K3a4. vacuum deposition - is much like the vacuum metalizing process described in section 8F3. The material is evaporated in a vacuum, often by electron beam, and the vapor condenses as a film on the wafer surface. The process is also referred to as evaporation . It is used for the deposition of thin film metallic conductors. Aluminum is the most frequently used metal. Gold can also be deposited with this method. Fixturing that moves the wafer during evaporation is commonly utilized, to ensure uniform metal deposition on the wafer. The process is used for integrated circuits having broader wiring paths. It is also used to deposit gold on the back sides of wafers to facilitate adhesion of the chips in packages.

K3a5. sputtering - Sputtering is another vacuum method and is used for metals, alloys, semiconductor materials, and dielectrics, including glass. High-melting-point metals, such as tungsten is deposited by sputtering which is also known as Physical vapor deposition (PVD). The operation is performed in a vacuum, and is described in section 8F3a and illustrated schematically in Fig. 8F3a. The deposition material is taken from a wide source and therefore covers steps in the substrate surface. Adherence of the film to the substrate is superior to that achieved with vacuum deposition.

K3a6. adding thick films - involves the printing and then firing of a coating on a substrate material. The film material may provide conductance, resistance, or a dielectric on wafer surfaces, hybrid circuits, or multichip substrate materials. The materials can be ceramic, glass, quartz, sapphire, or metal coated with porcelain enamel. Film thicknesses are typically 0.0005 to 0.0015 in (13 to 38 microns). 9 Resistors, capacitors and inductors can be formed on wafers or other substrates of dielectric materials. These components are used, along with other electronic devices, particularly integrated circuits, in hybrid microcircuits. 1 The thick films are also used in the fabrication of multichip modules, resistors, potentiometers, magnetic devices, circuit protection devices, electroluminescent devices and membrane switches. Thick films are normally in paste form and contain three basic ingredients: a functional material (resistor, conductor, dielectric), a binder (glass powder), and a vehicle (solvents, plasticizers, etc.) The paste is applied by screen or stencil printing . After printing, the paste is allowed to settle for 5 to 15 minutes at room temperature and is then oven dried at 210 to 300 ° F (100 to 150 ° C) for 10 to 15 min. Firing temperatures are typically 930 to 1850 ° F (500 to 1000 ° C). 1 Various layers are added, depending on the function involved and circuit devices needed. Hybrid circuits may have a mixture of thick and thin film layers.

Thick film capacitors 9 are made in one of two ways: The first is by printing conductive material to form the base electrode and its termination, then depositing a dielectric material and firing it, and then depositing conductive material for the other electrode and its connection. The dielectric film, commonly barium titanate or titanium dioxide in a vitreous mixture, is fired at about 1560 ° F (850 ° C). The second method is to print the metal electrodes on opposite sides of the ceramic substrate. In both methods, capacitance may be adjusted to more precise values, if needed, by laser trimming the electrode material. When trimming is required, the amount of material printed provides slightly more capacitance than needed and the trimming of appendages or small parallel trimming capacitors reduces the capacitance of the unit.

Thick film resistors 9 are made by first printing the resistor terminations on the substrate with a conductive ink (usually a metallic paste). The substrate, with terminations, is then fired at 1470 to 1700 ° F (800 to 930 ° C). Then the resistance material, also in paste form, is printed on the substrate and dried at about 300 ° F (150 ° C). If a resistor network is involved, several different resistance materials are normally used in the network. After drying, the device is fired again at about 1560 ° F (850 ° C) if made from mixtures of precious metals, metal oxides, and glass binder, or at up to 600 ° F (315 ° C) if carbon. The finished resistors may be trimmed by laser to provide more precise resistance values. When trimming is needed, the amount of resistance material printed is slightly more than needed and the trimming reduces the width of the material, increasing its resistance.

Thick film inductors 9 are made with thick films by printing a spiral pattern of conductive inks on the substrate. Because of size limitations on circuit devices, this method of inductor making is limited to circuits operating at high frequencies, 10MHz or higher.

K3a7. adding protective layers - Layers of silicon dioxide or silicon nitride are added to provide insulation between devices on the integrated surface and to provide protection to the existing layers. Protection may be needed from chemical action or handling damage. The insulating layers are grown by thermal oxidation (See K3a1.), nitridation (K3a2), or chemical vapor deposition. Adding oxide or nitride layers for protection after the chip is fabricated is called passivation . It provides protection for the chip during testing, packaging and use.

K3b. patterning - is the series of operations that incorporates the circuit layout from a photomask or reticle into the surfaces of the wafer. The purpose of the process is to provide the correct locations and spaces for fabricating circuit devices and the necessary wiring paths to connect them.

K3b1. lithography - is a means for etching patterns in integrated circuit surfaces corresponding to the elements of the integrated circuit. The size and location of the elements is measured in microns. Photolithography uses photographic techniques and ultraviolet light to provide such small size and precise positioning. One key element of photolithography is the use of a photoresist. Electron-beam lithography and X-ray lithography can provide even more precise positioning but have some process disadvantages. The resist film has two basic vital properties: 1) It changes its solubility (becoming either less soluble or more soluble, depending on its material) when exposed to light or other radiation and, 2) It resists the attack of an etchant that will remove substrate material. The steps of photolithography are illustrated in Fig. 13K3b1 and are as follows: 1) Preparation of the wafer substrate with an oxide layer. 2) Resist application - A thin film of photoresist material is applied to the substrate that is to be processed, 3) Exposure - Optical light, ultraviolet light, or other radiation is projected through a transparent mask plate. (The mask plate, made of glass or other transparent material, already has the circuit pattern printed on it.) Electron beam radiation produces the finest resolution, but is very slow. 11 The areas of the substrate that received the radiation are either softened (positive resist) or hardened (negative resist), depending on the material involved. The image on the substrate is reduced in size considerably from that on the mask using special optical reducing lenses. Circuit path widths of less than 100 nanometers (0.1 micron) have been achieved. The exposure machine exposes only a small portion of the wafer and then steps to the next position and exposes again. With each exposure, the mask’s circuit pattern is duplicated on the wafer surface. The process is repeated until the entire wafer surface is exposed. 4) Development - The substrate is washed with a solvent that dissolves the softer photoresist material. (With a positive resist , the softer material is that which was exposed to the radiation; with a negative resist , it is the material that did not receive the radiation.) 5) Etching - An etchant is applied to the substrate. Part of the substrate is removed from those areas not protected by the photoresist; the covered areas are unaffected. 6) Resist removal - The resist material is removed, leaving the substrate with a surface etched with the pattern that existed on the mask plate.

Fig. 13K3b1 The sequence of the lithography process: 1) wafer with a film layer is made ready for the operation, 2) resist is applied to the surface, 3) resist is subjected to radiation of ultraviolet light, x-rays or electron beams, through a mask, 4) the more soluble portion of the resist is removed. With a positive resist, the exposed portion becomes more soluble; with a negative resist, the exposed portion is made less soluble. 5) the layer underlying the resist is etched away, 6) the resist layer is then removed.