The phase shift effect in Alternating Phase Shift Masks (AAPSMs) and chrome-less phase shift masks is created by etching trenches directly into the quartz substrate. Since the phase shift is critically dependent on the etch depth, the quartz etch process must be tightly controlled. In the absence of an etch stop for the process, an integrated metrology solution is desirable on the mask tech tool. Traditional methods for measuring etch depth or phase shift, such as interferometry, profilometry, AFM, and SEM, are expensive, slow, and/or destructive. In addition, traditional methods cannot measure quartz etch depth without removing the resist and in some cases the chrome mask, making them unsuitable for integration into the etch process. This paper will present measurements of trench depth and phase shift on quartz phase shift mask using the n&k Analyzer 1512-RT. The n&k Analyzer measures reflectance (R) and transmittance (T) from 190-1000nm, which is analyzed according to the Forouhi-Bloomer dispersion relations to simultaneously determine n, k, film thicknesses, trench depth, and phase shift. The measurement is non-destructive and fast, typically taking 2-3 seconds per measurement point. No special test structures are required for the measurement. In addition, the n&k Analyzer can measure quartz etch depth with the chrome mask, ARC layers, and resist still intact. The n&k Analyzer measurements show good correlation with 193nm interferometer measurements, and good repeatability. The small footprint, ease of use, measurement speed, and the ability to measure quartz depth in the presence of chrome and resist make the n&k Analyzer an ideal candidate for integrated metrology applications on mask etch tools for advanced proces control (APC). The Applied Materials' Tetra II phootmask etch system has the unique capability to accommodate integrated metrology modules through the factory interface. Applications of APC with integrated phase shift measurements will be discussed.
Accurate determination of endpoint is important for creating a repeatable process that maximizes sidewall profile angle and resist selectivity while maintaining a low etch bias. An Applied Materials EyeD (TM) spectrometer on the Tetra(TM) II photomask etch system is used to examine several endpoint methods to maximize flexibility and productivity. These methods include: slope changes to a single line, slope changes via a ratio of product and etchant species and slope changes of a linear combination of all slope changes. Endpoint identification is typically performed with a single spectral line. In addition, a method using neural networks, or principal component analysis (PCA) has also been created in order to fully optimize and characterize exact endpoint definition. Comparison between these methods will be discussed.
Asymmetrically loaded patterns have been used to develop and optimize the chrome etch process on the TetraO II, the next -generation tool offered by Etec Systems. These asymmetrically loaded patterns offer unique challenges to the dry etch process by concentrating much of the chrome load in one section of the mask (usually one quadrant) while leaving the rest of the mask uniformly loaded. Numerical analysis of both the final chrome and the point-by-point etch
contribution has been implemented to allow accurate interpretation of etch results.