During integrated circuit fabrication processes, the integrated circuits typically receive a variety of different surface inspections and measurements, such as optical inspections and measurement. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices.
The term “tool” as used herein generally refers to inspection or measurement systems used in the integrated circuit fabrication industry. The term “investigation” as used herein generally refers to the process of inspection or measurement as used in the integrated circuit fabrication industry. As used herein, the term “substrate” refers to the substrates on which the integrated circuits are fabricated, the masks or reticles from which the patterns used to form the integrated circuits are transferred, and other types of substrates as used in the integrated circuit fabrication industry.
Current methods of investigation typically reference the backside of the substrate to a chuck while inspecting the front side of the substrate. In other words, the backside of the substrate is placed on the surface of a chuck, which is then brought into some kind of alignment with the operative head of the tool. The tool is most frequently designed to investigate the top surface of the substrate. Assumptions are made in the operation of the tool, such as that the chuck is flat and moves in a level manner, that the substrate is of a known and uniform thickness, and other such. These assumptions are used to align the head of the tool to the top surface of the substrate, when the position of the chuck is known.
This method is vulnerable to inconsistencies in the flatness of the chuck, variances in substrate geometry that affect the height of the substrate, and other problems that make the assumptions invalid. For example, if the substrate thickness varies from what is assumed, then the distance between the head and the top surface of the substrate will vary across the substrate—unbeknownst to the tool. Similarly, if the chuck height varies from what is assumed, then the distance between the head and the top surface of the substrate will vary across the chuck—unbeknownst to the tool.
This situation is typically resolved by using an active focusing mechanism to compensate for height changes as the substrate is moved with respect to the tool head. A typical auto-focus mechanism, including a control system, can cost thousands of dollars to implement, and many times that to engineer, especially when considering software development costs. History has proven that these are problematic mechanisms when implemented with the accuracy and repeatability that are expected by the customer. As customers demand faster through-put, the auto-focus mechanism needs to respond faster as well. Because they are mechanical in nature, these mechanisms have limited response times, which often are not sufficient to meet the through-put demanded. Some inspection systems have such large optical elements that moving them to track height variations at a high speed is just not a realistic option.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.