Variable ring width SDD

A silicon drift detector (SDD) comprising electrically isolated rings. The rings can be individually biased doped rings. One embodiment includes an SDD with a single doped ring. Some of the doped rings may not require a bias voltage. Some of the rings can be field plate rings. The field plate rings may all use the same biasing voltage as a single outer doped ring. The ring widths can vary such that the outermost ring is widest and the ring widths decrease with each subsequent ring towards the anode.

FIELD OF THE INVENTION

The present invention relates to silicon drift detectors.

BACKGROUND

Various types of radiation detectors, such as silicon PIN diode detectors or silicon drift detectors, are used for measuring the energy of incoming x-ray photons. A PIN diode can be used for collecting charge carriers that are proportional in number to the energy of the x-ray photon. An example of a PIN diode is illustrated inFIG. 1, shown generally at10. The PIN diode is comprised of a substrate12, often referred to as an intrinsic region, an anode11on one surface of the substrate, and a cathode13on the opposite surface of the substrate. A PIN diode may be used for collection of electron-hole pairs released in response to an x-ray photon. For example, an x-ray photon may interact with an atom in the substrate, resulting in the generation of an electron-hole pair. This initial electron-hole pair may then quickly give rise to a cloud of electron-hole pairs. The electrons can travel to the anode11and the holes to the cathode13. A disadvantage of the PIN diode is the large capacitance due to the large anode size. Such capacitance can result in undesirable electronic noise, resulting in poor resolution.

An example of a silicon drift detector is illustrated inFIG. 2and shown generally at20. A silicon drift detector, hereinafter SDD, has a small anode25(small relative to a PIN diode anode) at one surface of the substrate12and an entrance window layer26at the opposite surface of the substrate. Use of a smaller anode results in lower capacitance and thus less undesirable electronic noise, resulting in improved resolution. The anode can be surrounded by multiple doped rings21. The doped rings are biased in such a way that they result in an electric field which causes electrons to flow towards the anode. The doped rings21can have the same doping or conduction type as the entrance window layer26. The anode25can have the same doping or conduction type as the substrate12, but usually the anode25is more highly doped than the substrate12.

The doped rings21can be electrically coupled within the SDD. For example, a MOSFET structure27on an SDD is shown inFIG. 3. Conductive contacts37can be attached to the doped rings21. The conductive contacts can be metallic. An insulative layer38separates the conductive contacts37from the substrate12. The overlap39bandcof the conductive contacts37over an adjacent doped ring21can induce a region of charge carriers36in the substrate12beneath the insulative layer38. The charge carriers36in this region are of the same type as the majority carriers in the doped rings21and thus form a conductive path between the doped rings. For example, as shown inFIG. 3, doped ring21cis attached to a conductive contact37c. The conductive contact37coverlaps an adjacent doped ring21bat overlap39c. Due to the conductive contact37attachment to one ring and overlap of an adjacent ring, when a voltage is applied across a series of doped rings, the region of charge carriers36can be induced in the substrate resulting in a conductive current path between the rings.

The prior art embodiment just described, in which a conductive contact37, that is attached to one doped ring21, overlaps an adjacent doped ring, is one method of electrical coupling. Another method of electrical coupling is shown inFIG. 4. A doped region41, having the same conduction type as the doped rings21, is created in the surface of the substrate and connects the doped rings21. Conductive contacts47can be attached to the doped rings21. The doped region41provides electrical coupling between the doped rings21.

As shown inFIG. 2, one voltage bias V1can be applied to the innermost doped ring that is closest to the anode25and another voltage V2can be applied to the outermost ring. Because the rings are electrically coupled, the voltages at the innermost and outermost rings can create a voltage gradient across all of the rings. Another voltage V3can be applied to the entrance window layer26.

The voltage applied to the entrance window layer V3can be similar in magnitude to the voltage V2on the outermost ring. The voltage on the innermost ring V1can have a lower absolute value than the voltage at the outermost ring V2or at the entrance window V3. Due to the voltage gradient across the rings and the voltage applied to the entrance window26, the charge carrier can be drawn towards the anode25. If V2and V3are more negative than V1and V1is more negative than the anode, then an electron cloud resulting from an impinging x-ray photon can be directed to the anode. Although the prior art SDDs can have reduced electronic noise compared with the prior art PIN diode, such SDDs with electrically coupled rings can be costly to manufacture.

SUMMARY OF THE INVENTION

It has been recognized that it would be advantageous to create a radiation detector having a small anode area for reduced capacitance. It has been recognized that it would be advantageous to create a radiation detector that is relatively less expensive to manufacture.

The present invention is a silicon drift detector comprised of a substrate with a first conduction type. The substrate includes a top surface and a bottom surface. A layer, having a second conduction type, is disposed at the bottom surface of the substrate. An island region having the first conduction type is disposed at the top surface of the substrate. At least two rings can be disposed at the top surface of the substrate and substantially circumscribe the island region. The rings are electrically isolated from each other. Due to the lack of electrical coupling between the rings, the manufacturing cost can be reduced. This invention includes a relatively small anode for reduced capacitance.

DEFINITIONS

“Electrically coupled” or “electrical coupling” means that there is an electrical current path, not impeded by an electrical insulator, from one ring to another ring within the SDD. A voltage supply, external to the SDD, may be required to create such electrical coupling.

“Electrically isolated” means that there is no electrical current path from one ring to another ring within the SDD. In other words, electrically isolated rings are separated from each other, within the SDD structure itself, by electrically insulative material, such as an oxide or depleted silicon. There may be a temporary electrical current flow between adjacent electrically isolated materials. For example, if there are three adjacent doped rings separated by silicon of opposite doping type, and two different voltages are applied to the inner and outer rings, initially there may be an electrical current flow towards the unbiased ring in the center. Once a steady-state voltage is reached at this center ring, however, there is no more, or insubstantial, electrical current flow between the rings. This is in contrast to electrically coupled rings in which the electrical current flow can continue.

DETAILED DESCRIPTION

Referring toFIG. 5, a silicon drift detector (SDD), indicated generally at50, in accordance with an exemplary embodiment is shown. The SDD includes a substrate12having a first conduction type, and with top and bottom surface. The top and bottom surfaces are opposite one another and can be oriented in any direction. A layer26(FIGS. 7-9) having a second conduction type can be disposed at the bottom surface of the substrate, and can define an entrance window layer. Typically, the entrance window layer, or bottom surface, faces incoming x-rays. An island region25(FIGS. 6-9) having the first conduction type can be disposed at the top surface of the substrate. The island region can be an anode. In addition, the SDD can include at least two rings51disposed at the top surface of the substrate and substantially circumscribing the island region. The at least two rings51can be doped and are electrically isolated from each other. In another aspect, the SDD can have at least three rings, which can be doped. In another aspect, the SDD can have at least four rings, which can be doped. In another aspect, the SDD can have five rings, which can be doped. In another aspect, the SDD can have one or more field plate rings, as described in greater detail below. There is no overlap of conductive contact57over an adjacent doped ring. Also, there is no doped layer in the substrate12connecting one doped ring to another doped ring.

As illustrated inFIGS. 6 and 7, an SDD shown generally at60, includes a substrate12, an entrance window layer26at one surface of the substrate12b, and an anode25at the opposite surface12aof the substrate. In one embodiment, a single doped ring51ecircumscribes the anode25. The single doped ring is situated near the outer perimeter12cof the SDD. A substantial annular portion of the substrate12separates the single doped ring51efrom the anode25. This embodiment offers the advantage of the need to make only a single doped ring51eand apply only a single ring voltage V5.

Another embodiment of the present invention, also illustrated inFIGS. 6 and 7, includes additional doped rings51a-d. Note that doped rings51a-dare shown with dashed lines as boundary to indicate that they are optional and are not included in the previous embodiment. All rings are electrically isolated from each other. Addition of such additional rings can improve the electron flow to the anode.

Voltages V1-5can be applied to the rings. It may, however, be desirable, in order to allow for simpler bias voltage circuitry, to apply voltages only to some of the rings and not to other rings. For example, a voltage V5may be applied only to the outermost ring51e. This outermost voltage V5can induce a voltage on the inner rings. The voltage can be induced by a temporary flow of current between the outermost ring51eand the other rings51a-d. Because the rings are electrically isolated, once a voltage is induced in the inner rings, the electrical current can stop.

Alternatively voltages may be attached to multiple, but not to all, rings. For example, voltages V1, V3, and V5may be applied to rings51a,51b, and51eand no voltages applied to rings51band51d. The voltages V1, V3, and V5applied to rings51a,51b, and51ecan induce a voltage on rings51band51d. Although for optimal SDD performance it may be desirable to have a separate bias voltage applied to each individual ring, simpler bias voltage circuitry allowed by fewer rings may dictate that in some circumstances it is better to trade optimal performance for simpler circuitry. Although inFIGS. 6 and 7, an SDD with five doped rings is shown, either more or less rings may be used, depending on design requirements.

The substrate12and the anode25can be one conduction type and the entrance window26and the doped rings can be an opposite conduction type. For example, if it is desirable for electrons to the drawn to the anode, the substrate12and the anode25can be n doped and the entrance window26and the doped rings can be p doped. Normally the anode25is more highly doped than the substrate12. Alternatively, if it is desirable to draw positive charges (i.e., holes) to the anode, then the substrate12and the anode25can be p doped and the entrance window26and the doped rings can be n doped. Either configuration of doping is applicable to all embodiments of this invention. For simplicity, future discussion will describe electrons as the desired charge that is drawn to the anode. It will be appreciated, however, that by reversing the doping and changing the voltages, that positive charges can be drawn to the anode.

Also shown inFIG. 7is an electron drift path71. The distance d3, between the electron and the ring51b, when the electron is near the anode25, is shorter than the distance d1, between the electron and the ring51e, when the electron is near the outer perimeter12c. Due to the relatively larger distance d1, the ratio of distances d2:d1between the electron and adjacent rings51dand51ewhen an electron is near the outer perimeter12cis smaller than the ratio of distances d4:d3between the electron and adjacent rings51aand51bwhen an electron is near the anode25. As a result of this smaller ratio of distances between the electron and the rings when an electron is near the outer perimeter12cthan the electron is near the anode, the voltage differential between the rings results in a stronger electric drift field directly below a given ring when the electron is nearer the outer perimeter. Therefore, the electron drift speed can decrease when the electron is directly below a ring as the electron nears the anode25. This decrease in speed results in greater spreading of the electron cloud, which in turn results in poorer resolution at short peaking times due to ballistic deficit.

Electron travel time to the anode can be reduced with variable ring widths. As shown inFIG. 8, the ring width can be widest w5at the outermost ring81e. Ring width can decrease with each subsequent ring that is nearer the anode25such that the ring width w1of the innermost ring81ais the narrowest. Use of narrower rings near the anode can result in similar ratios of distance between the electron and adjacent rings when the electron is near the outer perimeter12cas when the electron is near the anode25. As a result, the electron drift speed as it approaches the anode can be more uniform along the entire drift path and the resolution at short peaking times can be improved. The SDD80, shown inFIG. 8, similar to the SDD60, shown inFIGS. 6 and 7, can have more or less rings than the five rings shown and bias voltages can be applied to one, some, or all of the rings.

As shown inFIG. 9, and indicated generally by90, an SDD may be made with field plate rings94a-din addition to at least one outer doped ring91. Field plate rings can be electrically isolated from each other. Field plate rings, unlike a doped ring, can be electrically isolated from the substrate by an insulating layer98, such as an oxide layer. Each field plate ring94a-dcan be individually biased V1-4.

An advantage of the field plate rings is that SDD performance with a single bias voltage for all field plate rings and the doped ring can be comparable to the performance realized with multiple bias voltages, thus allowing good performance with a single bias voltage. Use of a single bias voltage allows simpler circuitry in the bias voltage supply. A single bias voltage can be used because the insulating layer between the field plate ring and the substrate results in an inversion layer in the silicon adjacent to the insulative layer. As a result of this inversion layer, and the effect of the voltage V5on the outer doped ring91, once a high enough bias voltage on the field plate ring is obtained, any higher voltages merely result in a greater voltage drop through the insulating layer, but the voltage on the substrate side of the insulating layer remains substantially unchanged. For example, in the SDD90ofFIG. 9, use of voltages V1=−20 volts, V2=−40 volts, V3=−60, V4=−80, and V5=−100 can result in comparable SDD performance as would be obtained by use of a single bias voltage of V5=V4=V3=V2=V1=100 volts.

Although four field plate rings94a-dare shown inFIG. 9, the actual number of field plate rings may be more or less than four. Although one doped ring91is shown inFIG. 9, there may be additional doped rings. Also, as discussed above regarding SDD80, electron drift speed towards the anode25may be improved by creating the widest ring at the outer perimeter12cand using gradually decreasing ring widths towards the center.

Referring toFIG. 10, the above described various embodiments of silicon drift detectors can be utilized with a radiation detection system100. The radiation detection system can include an SDD101. The SDD101can be any previously described SDD embodiment. A hermetically sealed container102can surround the SDD101. A window104in the container102can allow x-rays, represented by line103, to pass into the container and impinge upon the silicon drift detector101.

How to Make

The SDD can be made by standard semiconductor manufacturing processes and can be made of standard semiconductor materials such as silicon, germanium, gallium arsenide, etc. The dopants can be standard doping materials. For example, boron or boron difluoride may be used for p type doping and phosphorous or arsenic may be used for n type doping.

Doped rings can be made by masking off desired areas of the substrate with a photoresistive mask and doping the rings by a standard method such as ion implantation. The anode can be created by a similar method. To form the field plate rings, an insulating layer can be added on top of the substrate by any suitable method such as thermal oxidation. A mask can then be applied and the oxide etched away in regions where it is not desired by any suitable method, such as a wet etch. The desired conductive material for the field plate rings can be sputtered onto the surface, followed by the application of a mask and the etching away of the conductive material from undesired regions by a suitable means, such as a wet etch. The field plate rings can be made of an electrically conductive material. For example, the field plate rings may be metallic or a metallic alloy.