Integrated circuit fabrication is typically accomplished by forming many different layers on a substrate. 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, InP, 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. Because the design tolerances of an integrated circuit are so strict, it is desirable to monitor the properties, such as thickness and elemental composition, of the various layers as they are formed. One way to measure the properties of film layers is to use a technique called electron stimulated x-ray metrology.
In very general terms, electron stimulated x-ray metrology works by directing a beam of electrons through a low pressure environment toward a sample surface. The electrons excite the atoms of the sample as they impinge against it. The excited atoms produce x-rays having properties that are characteristic of the properties of the sample, such as layer composition and layer thickness. Electron stimulated x-ray metrology is a highly favored technique, because in principle, it can be performed on production integrated circuits without damaging them.
Unfortunately, there are some problems with the technique. For example, deterioration of the x-ray signal over time, due to atom ejection, generally referred to as trending herein, requires that relatively short electron beam exposure times be used in order to get a reading that is closest to the initial value. However, a short acquisition time tends to result in a relatively large error bar spread. Thus, while repeated measurements at different points tend to result in a reduced precision, trending causes repeated measurements at the same point to result in a precision that is even further reduced.
What is needed, therefore, is a system that overcomes problems such as those described above, at least in part.