CCD coatings

To make use of the properties of CCDs in wavebands for which silicon is not sensitive or to enhance performance at specific wavelengths, CCDs can be and have been coated with various materials.1 These coating materials allow CCDs to become sensitive to photons normally too blue to allow absorption by the silicon. They generally consist of organic phosphors that down-convert incident UV light into longer wavelength photons, easily detected by the CCD. These vacuum deposited coatings are often used with front-side illuminated, thick CCDs (although not always; the Hubble Space Telescope WF/PC I CCDs, which were thinned devices, were phosphor coated) and can be viewed as an inexpensive method of increasing blue response without the cost and complexity of thinning the device.

A coronene phosphor has been commonly used in recent years to convert photons shortward of 3900 A to photons at a wavelength near 5200 A. The use of such a coating causes a "notch" in the overall QE curve as the CCD QE is falling rapidly near 4000 A but coronene does not become responsive until 3900 A. Another common phosphor coating, lumogen, eliminates this QE notch, as it is responsive to wavelengths between 500 and 4200 A (see Figure 2.9). An interesting side note is that lumogen is the commercial phosphorescent dye used in yellow highlighting pens. The QE of a coated CCD may increase in the UV and shortward regions by amounts upwards of 15-20%. Transparent in the visible and near-IR regions, properly deposited coatings can also act as antireflection (AR) coatings for a CCD. More on the use of coated CCDs is contained in Chapter 7.

The main problem encountered when using coated CCDs is that the deposited material tends to evaporate off the CCD when under vacuum conditions (see Chapter 7). A less important problem is the loss of spatial resolution due to the finite emission cone of the light generated in the coating. The specifics of coating materials and the process used to coat CCDs are both beyond the scope of this book. Appendix A and Janesick & Elliott (1992), Lesser (1990), Schaeffer et al. (1990), and Schempp (1990) provide further readings in this area.

1 Non-optical operation of a CCD can also be accomplished in this manner. See Chapter 7.

2000 4000 6000 8000

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BIASED GATE CCD 307 - 6/23/88

BIASED GATE CCD 307 - 6/23/88

D—□ LUMOGEN AREA CORONENE AREA J_I_i_I_I_I_I_I_

D—□ LUMOGEN AREA CORONENE AREA J_I_i_I_I_I_I_I_

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Fig. 2.9. The top plot shows QE curves for a Hubble Space Telescope WF/PC prototype CCD before and after being coated with lumogen. Note the increased UV response of the coated CCD. The bottom plot shows the QE properties of a WF/PC prototype in the far-UV spectral region. Presented are two curves, one for a coronene coated CCD and one for a lumogen coated CCD. From Trauger (1990).

Current processing techniques have reached a state where a given CCD can be "tuned," via its make-up, resistivity, thickness, and operating temperature, to provide a desired response at a specific wavelength (Lesser, 2004). Basically, one can build a pixel structure, QE, and well depth to suit a specific need. Much of the ability to tune CCDs is limited to the red end of the optical spectrum. These topics will be discussed in the next chapter.

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