Position sensitive solid state detector with internal gain

Details Position sensitive solid state detector with internal gain Position sensitive solid state detector with internal gain

2001-11-01

2004-08-24

Hannaher, Constantine (Department: 2878)

Radiant energy

Invisible radiant energy responsive electric signalling

Semiconductor system

C250S370110, C257S429000

Reexamination Certificate

active

06781133

ABSTRACT:

BACKGROUND
1. Field of Invention
This invention describes methods of obtaining position of incidence information from solid state devices, such as avalanche photodiodes, without introducing any dead space to the detector's active area.
2. Discussion of Prior Art
Many applications in science and industry require detectors that are capable of reporting time and position of incidence information for discrete quantum units of radiation such as single photons and beta particles. A single photon is understood to be a unit of radiation with an energy described by E=hc/&lgr;, where &lgr; is the wavelength of the radiation. In some cases it is most expedient to convert a high energy photon into a group of multiple lower energy photons and then detect the group of lower energy photons as a single event corresponding to the lower energy photons. This is typically achieved using fluorescent materials such as scintillators.
Detectors for these applications will ideally have an output that gives a rapid position sensitive readout with a good signal to noise ratio. In order to achieve a good signal to noise ratio, it is beneficial for the detector to have internal gain. The detector should also have good detection efficiency over a large active area and a wide dynamic range. Furthermore, the active area should cover a significant portion of the detector's physical footprint and allow for efficient tiling to cover areas greater than the practical size of a discrete detector. In some applications, it is desirable for the detector to be capable of operating effectively in a high magnetic field. It is also beneficial if the detector has low power requirements, especially for applications that require many detector elements. A number of technologies have been developed in an effort to satisfy these requirements. These technologies fall into two main categories: vacuum tube detectors and solid state detectors.
Vacuum Tube Detectors
Vacuum tube detectors include photomultiplier tubes, image intensifiers, and imaging photon detectors. These detectors have a photocathode that converts incident radiation outside the detector envelope into electrons inside the detector envelope. Electrons from the photocathode are then amplified inside the detector envelope, typically using a system of dynodes or microchannel plates that confine the amplification process to remain spatially centered about the position at which the electrons originated from the photocathode. The bundles of electrons resulting from the amplification process are then collected on an anode structure that can provide a position sensitive readout, and the position of the incident radiation is then determined from this readout.
Vacuum tube detectors can achieve gains in excess of 10
6
with relative ease, and can provide sub-nanosecond readout. However, they are limited by the quantum efficiency of the photocathode material, which in practice is typically in the range of 10-20%. In addition, the input window on which the photocathode is formed is generally made of glass or a fiber optic faceplate that is a few millimeters thick. Both methods introduce optical losses when the detector is used with proximity-focused scintillator arrays. Detectors that use microchannel plate structures for internal amplification suffer from a localized dead time on the order of 10-100 milliseconds, which severely limits the realizable dynamic range of the detector for detecting sequential pulses of radiation. Vacuum tube detectors are also frequently constructed in a round enclosure, which is inefficient for tiling to cover large areas. Furthermore, magnetic fields that are not parallel to the electron transit path inside the vacuum enclosure will always cause geometric distortion in a position sensitive readout and may affect gain as well.
Solid State Detectors
There are two main types of solid state detectors that are used in the radiation detection applications described above: photodiodes and avalanche photodiodes (APDs). The fundamental difference between these two types of detectors is that avalanche photodiodes have internal gain, while photodiodes have no gain. This makes APDs a better choice than photodiodes in applications where small signals with low background must be detected with wide bandwidth at high frequencies. Positron Emission Tomography (PET) is a classic example of this type of application, where the timing coincidence of individually detected gamma rays must be measured to within a few nanoseconds while maintaining good energy resolution and high signal throughput. Similar applications exist in high energy physics, LIDAR, and LADAR.
Owen (“One and Two Dimensional Position Sensing Semiconductor Detectors”, IEEE Trans. Nucl. Sci. NS-15, p.290+, 1968), Kelly (“Lateral-Effect Photodiodes”, Laser Focus, Mar. 1976, pp. 38-40) Kurasawa (“An Application of PSD to Measurement of Position”, Precision Instrument, Vol 51, No. 4, 1985, pp. 730-737) and others have shown methods of obtaining position sensitive information from solid state detectors with no internal gain. A number of companies including Hamamatsu, UDT, and Silicon Sensor sell ‘lateral effect’ position sensing photodiode products that use similar methods. However, because they are photodiodes that have no internal gain, all of these detectors are limited to applications that have relatively low bandwidth requirements and a relatively high background when compared to what is possible with avalanche photodiodes.
An APD is a semiconductor device that is constructed in such a way that a large electric field can be created inside the semiconductor material with a very low leakage current. Any free carriers that enter the electric field region will be accelerated out of it. If the size of the electric field region is large relative to the mean fire path of the carriers, then there is a high probability that a free carrier will gain enough energy to liberate other carriers in the space charge region, which will in turn be accelerated. This avalanche effect continues until the free carriers get accelerated out of the space charge region and either recombine or are extracted from the device. The device is designed such that when an electron-hole pair is created in the top layer, a charged carrier will drift into the high field region of the device and experience avalanche multiplication. The avalanche process gives APDs internal gain, which is very useful for detecting low levels of electromagnetic radiation.
There are a number of reasons why the prior art methods for extracting position sensitive information from photodiodes cannot be directly extended to work with APDs. Before considering how to obtain position sensitive information, however, it is important to recognize that substantially different approaches must be used to design and fabricate a non-position sensitive APD as compared to a non-position sensitive photodiode with the same active area. This is because the internal fields in APDs are much higher than the internal fields in photodiodes, so a field spreading structure is required to avoid edge breakdown when bias is applied to an APD. The details of these methods are well known to those skilled in the art.
The design of a position sensitive APD must give special consideration to the placement of contacts on the device in order to avoid electrical interaction with the field spreading structure. The contact method also affects the package design, which can in turn affect the usability of the detector in tiling applications. In addition, while photodiodes can receive uniform surface treatments to achieve a position sensitive readout, most surface treatments will need to be modified in order to be compatible with the field spreading structure in an APD. Furthermore, it can be advantageous to extract position sensitive information from the majority carrier signal on the cathode in order to avoid modifying the anode structure in ways that could significantly affect the sensitivity or response uniformity of the device. If position determining signals are only extract

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