Lithography process for patterning HgI2 photonic devices

Details Lithography process for patterning HgI2 photonic devices Lithography process for patterning HgI2 photonic devices

2000-11-13

2004-11-23

McPherson, John A. (Department: 1756)

Radiation imagery chemistry: process, composition, or product th

Imaging affecting physical property of radiation sensitive...

Making electrical device

C430S319000, C430S321000, C250S370120

Reexamination Certificate

active

06821714

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods for obtaining improved spatial and energy resolution in room temperature HgI
2
radiation detectors, and more specifically, it relates to a method for lithographically patterning HgI
2
crystals.
2. Description of Related Art
Semiconductor materials exhibit a band gap between their valence and conduction bands of typically a few eV. Because this energy gap is so low, as the temperature of the crystal is increased, electrons are thermally excited and easily move from the valence band to the conduction band. The electrical properties of these materials, therefore, are affected not only by the movement of electrons into the conduction band but also by the formation of vacant sites or “holes” in the valence bands left behind by the departing electrons. Both can conduct current.
Holes also may be created by the interaction of energetic radiation, such as X-rays, gamma rays, and the like, with intrinsic semiconductors and therefore one should be able to use these materials as detectors for measuring high-energy radiation. In fact, high-resistivity semiconductor radiation detectors are widely used for detecting ionizing radiation due to their ability to operate at room temperature, their small size and durability. Such detectors are used in a wide variety of applications, including medical diagnostic imaging, nuclear waste monitoring, industrial process monitoring, and space astronomy. Ionizing radiation includes both particulate radiation such as alpha or beta particles and electromagnetic radiation, such as gamma or x-rays.
If all the electrons and holes generated by the ionizing radiation reach their respective electrodes (i.e., the electrons reach the anode and the holes reach the cathode), the output charge signal will exactly equal the charge from the ene deposited within the crystal by the radiation. Because the deposited charge is directly proportional to the energy of the ionizing radiation, the semiconductor detector provides a means for measuring the energy of the ionizing radiation.
Room temperature detectors, however, suffer from a serious drawback. Because of limitation in the transport properties of the bulk semiconductor crystal, some of the electrons and, more particularly, some holes are generally lost by being trapped as they move toward the respective electrodes under the influence of the external electrical field. This is particularly evident for semiconductors wherein the transport properties of one carrier type (e.g., electrons) are much better than those of another type (in this example the “holes”). Therefore under such circumstances the amplitude of the output charge signal becomes dependent on the position within the crystal at which the ionizing radiation is absorbed. Generally speaking, the amplitude is less than the charge deposited by the ionizing radiation and results in a corresponding reduction of energy measurement accuracy, poor resolution, and reduced peak efficiency. This loss (or trapping) of charge in a radiation detector results in distorted and asymmetrical spectral peak shapes known as “hole tailing” or “hole trapping.”
The inability to eliminate “hole” drift current is a major impediment for the use of room temperature semiconductors as detectors. Gamma-ray spectroscopy is particularly encumbered because pulse height spectra produced by these devices are distorted by this process. Mono-energetic gamma rays produce charge signal responses of different pulse height because the total combined distance drifted by the electrons and holes is dependent on the position of gamma-ray interaction. This phenomenon is well known in the prior art and has been described by many researchers. It is widely understood to be the major deficiency limiting the effectiveness of room temperature semiconductor materials.
Due to the deleterious effects of hole-trapping in semiconductor detectors, much effort has gone into attempting to solve this problem. U.S. Pat. Nos. 4,253,023 and 4,996,432 recognized the problem and proposed early remedies. The first of these included a method to de-convolute the contribution of the electron motion from the acquired signal. The second approach proposes a method relying upon use of a thick crystal and a crystal orientation placing the detector anode surface facing the source of radiation, thereby reducing the positional dependence of the radiation interaction with the crystal and restricting it only to that part of the crystal immediately behind the anode. Neither of these approaches directly addresses the problem of eliminating hole-trapping.
U.S. Pat. No. 5,677,539 provides another approach and a comprehensive review of much of the pertinent prior art. A particularly relevant approach, described therein, employs an anode patterned into an interleaving grid structure, with the, cathode remaining planar. (See, e.g., P. N. Luke, “Unipolar Charge Sensing with Coplanar Electrodes—Application to Semiconductor Detectors,” IEEE Tran. Nucl. Science, vol. 42, No. 4, at pages 207-213 (1995)). In this approach, one set of anode grids is maintained at a slightly higher voltage than the other. A train of signal conditioning electronics is connected to each set of grids, and the difference between the outputs from these trains constitutes the final output signal. With this arrangement, when the charge cloud is far from the grids, the difference-signal between the grid outputs is zero. As the cloud approaches the grids, the induced charge on one grid rises rapidly, while the charge induced in the other grid drops rapidly. The difference signal is then a measure of the full charge in the electron cloud, independent of the position of the ionizing event.
This approach, however, also suffers from various drawbacks. First, the grid structure is relatively complex and would be difficult, if not impossible, to use in detector arrays. Second, the grids require two separate amplifying chains, plus a difference amplifier, adding significantly to the complexity and cost of manufacture. This circuitry also would be difficult to implement in the multichannel type integrated circuits needed in detector array structures.
A relatively simpler structure is a variation on a technique devised by Frisch for use in gas detectors. The hole-trapping phenomenon observed in semiconductor detectors is analogous to the trapping behavior of positive ions in gas detectors. Frisch proposed, and later developed, detectors that contained a grid of conductive wires between the two electrodes of a conventional gas detector.
In the Frisch grid type detector, the signal is measured between the anode and the grid. The negative charge carriers, electrons in the case of a semiconductor detector, usually drift all of the way to the anode. Thus, any radiation interactions occurring between the cathode and the grid will produce electrons that drift past the grid and on to the anode. When the electrons drift across the gap between the grid and the anode, they will induce a current that is due solely to the motion of electrons. The current induced by positively charge carriers (“holes” in the case of semiconductors) traveling in the opposite direction is shielded by the grid. The main advantage, then, of the Frisch grid is that the signal produced from the device is independent of the position of interaction between the cathode and grid (completely solving the hole trapping problem in the cathode to grid region).
An approach to reducing hole-trapping in a semiconductor detector using a modified Frisch grid approach has been proposed by D. S. McGregor. McGregor's scheme, described as an “etch trench” device, entailed building a Frisch Grid device on a semiconductor crystal and placing electrodes at the bottom of these trenches. The primary disadvantage of McGregor's etch-trench design is that it is difficult to execute. In other words, it might be very difficult to produce the requisite trenches, particularly since the fabrication technology for room temperature semiconductor materi

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