Người ta phủ lên trên bề mặt phiến silicon có tính chất nhậy sáng đặc biệt gọi là chất cản quang (photoresist) – thường được gọi là chất cảm quang. Chất cảm quang phải bảo đảm hai tính chất: - Nhậy quang; - Bền vững trong các dung môi axít hoặc kiềm. | I w _ T T 3 10 L Although the W L ratio is constrained the values for w and L remain to be specified within the design rules of the process. It is a good practice to make the resistors large if the area is available to minimize edge roughness effects increase power dissipation capability and make trimming easier. Serpentined patterns such as shown in Problem should also be avoided with thick film technology. This is due to the increased current density that will result from current crowding at the inner comers of serpentined structures - A capacitor is constructed by screening a conductive layer followed by a dielectric followed by another conductor. The dielectric is generally applied in two coats to minimize pinholes which would short the capacitor plates together. With the two layers of dielectric the chances of a pinhole coincident with both layers are greatly reduced. Since a thick film capacitor is actually a parallel plate capacitor the capacitance is given by C eRe0 where pF m. A is the area of the capacitor plates t is the dielectric thickness and eR is the relative dielectric constant. Inks with relative dielectric constants from 10 to 1000 are available. The high dielectric constants offer a reasonable capacitance density at the expense of large and nonlinear temperature coefficients. The lower dielectric materials offer improved performance but are restricted to applications requiring small capacitors due to a low capacitance density. As in the MOS and bipolar processes the upper plate of thick film capacitors is typically a little smaller than the lower to minimize capacitance changes caused by minor misalignments. The two conductive layers used for the capacitor plates also serve as interconnects. If crossovers of two conductors are required the dielectric layer can be used as an insulator at the expense of creating a small parasitic capacitor at the crossover. The screen geometries along with a cross-sectional view of a .
