Testing of solid state imagers takes up a substantial portion of the total time needed, on average, for making each imager in mass production. Vertical and horizontal modulation transfer functions are two vital parameters to be measured. Modulation transfer function (MTF) measures the relative responses of a camera system component, such as a solid state imager, to patterns of sinusoidal intensity variation of given spatial frequency as that spatial frequency increases. Response falls off at higher spatial frequencies in a solid state imager using charge coupled devices (CCDs) for transporting charge packets descriptive of image samples owing to inefficiencies in charge transfer. Charge transfer is inefficient to the degree it is incomplete.
RCA Corporation manufactures CCD imagers that have thinned substrates of semiconductive material. These imagers are mounted on glass backing plates and are back-illuminated, which is to say the light image impinges on the substrate surface cemented to a glass backing plate by an optically transparent cement. Vertical and horizontal MTF measurements are made while the imagers are on the wafer, before being separated into individual imagers by the dicing steps preparatory to packaging. These MTF measurements have been made by projecting a test image through the front surface of the substrate--i.e., through the surface on which the CCD gate electrode structures are disposed. These gate electrodes are thin polysilicon, so light will pass through them.
Filter plates with sinusoidal variations in optical transmission are difficult to make accurately, while filter plates with alternately transparent and opaque stripes are relatively simple to make. So, in practice, modulation transfer function is calculated from measurements of contrast transfer function (CTF). Refer to RCA Electro-Optics Handbook, Technical Series EOH-11, Section 8.3, "MTF (Modulation Transfer Function) and CTF (Contrast Transfer Function)", pp. 114-117, published by RCA Commercial Engineering, Harrison, NJ 07029, copyright 1974. CTF measures the relative responses of a camera system component to square wave patterns of given spatial frequency and orientation. The CTF measurements are converted to MTF specifications to facilitate calculating the overall MTF of a proposed system by multiplying the respective MTFs of the optical and electrical components which are to be in cascade arrangement in the system.
Owing to MTF losses in the optical system, the front-illumination test method described above can only give an appropriate measurement of modulation transfer function and may leave subtle variations undetected. In addition accurate alignment of the pattern with respect to the pixel positions is difficult, preventing accurate automatic testing.