Quantum Efficiency Model and Fitting Results

In this section we present the best-estimate absolute quantum efficiency, as of this writing, for each flight device. In each case the absolute quantum efficiency has been derived from measurements of the relative quantum efficiency of the flight devices, with respect to reference detectors, described above in section 4.7.2, together with the absolute quantum efficiency models of the reference detectors described in section 4.6.1.

We are confident that we understand the accuracy of quantum efficiency models for the front-illuminated (FI) detectors, and we discuss this subject at some length here. On the other hand, the models we present for back-illuminated (BI) devices are not particularly accurate, and, perhaps worse, the errors in the back-illuminated models are not well characterized. Thus the BI models given here must be regarded as interim products. Fortunately, we have very recently identified the origin of at least a large fraction of the fairly large, (i.e. 10-20%) systematic errors in our current analysis of BI CCD data. In short, we now believe it is essential to use accurate models of both source spectrum and detector spectral redistribution to obtain reliable quantum efficiency measurements for these devices. This more accurate analysis has not been completed at this writing, but we are confident that it can be done. See section 4.7.3 for a brief quantitative discussion of these errors.

We have used the so-called ``slab and stop'' model of the ACIS CCD gate structure, described in section 4.1, to model detector quantum efficiencies. For clarity, the various slab-and-stop model parameters used in the model of FI detector quantum efficiency are represented in Table 4.62. Note that only four model parameters were varied in fits to measurements made with the flight detectors. The remaining three parameters were constrained by mesh measurements (see section 4.5) or by scanning electron micrograph (SEM) measurements of siblings of the flight devices.

 

Table 4.62: Model Parameter Determination for Absolute Quantum Efficiency of Front-illuminated Flight Devices

Parameter	Determined from:	See Section:
Poly-silicon gate thickness	Fit to flight-device relative QE data	4.7.2
SiO2 insulator thickness	Fit to flight-device relative QE data	4.7.2
Si3N4 insulator thickness	Fit to flight-device relative QE data	4.7.2
Depletion depth	Fit to flight-device branching ratio data at 5.9 keV	4.6.2
Channel Stop width	Mesh and SEM measurements$^{\ddag }$ on sibling devices	4.5
Channel Stop SiO2 thickness	Mesh and SEM measurements on sibling devices	4.5
Channel Stop Si P+ thickness	Mesh measurements on sibling devices	4.5
Note: The slab-and-stop model is described in detail in section 4.1