A backside illuminated image sensor includes a substrate, a backside passivation layer disposed on backside of the substrate, and a transparent conductive layer disposed on the backside passivation layer.

Backside Illuminated Image Sensor

Low-temperature silicon molecular beam epitaxy is used to grow a delta-doped silicon layer on a fully processed charge-coupled device (CCD). The measured quantum efficiency of the delta-doped backside-thinned CCD is in agreement with the reflection limit for light incident on the back surface in the spectral range of 260-600 nm. The 2.5 nm silicon layer, grown at 450 C, contained a boron delta-layer with surface density of about 2 x 10 exp 14/sq cm. Passivation of the surface was done by steam oxidation of a nominally undoped 1.5 nm Si cap layer. The UV quantum efficiency was found to be uniform and stable with respect to thermal cycling and illumination conditions.

Growth of a delta-doped silicon layer by molecular beam epitaxy on a charge-coupled device for reflection-limited ultraviolet quantum efficiency

We have developed low noise, high quantum efficiency photodiode arrays for use with positron-emission tomography (PET). A fabrication process developed for high-energy physics detectors was modified to allow for back-side illumination. A back-side contact consisting of a thin (10 nm) n/sup +/ polysilicon layer covered by an indium tin oxide (ITO) antireflection coating (57 nm) results in >70% quantum efficiency over the wavelength range of 400-1000 nm. The photodiodes are operated fully depleted (300 /spl mu/m thick) resulting in a measured capacitance of 3.2 pF and typical leakage currents of 20-50 pA for a 3 mm square element. At room temperature the noise measured at a shaping time of 4 /spl mu/s is 140 e/sup -/ rms. When coupled to a CsI(TI) scintillator and excited with 141 keV gamma rays, the energy resolution is 12% fwhm.

Development of low noise, back-side illuminated silicon photodiode arrays

We have used Molecular Beam Epitaxy (MBE)-based delta doping technology to demonstrate near 100% internal quantum efficiency (QE) on silicon electron-multiplied Charge Coupled Devices (EMCCDs) for single photon counting detection applications. Furthermore, we have used precision techniques for depositing antireflection (AR) coatings by employing Atomic Layer Deposition (ALD) and demonstrated over 50% external QE in the far and near-ultraviolet in megapixel arrays. We have demonstrated that other device parameters such as dark current are unchanged after these processes. In this paper, we report on these results and briefly discuss the techniques and processes employed.

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Silicon Detector Arrays with Absolute Quantum Efficiency over 50% in the Far Ultraviolet for Single Photon Counting Applications

We have used molecular beam epitaxy (MBE) based delta-doping technology to demonstrate nearly 100% internal quantum efficiency (QE) on silicon electron-multiplied charge-coupled devices (EMCCDs) for single photon counting detection applications. We used atomic layer deposition (ALD) for antireflection (AR) coatings and achieved atomic-scale control over the interfaces and thin film materials parameters. By combining the precision control of MBE and ALD, we have demonstrated more than 50% external QE in the far and near ultraviolet in megapixel arrays. We have demonstrated that other important device performance parameters such as dark current are unchanged after these processes. In this paper, we briefly review ultraviolet detection, report on these results, and briefly discuss the techniques and processes employed.

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Delta-doped electron-multiplied CCD with absolute quantum efficiency over 50% in the near to far ultraviolet range for single photon counting applications.

Delta-doped CCDs, developed at JPL's Microdevices Laboratory, have achieved stable 100% internal quantum efficiency in the visible and near UV regions of the spectrum. In this approach, an epitaxial silicon layer is grown on a fully-processed commercial CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30% of a monolayer of boron atoms are deposited on the surface, followed by a 15 $angstrom silicon layer for surface passivation. The boron is nominally incorporated within a single atomic layer at the back surface of the device, resulting in the effective elimination of the backside potential well. The measured quantum efficiency is in good agreement with the theoretical limit imposed by reflection from the Si surface. Enhancement of the total quantum efficiency in the blue visible and near UV has been demonstrated by depositing antireflection coatings on the delta-doped CCD. Recent results on antireflection coatings and quantum efficiency measurements are discussed.

Delta-doped CCDs: high QE with long-term stability at UV and visible wavelengths

We have used low temperature molecular beam epitaxy to grow p+ silicon on a backside-thinned Reticon 512x5 12 CCD. The techniques for preparing the CCD for the growth and the processing conditions are discussed. A 50 A layer of silicon doped with 3x102 B/cm3 was grown at a substrate temperature of 450C. The ultraviolet quantum efficiency of the modified CCD was significantly higher than that of a CCD with an untreated back surface. Charging the back surface of the modified CCD with a Uv flood did not affect the quantum efficiency indicating that the bands were pinned by the added p+ layer. Gold contamination measured by secondary ion mass spectrometry to have a concentration of 1 x 1 0 18 cm3 near the back surface caused the UV quantum efficiency to be lower than optimum by reducing the minority carrier lifetime. 2.

Epitaxial growth of p+ silicon on a backside-thinned CCD for enhanced UV response

Thin, backside-illuminated CCDs are modified by growing a delta-doped silicon layer on the back surface using molecular beam epitaxy. Delta-doped CCDs exhibit stable and uniform 100% internal quantum efficiency. The process consists of growth of an epitaxial silicon layer on a fully processed commercial CCD die in which 30% of a monolayer of boron atoms are incorporated into the lattice nominally in a single atomic layer. Long term stability was tested and showed no degradation of the device quantum efficiency over sixteen months. Reduction of the reflectivity of the Si surface by deposition of HfO2 on the CCD back surface further increased the QE, with measured QE over 80% in some regions of the spectrum. We discuss these results as well as the delta-doped CCD concept and process.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

DELTA-DOPED CCDs FOR ENHANCED UV PERFORMANCE

Delta-doped CCDs have achieved stable quantum efficiency, at the theoretical limit imposed by reflection from the Si surface in the near UV and visible. In this approach, an epitaxial silicon layer is grown on a fully-processed commercial CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30% of a monolayer of boron atoms are deposited nominally within a single atomic layer, resulting in the effective elimination of the backside potential well. These devices are highly uniform and have exhibited long-term stability. To achieve significantly higher total quantum efficiency, antireflection layers can be directly deposited on the device. This was demonstrated in the 250-400 nm region.© (1994) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Robert W. Terhune

Papers

Growth of a delta-doped silicon layer by molecular beam epitaxy on a charge-coupled device for reflection-limited ultraviolet quantum efficiency

Delta-doped CCDs: high QE with long-term stability at UV and visible wavelengths

Epitaxial growth of p+ silicon on a backside-thinned CCD for enhanced UV response

DELTA-DOPED CCDs FOR ENHANCED UV PERFORMANCE

Delta-doped CCDs as stable, high-sensitivity, high-resolution UV imaging arrays