Low dark current, back-illuminated charge coupled devices

Abstract

Abstract:Dark current for back-illuminated (BI) charge-coupled-device (CCD) imagers at Lincoln Laboratory has historically been higher than for front-illuminated (FI) detectors. This is presumably due to high concentrations of unpassivated dangling bonds at or near the thinned back surface caused by wafer thinning, inadequate passivation and low quality native oxide growth. The high dark current has meant that the CCDs must be substantially cooled to be comparable to FI devices. The dark current comprises three components: frontside surface-state, bulk, and back surface. We have developed a backside passivation process that significantly reduces the dark current of BI CCDs. The BI imagers are passivated using molecular beam epitaxy (MBE) to grow a thin heavily boron-doped layer, followed by an annealing step in hydrogen. The frontside surface state component can be suppressed using surface inversion, where clock dithering reduces the frontside dark current below the bulk. This work uses surface inversion, clock dithering and comparison between FI and BI imagers as tools to determine the dark current from each of the components. MBE passivated devices, when used with clock dithering, have dark current reduced by a factor of one hundred relative to ion-implant/laser annealed devices, with measured values as low as 10-14 pA/cm 2 at 20°C.

Testing and characterization of the TESS CCDs

“…The CCID-80s have sufficiently low enough dark current that they can image at room temperature without saturating. Dark room temperature images consistently show a "brickwall" pattern (see Figure 4), which was a result of the ion implant/laser annealing process done to passivate each surface of the CCDs and reduce the dark current (Westhoff et al, 2009). The pattern follows the laser treatment as it was stepped across the surface of the CCD.…”

The Noise Performance of Electron-Multiplying Charge-Coupled Devices at Soft X-Ray Energy Values

“…In this experiment, a substantial increase in the dark current over the course of the experiment would lead to an increase in the noise on the system and could affect the result; therefore, the effect must be considered. If a device is run in inverted mode, the increase in dark current can be suppressed, but in this experiment the device was run non-inverted [17].…”

Development of CCDs for REXIS on OSIRIS-REx

“…After the MBE, an H 2 sinter is done to passivate the interface states at the silicon/oxide interface of the CCD. More details of the MBE process can be found in [6] [7].…”

“…While many methods exist to passivate the back surface [6], MBE is an attractive process because of its controllability and ability to make ultra-shallow junctions on the order of a hundred atomic layers. MBE deposits an epitaxial silicon layer at rates of one-tenth atomic layer per second.…”