Photolithography is used to produce windows in the oxide layer of the silicon wafer, through which diffusion can take place. For this purpose photomask is required. In this section we shall discuss various techniques of mask fabrication. The pattern appearing on the mask is required to be transferred to the wafer. For this purpose various exposure techniques are employed. We will also discuss these techniques.
Mask Making
IC fabrication is done by the batch processing, where many copies of the same circuit are fabricated on a single wafer and many wafers are fabricated at the same time. The number of wafers processed at one time is called the lot size and many vary between 20 to 200 wafers. Since each IC chip is square and the wafer is circular, the number of chips per wafer is the number of complete squares of a given size that can fit inside a circle.
The pattern for the mask is designed from the circuit layout. Many years ago, bread boarding of the circuit was typical. In this, the circuit was actually built and tested with discrete components before its integration. At present, however, when LSI and VLSI circuits contain from a thousand to several hundred thousand components, and switching speeds are of such high order where propagation delay time between devices is significant, bread boarding is obviously not practical. Present-day mask layout is done with the help of computer.
The photographic mask determines the location of all windows in the oxide layer, and hence areas over which a particular diffusion step is effective. Each complete mask consists of a photographic plate on which each window is represented by an opaque are, the remainder being transparent. Each complete mask will not only include all the windows for the production of one stage of a particular IC, but in addition, all similar areas for all such circuits on the entire silicon as shown in the figure below.
It will be obvious that a different mask is required for each stage in the production of an array of IC’s on a wafer. There is also a vital requirement for precise registration between one mask and the other in series, to ensure that there is no overlap between components, and that each section of a particular transistor is formed in precisely the correct location.
To make a mask for one of the production stages, a master is first prepared which is an exact replica of that portion of the final mask associated with one individual integrated circuit, but which is 250x [say] enlargement of the final size of IC. The figure below shows a possible master for the production of a mask to define a particular layer of diffusion for a hypothetical circuit. Art work at enlarge size avoids large tolerance errors. Large size also permits the art work to be dealt easily by human operator. In the design of the art work, the locations of all components that is, resistor, capacitor, diode, transistor and so on, are determined on the surface of the chip. Therefore, six or more layout drawings are required. Each drawing shows the position of Windows that are required for a particular step of the fabrication. For complex circuit the layout is generated by the use of computer-aided graphics.
Photomask Fabrication
The master, typically of order 1 m x 1 m, is prepared from cut and strip plastic material which consists of two plastic films, one photographically opaque called Rubilith and the other transparent [mylar], which are laminated together. The outline of the pattern required is cut in the red coating of Rubilith (which is opaque) using a machine controlled cutter on an illuminated drafting table. The opaque film is then peeled off to reveal transparent areas, each representing a window region in die final mask.
The next step is to photograph the master using back illumination, to produce a 25 x reduced sub-master plate. This plate is used in a step and repeat camera which serves the dual purpose of reducing the pattern by a further 10 x to finished size and is also capable of being stepped mechanically to produce an array of identical patterns on the final master mask, each member of the many corresponding to one complete IC. Instead of the photographic plate being transported mechanically in discrete steps, better accuracy may be achieved by using continuous plate movement; discrete exposures then being made by an electronically synchronized flash lamp which effectively freezes the motion.
The entire sequence just described can be done with plates containing a photosensitive emulsion; typically the emulsion is considered too vulnerable to abrasion and tears. For this reason, masks are often made of harder materials such as chrome or iron oxide.
For very complex circuits automated mask generation equipment is used. In this, a computer controlled light flashes to build up the pattern on a photographic film by a series of line or block exposures, the resulting film is then reduced and handled in a step and repeat system to create the production mask. Alternatively, the master mask can be generated by an electron beam exposure system, again controlled by computer.