Photolithographic techniques are often used to pattern features of miniature devices.  In a typical application of the photolithographic technique, a layer of photoresist is deposited on a substrate or other device layer and then exposed to radiation of appropriate wavelength through a patterning mask.  Certain regions of the photoresist layer are exposed, and others are not exposed, according to the pattern defined in the mask.  Exposing the photoresist to radiation changes its solubility.  After exposure, solvent is used to remove regions of higher solubility photoresist, leaving regions of "hardened" photoresist at sites on the device layer as dictated by the patterning mask. 
The hardened photoresist protects the directly underlying device layer while the unprotected regions of the device layer surrounding such protected regions are removed by etching or some other suitable method.  After etching the device layer, the hardened photoresist is removed.  In this manner, a feature is created in the device based on the pattern defined in the mask. 
It is clear that the photoresist layer must be accurately patterned to form features to exacting specifications in a device.  It is desirable, therefore, to monitor the photolithographic process at various stages and on a periodic basis. 
For example, it may be desirable to measure the thickness of photoresist prior to exposure and patterning.  This may be done by subjecting the photoresist to ultraviolet light having a wavelength in the range of 300 to 800 nanometers (nm) and measuring the reflected radiation.  The reflected radiation may be correlated to photoresist thickness.  The general principle of this measurement technique is that the measured light reflected from a substrate is modulated by constructive and destructive optical interference from an overlying semi-transparent material such as photoresist.  See Chopra, K. L., Thin Film Phenomena, p. 99 (McGraw Hill, 1969).  The periodicity of the reflectance spectra can also be used to determine optical properties, such as refractive index. 
Furthermore, it is useful to take measurements after patterning the photoresist.  For example, in a typical application, a plurality of stripes of photoresist may be patterned on the surface of a substrate.  The stripes are spaced from one another so that there are alternating regions of substrate covered by hardened photoresist and regions that are not covered by hardened photoresist.  It may be desirable, for example, to measure the width of the stripes of photoresist.  Such measurements have been typically performed by scanning electron microscopy (SEM). 
The aforementioned arrangement of spaced stripes is usually referred to as a line and space pattern.  Such micro-size line and space patterns are useful for forming diffractive elements such as lenses or gratings in semiconductors or glass, forming fluid flow microchannels in silicon or, more generally, for providing a variety of mechanical features in a substrate. 
More recently, attempts have been made to measure patterns using scatterometry.  In this technique, a pattern is subjected to light, such as from a laser, typically having a single wavelength.  The light is usually directed toward the pattern at some angle to the normal.  The light reflected from the pattern is reflected at various orders, i.e., angles relative to the incident light.  The amount or intensity of light reflected at the various orders is measured.  It may be possible to use such data to obtain quantitative information about the pattern, such as the line width of a line and space pattern.  However, scatterometry is very sensitive to changes in the profile of the pattern, i.e., the height of lines, and requires relatively sophisticated correlation work to relate the reflected radiation to features of a pattern. 
Other examples of characterization methods pertaining to photolithography, and equipment suitable for accomplishing such methods, are described in U.S. Pat.  Nos.  5,363,171, 5,184,021, 4866,782 and 4,757,207.  These patents, and all others mentioned in this specification, are incorporated herein by reference in their entirety. 
Thus, there is a need for a method and arrangement for characterizing features of a micro-size patterned layer, such as a patterned photoresist, using reflected radiation.