Semiconductor device manufacturing: process – Making device or circuit responsive to nonelectrical signal – Responsive to electromagnetic radiation
Utility Patent
1997-11-26
2001-01-02
Bowers, Charles (Department: 2813)
Semiconductor device manufacturing: process
Making device or circuit responsive to nonelectrical signal
Responsive to electromagnetic radiation
C438S085000, C438S767000, C148SDIG001, C148SDIG001, C148SDIG009, C148SDIG009, C148SDIG006
Utility Patent
active
06168967
ABSTRACT:
BACKGROUND OF THE INVENTION
Cadmium zinc telluride, Cd
x
Zn
1-x
Te, is a wide-bandgap, compound semiconductor that is used for gamma-ray spectroscopy and imaging at room temperature. Spectrometer-grade Cd
x
Zn
1-x
Te material (x≅0.1) is grown commercially using the high-pressure Bridgeman process. At present, small-volume (<0.1 cm
3
) substrates for spectrometry and thin, wide-area substrates for imaging are routinely manufactured for use in a variety of applications, including medical imaging. Larger substrates (<2 cm
3
) have been manufactured that have high bulk resistivity and relatively uniform electronic properties. However, the availability of such samples is presently low.
A principal limitation of CdZnTe is poor hole collection, that is, the trapping length for holes in CdZnTe under typical bias conditions (approximately 1000 V/cm) is routinely less than 1 mm. Electrons, by contrast, have a trapping length of approximately 5 cm so that single-carrier (electron-only) devices up to 2-cm thick can be made. However, in order for these devices to function as spectrometers, compensation for incomplete hole collection is essential. One currently employed compensation method involves the use of coplanar grids. See, for example, “Performance of CdZnTe Coplanar-Grid Gamma-Ray Detectors,” by P. N. Luke and E. E. Eissler, Trans. Nucl. Sci. 43, 1481 (1996). Parallel gold or platinum strips are deposited on the surface of the detector and are alternately connected to separate preamplifier circuits. The strips form the anode of the device. One set of strips is biased slightly higher than the other so that it collects the charge. The other set of strips serves to sense charge motion in the device. Subtracting the preamplifier output signals eliminates the effect of charge motion within the device. The difference circuit responds only to charge motion near the electrodes. Because the signal measured by the difference circuit is the same independent of where the electrons originate, excellent resolution and peak-shape are achieved.
To date, the best reported resolution using coplanar grids is about 2% full-width-at-half-maximum (FWHM) at 662 keV for a 1 cm
3
detector. The resolution of large-volume (>2 cm
3
) detectors with coplanar grids is highly variable, but is typically less than 5% FWHM at 662 keV. This is significantly better than the commonly used NaI(TI) detectors in combination with photomultiplier tubes. The energy resolution that can be achieved using CdZnTe is superior to NaI and since CdZnTe is a high-resistivity, wide-bandgap semiconductor which requires minimal power and no cooling, the possibility of increased detector volume and efficiency has lead to renewed interest in CdZnTe.
Manufacturing CdZnTe substrates having uniform electronic properties for gamma-ray spectrometry and imaging is difficult. Attempts to improve CdZnTe for spectroscopy applications have focused on crystal growth to eliminate the effect of variable zinc concentration, grain boundaries, and other structures, such as pipes, which often cause nonuniform charge collection and results in poor detector performance. Some samples have been made which have relatively uniform bulk-electronic properties and perform well as gamma-ray spectrometers. However, a significant number of these samples have high surface leakage current which is a significant source of parallel noise in the charge measurements made in spectrometry. This problem is exacerbated by the use of fine grid structures across which bias is applied; for example, coplanar grids. Additionally, surface leakage may be a significant problem for detectors used for imaging which employ pixels to determine the position of a gamma-ray interaction, as an example. The magnitude of the noise caused by leakage is known to increase with leakage current. Consequently, a direct reduction in the noise can be realized by reducing the leakage current. Other sources of noise limit the magnitude of the reduction that can be achieved through surface treatment; however, in many devices, the leakage noise is sufficiently large that surface passivation is beneficial.
Surface leakage currents are usually dominated by two effects: (1) the presence of localized surface features where the electric field is significantly enhanced; and (2) the presence of a thin surface layer of nonstoichiometric CdZnTe material having an electrical resistivity that is much lower than the underlying CdZnTe substrate. One would expect an electric field enhancement in regions where sharp edges, corners, or surface roughness exist. Since the leakage current is proportional to the localized electric field, these regions will have higher current values than the surrounding smooth areas. For this reason, coplanar-grid detectors, particularly those with fine grid structures, may have high surface-leakage currents.
Chemical etching processes have recently been shown to increase the planarity (smoothness) of CdZnTe crystals. See, e.g., A. Burger et al., “Oxidation Of CdZnTe Materials,” Scientific Symposium On Room Temperature X-Ray, Gamma-Ray, and Neutron Detectors,” Livermore, Calif., Mar. 11-12, 1997. Improved planarity makes oxidation and passivation requirements for CdZnTe surfaces an even more critical issue. Previous studies reveal that bromine-methanol solutions leave a tellurium-rich surface on CdZnTe crystals consisting of tellurium precipitates. This tellurium-rich layer has an electrical conductivity that is much greater than the underlying stoichiometric CdZnTe (resistivity of >10
10
&OHgr;-cm). Since the leakage current is proportional to the electrical conductivity, it would be beneficial to have a surface passivation process that reduces the surface electrical conductivity to as small a value as possible.
In “Study Of Oxidized Cadmium Zinc Telluride Surfaces,” by K. -T. Chen et al., J. Vac. Sci. Technol. A 15, 850 (1997), the authors describe a method for decreasing surface leakage current for CdZnTe materials using various surface oxidation treatments. In particular, a 15% aqueous solution of hydrogen peroxide was used in the oxidation of detector-grade, Cd
0.9
Zn
0.1
Te wafers produced a 37% decrease in surface leakage current.
Plasma oxidation processes have been used to form oxide films on Hg
1-x
Cd
x
Te. See, e.g., Plasma Anodization Of Hg
1-x
Cd
x
Te,” by Y. Nemirovsky and R. Goshen, Appl. Phys. Lett. 37, 813 (1980). However, alternative methods have been explored in order to avoid excessive fixed charge, long term instabilities and substantial depletion of Hg in the semiconductor near the surface. See, e.g., “Summary Abstract: Photochemical Oxidation Of (Hg,Cd)Te,” by S. P. Buchner et al., J. Vacuum Technol. 21, 446 (1982).
In “New Surface Treatment Method For Improving The Interface Characteristics Of CdTe/Hg
1-x
Cd
x
Te Heterostructure,” by Seong Hoon Lee et al., J. Electron. Mater. 26, 556 (1997), the authors describe chemical oxidation of Hg
0.7
Cd
0.3
Te with nitric acid and the subsequent removal of the oxide with ammonium hydroxide in order to prepare the surface for the electron-beam deposition of CdTe. Clearly, the oxide coating formed by chemical means is sufficiently thin to be unable to effectively passivate the surface of the material, and the deposition of CdTe (after removal of the oxide) is responsible for the observed substantial improvement in the electrical properties.
In “Effects Of Passivation And Extraction Surface Trap Density On The 1/f Noise Of HgCdTe Photoconductive Detector,” by C. T. Lin et al., IEEE Photonics Technology Letters 9, 232 (1997), the authors describe their previous work on the growth of native oxides on HgCdTe substrates. Because of the extreme temperature sensitivity of the HgCdTe substrate, surface passivation processes are limited to low temperature. Therefore, photo-enhanced native oxides, which were observed to have low leakage current, were formed at 50° C. in a direct photochemical vapor deposition process using D
2
lamps as the light source. However, additional passivating layers of ZnS were required to obta
Hoffbauer Mark A.
Prettyman Thomas H.
Bowers Charles
Freund Samuel M.
Kilday Lisa
The Regents of the University of California
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