Xử lý bề mặt - đó là viiệc đầu tiên người làm công nghệ cần thực hiện trong phòng sạch. Công đoạn làm sạch bề mặt phiến (silicon) thường được thực hiện nhờ các axit mạnh, các chất có tính ôxi hoá như HNO3, H2SO4 H2O2 và HF. | The sheet resistance of a thin layer of thickness z constructed from a material that has a resistivity p is given by - z The resistance of thin rectangular regions of length L and width w on an integrated circuit can be readily obtained from the sheet resistance by counting the number of square blocks of length w that can be placed in the rectangular region. If N blocks can be placed adjacently in the region then the resistance in terms of the sheet resistance is given by R An example illustrating this technique is depicted in Fig. Occasionally it is necessary to determine the resistance of nonrectangular regions such as that shown in Fig. d or the serpentined pattern of Problem . The problem of determining resistance of irregular regions is difficult but for rectangular regions containing the right angles shown the rule of thumb of adding squares for each comer is often used. It is often necessary to specify the temperature characteristics of resistors and capacitors. The Temperature Coefficient of Resistance TCR and Temperature Coefficient of Capacitance TCC are typically used for this purpose. These temperature coefficients which are generally expressed in terms of ppm CT are defined by TC 106ppm C x dT where JC is the temperature-depend ent value of either the resistor or the capacitor. If the temperature coefficient is independent of temperature then the value of the component at a temperature T can be obtained from its value at temperature T by the expression x r2 x T1kITO7 - r l i which is often closely approximated by x T2 - x T l T - nXTC lO6 If the TC is a function of temperature then the previous expressions are good only in local neighborhoods of 71 . The value of TC which can be either positive or negative is determined by the material properties and is often quite small. The absolute values of TC are often less than 1000 ppm c. Unfortunately since integrated circuits are often expected to operate over
