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DEMODULATION PIXELS IN CMOS/CCD 101
This chapter is divided into three main sections: In Section 5.1 we introduce
different realizations of demodulation pixels. They have all been realized in Orbit
2 µm CMOS/CCD technology, a process offered as MPW. This process has two
different polysilicon layers (poly1 and poly2) that can both be used as transistor
gates. Additionally the design rules allow these poly gates to overlap over the thin
gate oxide. Hence it is possible to realize CCDs with this CMOS/CCD process (c.f.
Section 3.2). The realized demodulation pixels vary in operational architecture,
maximum demodulation frequency and fill factor, the latter being a very important
parameter for realizing highly sensitive devices. All pixels fulfill the four physical
demodulation demands described above. The characteristic features of the pixels
introduced are summarized in Table 5.1 and Figure 5.8.
Section 5.2 then summarizes the results of a detailed characterization of the
demodulation performance for the 1-tap pixel, which is the pixel-realization with the
best electrooptical properties (demodulation bandwidth and optical fill factor). This
pixel has been fabricated twice; (1) with surface channel (SCCD) CCD structures
and (2) with buried channel (BCCD) structures, allowing a direct comparison of both
CCD types. Especially we focus on the efficiency of the shutter mechanism
(sampling efficiency or demodulation contrast) and its dependency on sampling
frequency and wavelength. Also, we present the measured phase accuracy as a
function of the optical power received in the pixel and compare these results to the
theory presented in Section 4.2. The 1-tap pixel is the basis for two lock-in sensor
arrays we realized, a line-sensor with 108 pixels and an image sensor with 64x25
pixels. Both arrays are implemented as range cameras, which are introduced in
Chapter 6.
Motivated by the outstanding results presented in Section 5.2, we present a very
simple and only slightly modified pixel structure in Section 5.3 that overcomes the
restrictions of the 1-tap pixel (serial integration of the single sampling points) by
offering two optically identical photosites.