Prior art methods for performing the prior-to-bonding alignment of the chips which will make up the flip-chip are adequate for chips that only contain electronic circuits, because it is necessary to align the electrical contact pads of each chip with merely sufficient accuracy that an electrical connection can be made. One prior art alignment method involves inserting a visible wavelength sensitive bi-directional video camera between the two chips to be bonded, the bi-directional video camera being a video camera which can look up and down simultaneously through the use of appropriately placed prisms, and aligning opaque-to-visible-light fiducials in the chips to be bonded as seen by the bi-directional video camera. Another prior art alignment method takes advantage of the fact that silicon is transparent to infrared light. This method uses video camera that can see infrared light to look though both chips and align fiducials which are specially made on the chips to be opaque to infrared light while at least their surrounding area is transparent to infrared light.
Neither of the foregoing prior art methods can achieve the high precision required for making flip-chips that contain optical devices such as micro-electromechanical systems (MEMS) devices. The precision required for a MEMS flip-chip is considerably higher than for an flip-chip that contains only electrical devices because the alignment tolerance required for a MEMS flip-chip is much less than the size of an electrical contact pad. The former prior art method is unable to achieve such high precision because the chips can only be aligned when they are far apart enough to insert the bi-directional, sensitive-to-visible-light, video camera, and thereafter, as the chips are then moved toward each other for bonding they may become misaligned again. The latter prior art method is limited by the resolution of the infrared video camera, which is on the order of the alignment precision desired, e.g., several microns. Also, it restricts the types of materials that can be employed to those which ensure that the fiducials can be seen using infrared light.