation layer to form the transistor gate. Dry oxygen is used to form the gate and thin oxide layer. Vapor is used to form a thick oxide layer. The insulating oxide layer is usually around 1500nm, and the gate layer is usually between 200nm and 500nm.
(2) Chemical Vapor Deposition
Chemical vapor deposition (CVD) forms a thin film on the surface of a substrate through thermal decomposition and/or reaction of gaseous compounds.
There are three basic types of CVD reactors: ◈ Atmospheric chemical vapor deposition
◈ Low pressure CVD (LPCVD)
◈ Plasma enhanced CVD (PECVD)
The schematic diagram of the low pressure CVD process is shown below.
The main reaction processes of CVD are as follows
i). Polysilicon PolysiliconSiH4 -> Si + 2h2 (600℃)
Deposition rate 100 - 200 nm /min
Phosphorus (phosphine), boron (diborane) or arsenic gas can be added. Polysilicon can also be doped with diffusion gas after deposition.
ii). Silicon dioxide Dioxide
SiH4 + O2→SiO2 + 2h2 (300 - 500℃)
SiO2 is used as an insulator or passivation layer. Phosphorus is usually added to obtain better electron flow performance.
iii). Silicon nitride Siicon Nitride
3SiH4 + 4NH3 -> Si3N4 + 12H2
(Silane) (Ammonia) (Nitride)
(3) Sputtering
The target is bombarded with high-energy ions such as Ar+, and the atoms in the target will be moved and transported to the substrate.
Metals such as aluminum and titanium can be used as targets. (4) Evaporation
Al or Au (gold) is heated to the evaporation point, and the vapor will condense and form a thin film covering the surface of the wafer.
The following example will explain in detail how the circuit on the silicon wafer is formed step by step from photolithography, etching to ion deposition:
5. Semiconductor Manufacturing Process - Packaging Test (Post-processing)
(1) Wafer Test After the final circuit preparation is completed, the test devices on the wafer are tested using an automated probe test method to remove defective products.
(2) Wafer Dicing After the probe test, the wafer is cut into individual chips.
(3) Wiring and packaging ◈ Individual chips are connected to the lead frame, and aluminum or gold leads are connected by thermal compression or ultrasonic welding. ◈ Packaging is completed by sealing the device in a ceramic or plastic package. ◈ Most chips still need to undergo final functional testing before they are sent to downstream users.
