OVERVIEW of Semiconductor Processing Steps |
Semiconductor device fabrication is the process used to create chips, the integrated circuits that are present in everyday electrical and electronic devices. It is a multiple-step sequence of photographic and chemical processing steps during which electronic circuits are gradually created on a wafer made of pure semiconducting material. Silicon is the most commonly used semiconductor material today, along with various compound semiconductors. The entire manufacturing process from start to packaged chips ready for shipment takes six to eight weeks and is performed in highly specialized facilities referred to as fabs.
Wafers
A typical wafer is made out of extremely pure silicon that is grown into mono-crystalline cylindrical ingots (boules) up to 300 mm (slightly less than 12 inches) in diameter using the Czochralski process. These ingots are then sliced into wafers about 0.75 mm thick and polished to obtain a very regular and flat surface.
Once the wafers are prepared, many process steps are necessary to produce the desired semiconductor integrated circuit. In general, the steps can be grouped into two areas:
Front end processing
Back end processing
Processing
In semiconductor device fabrication, the various processing steps fall into four general categories:
Deposition, Removal, Patterning, and Modification of electrical properties.
Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies consist of physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others.
Removal processes are any that remove material from the wafer either in bulk or selective form and consist primarily of etch processes, both wet etching and dry etching such as reactive ion etch (RIE). Chemical-mechanical planarization (CMP) is also a removal process used between levels.
Patterning covers the series of processes that shape or alter the existing shape of the deposited materials and is generally referred to as lithography. For example, in conventional lithography, the wafer is coated with a chemical called a "photoresist". The photoresist is exposed by a "stepper", a machine that focuses, aligns, and moves the mask, exposing select portions of the wafer to short wavelength light. The unexposed regions are washed away by a developer solution. After etching or other processing, the remaining photoresist is removed by plasma ashing. Modification of electrical properties has historically consisted of doping transistor sources and drains originally by diffusion furnaces and later by ion implantation. These doping processes are followed by furnace anneal or in advanced devices, by rapid thermal anneal (RTA) which serve to activate the implanted dopants. Modification of electrical properties now also extends to reduction of dielectric constant in low-k insulating materials via exposure to ultraviolet light in UV processing (UVP). Many modern chips have eight or more levels produced in over 300 sequenced processing steps.
Front End Processing
"Front End Processing" refers to the formation of the transistors directly on the silicon. The raw wafer is engineered by the growth of an ultrapure, virtually defect-free silicon layer through epitaxy. In the most advanced logic devices, prior to the silicon epitaxy step, tricks are performed to improve the performance of the transistors to be built. One method involves introducing a "straining step" wherein a silicon variant such as "silicon-germanium" (SiGe) is deposited. Once the epitaxial silicon is deposited, the crystal lattice becomes stretched somewhat, resulting in improved electronic mobility. Another method, called "silicon on insulator" technology involves the insertion of an insulating layer between the raw silicon wafer and the thin layer of subsequent silicon epitaxy. This method results in the creation of transistors with reduced parasitic effects.
Silicon dioxide
Front end surface engineering is followed by: growth of the gate dielectric, traditionally silicon dioxide (SiO2), patterning of the gate, patterning of the source and drain regions, and subsequent implantation or diffusion of dopants to obtain the desired complementary electrical properties. In memory devices, storage cells, conventionally capacitors, are also fabricated at this time, either into the silicon surface or stacked above the transistor.
Metal layers
Once the various semiconductor devices have been created they must be interconnected to form the desired electrical circuits. This "Back End Of Line" (BEOL) the latter portion of the front end of wafer fabrication, not to be confused with "back end" of chip fabrication which refers to the package and test stages) involves creating metal interconnecting wires that are isolated by insulating dielectrics. The insulating material was traditionally a form of SiO2 or a silicate glass, but recently new low dielectric constant materials are being used. These dielectrics presently take the form of SiOC and have dielectric constants around 2.7 (compared to 3.9 for SiO2), although materials with constants as low as 2.2 are being offered to chipmakers.
Interconnect
Historically, the metal wires consisted of aluminium. In this approach to wiring often called "subtractive aluminium", blanket films of aluminium are deposited first, patterned, and then etched, leaving isolated wires. Dielectric material is then deposited over the exposed wires. The various metal layers are interconnected by etching holes, called "vias," in the insulating material and depositing tungsten in them with a CVD technique. This approach is still used in the fabrication of many memory chips such as dynamic random access memory (DRAM) as the number of interconnect levels is small, currently no more than four.
More recently, as the number of interconnect levels for logic has substantially increased due to the large number of transistors that are now interconnected in a modern microprocessor, the timing delay in the wiring has become significant prompting a change in wiring material from aluminium to copper and from the silicon dioxides to newer low-K material. This performance enhancement also comes at a reduced cost via damascene processing that eliminates processing steps. In damascene processing, in contrast to subtractive aluminium technology, the dielectric material is deposited first as a blanket film and is patterned and etched leaving holes or trenches. In "single damascene" processing, copper is then deposited in the holes or trenches surrounded by a thin barrier film resulting in filled vias or wire "lines" respectively. In "dual damascene" technology, both the trench and via are fabricated before the deposition of copper resulting in formation of both the via and line simultaneously, further reducing the number of processing steps. The thin barrier film, called Copper Barrier Seed (CBS), is necessary to prevent copper diffusion into the dielectric. The ideal barrier film is effective, but is barely there. As the presence of excessive barrier film competes with the available copper wire cross section, formation of the thinnest yet continuous barrier represents one of the greatest ongoing challenges in copper processing today.
As the number of interconnect levels increases, planarization of the previous layers is required to ensure a flat surface prior to subsequent lithography. Without it, the levels would become increasingly crooked and extend outside the depth of focus of available lithography, interfering with the ability to pattern. CMP (Chemical Mechanical Polishing) is the primary processing method to achieve such planarization although dry "etch back" is still sometimes employed if the number of interconnect levels is no more than three.
Wafer Test
The highly serialized nature of wafer processing has increased the demand for metrology in between the various processing steps. Wafer test metrology equipment is used to verify that the wafers are still good and haven't been damaged by previous processing steps. If the number of "dies" the integrated circuits that will eventually become "chips" on a wafer that measure as fails exceeds a predetermined threshold, the wafer is scrapped rather than investing in further processing.
Device Test
Once the Front End Process has been completed, the semiconductor devices are subjected to a variety of electrical tests to determine if they function properly. The proportion of devices on the wafer found to perform properly is referred to as the yield. The fab tests the chips on the wafer with an electronic tester that presses tiny probes against the chip. The machine marks each bad chip with a drop of dye. The fab charges for test time; the prices are on the order of cents per second. Chips are often designed with "testability features" to speed testing, and reduce test costs. Good designs try to test and statistically manage corners: extremes of silicon behavior caused by operating temperature combined with the extremes of fab processing steps. Most designs cope with more than 64 corners.
Packaging
Once tested, the wafer is scored and then broken into individual die. Only the good, undyed chips go on to be packaged. Plastic or ceramic packaging involves mounting the die, connecting the die pads to the pins on the package, and sealing the die. Tiny wires are used to connect pads to the pins. In the old days, wires were attached by hand, but now purpose-built machines perform the task. Traditionally, the wires to the chips were gold, leading to a "lead frame" (pronounced "leed frame") of copper, that had been plated with solder, a mixture of tin and lead. Lead is poisonous, so lead-free "lead frames" are now the best practice. Chip-scale package (CSP) is another packaging technology. Plastic packaged chips are usually considerably larger than the actual die, whereas CSP chips are nearly the size of the die. CSP can be constructed for each die before the wafer is diced.
The packaged chips are retested to ensure that they were not damaged during packaging and that the die-to-pin interconnect operation was performed correctly. A laser etches the chips' name and numbers on the package.
List of Steps:
This is a list of processing techniques that are employed numerous times in a modern electronic device and do not necessarily imply a specific order.
Wafer Processing - Wet cleans - Photolithography - Ion implantation (in which dopants are embedded in the wafer creating regions of increased (or decreased) conductivity) - Dry etching - Wet etching - Plasma ashing - Thermal treatments - Rapid thermal anneal - Furnace anneals - Thermal oxidation - Chemical Vapor Deposition (CVD) - Physical Vapor Deposition (PVD) - Molecular Beam Epitaxy (MBE) - Electrochemical Deposition (ECD) - Chemical-mechanical planarization (CMP) - Wafer testing (where the electrical performance is verified) - Wafer backgrinding (to reduce the thickness of the wafer so the resulting chip can be put into a thin device like a smartcard or PCMCIA card.) - Die Preparation - Wafer mounting - Die cutting - IC Packaging - Die attachment - IC Bonding - Wire bonding - Flip chip - Tab bonding - IC Encapsulation - Baking - Plating - Lasermarking - Trim and form - IC Testing
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