The stealth dicing of precision optical devices is essential in many scientific disciplines. Because the optical fibers transmit very little information, the motion of particles must be nearly zero while they are in motion. The particles must remain intact and invisible to the human eye. To achieve this goal, the precision dicer must have the ability to detect and record the smallest of changes in optical parameters. Such changes are then used for purposes such as testing the sensitivity of medical imaging systems, such as computed tomography (CT) scanners, magnetic resonance imaging (MRI) scanners and ultrasound scanners.
Dicing tape, the original optical tape used in scientific research, is applied to the inside of a wafer using a modified layer of tape that is thin enough to allow light to pass through but thick enough to prevent the penetration of surface film. When blade imaging ( Mechanical dicing ) is the desired method, the top surface of the modified tape is placed upon a glass plate that is heated to produce a beam of light. The beam of light then passes through the top surface of the tape, which is illuminated by a small but powerful laser. The light, however, is not visible to anyone sitting nearby because it is absorbed by the transparent sheet on which the image is being displayed.
Figuring out how to apply stealth dicing to an image display system thus became the focus of researchers such as John van de Ruit, Robert J. Trakman and Frank J. Furter. They realized that if the light was transmitted through the transparent film and focused on the image area, the beam would be visible. A variation on this theme is known as the optical tomograph, where the light is focused on a thin glass plate and the image on that plate is viewed through a very small eyepiece. Because the eyepieces in optical tomographs are so small, the phenomenon of stealth dicing is enhanced.
The key to success in stealth dicing is to make the pulses appear to be random. To achieve this goal, the pulses must have a periodic distribution with no discernable pattern or structure. In the early days of concealment device design, this was accomplished by placing the pulse energy at the stress points. The energy was spread over the surface of the wafer so that the stresses in the wafer would knock the stresses out of alignment. As the pulses moved through the wafer, this would cause the stresses to get out of alignment and spread out, which would cause the image to appear misaligned.
Another way to reduce or eliminate the apparent effects of stealth dicing is to alter the formation of the wafer. In its dry process, a thin metal layer imparts a certain amount of mechanical strength to the wafer. This mechanical strength, coupled with impregnation, causes the metal to bend, elongate, or even spring back into shape. If the shape change is subtle, the damage is not easily noticed. However, if the subtle bending is significant, the damage can be minimized by using a substrate that does not impart mechanical bending to the material.
The third way to prevent stealth dicing is to control the number of passes over the surface of the device during the manufacturing process. When using conventional curing methods, each successive pass causes a small amount of vaporization. The vapor pressure generated also causes a small amount of heat transfer, increasing the internal friction and decreasing the potential energy transfer from laser beams. This decrease in transfer potential energy allows for fewer passes over the surface area of the object.
The fourth way to avoid stealth dicing is to introduce silicon during the manufacturing process. Silicon is very insoluble in the solid state, but soluble in an liquid solution. In addition, the amount of silicon introduced to the final product can vary greatly depending on factors such as thickness, type of media used, and pass rates. It is important to note that although applying silicon during the process will most likely increase the manufacturing cost of the product, it will reduce the level of surface damage.
The fifth way to avoid stealth dicing is to use a cold dip or hot dip galvanic process. This method minimizes dicing due to contact between the object and the dielectric. This method is effective for high-impact situations, and is not affected by thermal conductivity. Although the overall cost of this method is much higher than conventional CMM, it has the potential to produce a level of performance comparable in price to CMM. Finally, this method has the ability to increase production efficiency since it allows for very little variations in the pass rates between passes.