In this post, we shall discuss, the fabrication of a standard, junction- isolated bipolar IC. Of course, many devices can be formed at a time over the surface of the water if appropriate patterns are provided. We shall show only one device, that is, a bipolar transistor as an example. The major steps in the IC bipolar process are listed in the diagram below.
The starting material is a p-type single crystal silicon wafer having 5 to 20 ohm-cm resistivity and thickness of approximately several hundred micrometers. The diameter can be 50, 75, 100, 125, or 150 mm. The most standard size is 100 mm or about 4 inches
- A thin layer of SiO2 is formed on all surfaces of a p-type silicon wafer by exposing it to oxygen or water vapour in an electric furnace. The first masking step defines the area for n+ buried layers, (also called sub-collector). The function of this layer is to reduce the collector resistance of the transistor. The SiO2 is removed in these areas by chemical etching. Thermal diffusion or ion implantation forms the desired heavily doped n-type, that is, n+ buried layer region. The resulting structure is shown in the figure below.
- The SiO2 masking layer is removed, exposing the entire silicon wafer surface. By epitaxial deposition, an n-type layer is grown, over the entire surface. It is n-type single-crystal silicon 2 to 5 micro meter thick with its resistivity in the range of 0.1 to 1 ohm-cm. During the epitaxial process, the n-type dopant previously introduced in the buried layer areas diffuses in all directions. This is shown in the figure below.
- The wafer with the epitaxial layer is then oxidized at an elevated temperature in an H2O ambient. This forms a layer of SiO2, approximately 0.5 micro meters thick over the entire surface of silicon. A second masking step defines a border completely enclosing n-type islands of silicon that are to be electrically isolated collectors of transistors. P-type diffusion into the border areas is continued until the entire epitaxial layer has been penetrated, as shown in the figure below. Thus, islands of n-type silicon are bounded on all sides by p-type Si. Isolation is achieved by applying voltages such that this p-n junction is always reverse- biased. The p-type diffusion uses boron as impurity. A new layer of thermal oxide is grown over the isolation areas.
- The third masking step defines base regions of n-p-n transistors. Patterns of resistors are formed simultaneously in separate isolated n-type regions. Boron is again diffused (but this time not as deeply) or implanted to forms bases and resistors. The n-type collector is converted to p-type when the density of p-type impurities exceeds that of n-type impurities. The resulting structure is shown in figure below.
- The fourth photolithographic step defines n-type transistor emitters and n-type regions for low resistance contacts to collector regions, as in the figure below. Again conversion of p-type base to n-type requires impurity compensation.
- An oxide is again thermally grown over the entire wafer and via photolithography, (5th mask) those regions where contact is to be made to the silicon are defined. Metal (AI) is then deposited by vacuum evaporation. The photolithographic process (6th mask) is then used to define the appropriate metallization inter-connection pattern, and the remaining metal is removed. The figure below shows the contact areas (defined by 5th mask) to collector, base, and emitter. The 6thmasking step is not shown in figure.
At this point, the ICs are in finished state. However before finished form, a protective passivating layer using glass is deposited over the entire wafer. This is known as die passivation or scratch protection, or glassivation. This protects the surface of the wafer from contamination. Glassivation is done using chemical vapour deposition. This added step paysoff in protection before and after packaging, in higher yields and in better reliability.
A final masking step removes the above insulating layer over the pads where contacts will be made. Now the IC chip undergoes a probe test. This is necessary because there are many faulty chips after such highly complicated fabrication steps. The probe test is automatically carried out by contacting the pads of every chip with microelectrode probes. Registration of each chip with respect to the probes is done automatically by final mechanical adjustment.
The chip is then tested using a set of test vectors, which consist of a sequence of input voltages, stimuli (to chip input pads) and expected output voltage responses (from chip output pads) that have been previously generated by the design engineer. If the chip passes all test vectors, namely all outputs provide the correct results for all input stimuli then the probes are automatically stepped to the next chip position and all test vectors are applied to that chip. This process is repeated until all chips have been tested. Chips that failed to pass all test vectors are marked with an ink dot. Due to computer controlled operation of wafer probe equipment, upto 16000 test vectors are tested for each chip of a wafer in some minutes.
Faulty chips will be thrown away later. Now, the entire wafer is broken into individual chips. This is discussed below.
The entire wafer is divided up into individual chips by “scribe-and-break” operation using any one of the following ways.
- Diamond-tipped scribe
- High-intensity laser beam (laser scribing), or
- High-speed circular saw
Since this process is similar to glass culling it is called scribing and breaking. In the diamond-tipped scribe method, the grooves are very shallow. In laser scribing method, the grooves are somewhat deeper, and may extend more than halfway through the wafer. In the high-speed circular saw method, the wafer will have a pattern of orthogonally oriented “scribing streets” which are kept clear of oxide and metal and are aligned along certain crystallographic directions to promote the easy and smooth cleavage of the wafer.
A popular process for chip separation is to use a wafer saw to cut entirely through the wafer. The wafer is mounted on adhesive-coated tape prior to the sawing operation so that after sawing the chips will remain in matrix form for convenience in further operations.
Faulty chips are identified using probe test mentioned above. Hence, only good chips are mounted in containers. The chips are bonded to either metal headers or ceramic substrate. The metal headers are usually gold-plated Kover. Kovar is an iron-nickel-cobalt alloy whose thermal expansion coefficient is a close match to that of silicon. The headers axe heated to temperatures in the range of 400 to 420°C in an inert-gas atmosphere (N2 or a mixture of about 90% N2 and 10% H2). The chips are then bonded to the headers by means of the formation of a gold-silicon alloy that results in a good mechanical bond and a low-resistance electrical contact. This contact will be the substrate of the IC chip. The same process is used for discrete components, such as transistor. In that case the contact will be the collector of the transistor.
Lead Bonding and Encapsulation
Connecting the pads (metallised contact areas) to the terminal (that is, pins) of container with gold (or aluminium) wires is referred to as bonding. For this purpose small-diameter (20 to 40 micro meters) gold wires are used. Aluminium wire is used especially for high-current power devices, where large-diameter round or flat ribbon leads may be used. Take a look at the figure below.
The IC chip is now encapsulated in a metal, ceramic or plastic package. The plastic package is the lowest in cost, but the metal and ceramic package offer the advantage of providing a hermetic seal and a higher operating temperature range.
Sometimes chips are mounted on ceramic sheets without containers. IC prepared in this way is called IC modules. Each IC package is finally tested with its external terminals by feeding electric signals to its input pins and analyzing those at its output pins by a computer. This is a package test.
It should be noted that the fabrication process, discussed above, treats simultaneously a number of wafers, each of which has tens to several hundreds of chips. In 1985, the standard IC utilized 100 mm wafer such as that shown in the figure shown below. The current trend is of 15 cm wafer and 20 cm technology. The wafer thickness, about 0.2 to 0.3 mm, provides the chip with sufficient mechanical rigidity. Each wafer may have 100 to 8000 rectangular chips having side 1 to 10 mm. If we process, twenty-five (say) 10-cm wafers in a single batch production, 200,000 ICs are manufactured at a time. If the average component count per IC were only 800, a batch would contain more, than 100 million components. If the yield, that is, the percentage of fault free chips, is 10% (say) 20,000 good chips are mass produced in a single batch of production. This shows a significant advantage of IC technology.
A variety of IC packages are available. The most commonly used are dual in-line pack (DIP), flat pack, leadless chip carriers (LCC), and pin grid arrays (PGA). Some of them are discussed in the appendix. The PGA provides a very high pin count in a minimum of area.
It should be emphasized that when circuits are implemented on a single chip and encased into a single package, as above it is called a monolithic integrated circuit. When a package contains more than one chip or a mixture of chips and discrete components (one component per container, i.e., one transistor, per container, for example), all of which are put on a large substrate (such as ceramic substrate), it is called a hybrid integrated circuit.
IC packages are usually placed on a PC board. The pins of IC packages and the holes on the pc board are soldered. Then the pc boards are inserted into mother boards which are placed in cabinets along with backplanes.
Another approach to assemble IC chips is to place chips on a ceramic carrier and then the ceramic carriers on a ceramic mother carrier. The ceramic mother carriers are placed on a pc board. This approach has the advantage of smaller space than IC packages are assembled on a pc board and a lower cost than hybrid ICs.