Abstract:
A semiconductor radiation detector crystal is patterned by using a Q-switched laser to selectively remove material from a surface of said semiconductor radiation detector crystal, thus producing a groove in said surface that penetrates deeper than the thickness of a diffused layer on said surface.

Description:
TECHNICAL FIELD 
       [0001]    The invention concerns in general the technology of solid-state semiconductor-based radiation detectors. Especially the invention concerns the problem of patterning a detector element, which essentially consists of a semiconductor piece with a PN junction on one side. 
       BACKGROUND OF THE INVENTION 
       [0002]    The use of solid-state radiation detectors is based on exposing a semiconductor piece to incoming radiation. The interaction of a received photon with the semiconductor material creates a number of electron/hole pairs, also referred to as free charge carriers. At least one species of free charge carriers are accumulated to electrodes connected to the semiconductor material, and the amount of electric charge thus collected is measured. The total amount of collected charge reveals the original energy of the received photon. If the detector has spatial resolution, detecting the location at which the charge cloud was created enables using the measurement results for imaging. The semiconductor piece is often referred to as the crystal, because detectors of best quality require a single-crystal semiconductor piece with as few lattice faults as possible. 
         [0003]    Patterning a detector crystal may have various aims. Dividing the detector crystal into discrete pixels enables making an imaging detector, in which the charge collected by each pixel can be read separately. Even if the detector is not meant for imaging, patterns on the crystal surface may be used as guard structures that control and limit the flow of charges in the crystal. 
         [0004]    Known methods for patterning a detector crystal include at least photolithography and ion beam based methods. The former refers to a family of processes, the common features of which include depositing a photoresist on a surface to be patterned, selectively hardening the photoresist through exposure to light, and utilizing the chemical differences between hardened and non-hardened photoresist to etch out desired portions of the surface beneath the photoresist. Ion beam deposition may also use a photoresist, but the eventual mechanism of actual pattern production is not chemical etching but ions abrading away the areas not covered by the photoresist. Focused ion beam techniques are also available and can be used for example to directly inject new material onto the surface to be patterned. 
         [0005]    The above-mentioned methods are not sufficient for good results in all patterning applications. As an example we may consider the case of  FIG. 1 , which shows a cadmium telluride (CdTe) detector crystal  101  with a diffused indium layer  102  on its back surface. The front surface has a thin contact layer  103  made of a metal, like gold, platinum or palladium. The terms “front” and “back” are here used in the conventional sense to tell that the radiation to be detected is meant to enter through that surface that has the metal contact layer  103 . The thicknesses of the layers are not to scale. In practice, the thickness of the detector crystal  101  could be in the order of millimetres, the thickness of the diffused indium layer  102  could be in the order of tens of micrometers, and the thickness of the metal contact layer  103  could be in the order of nanometers. 
         [0006]    It would be relatively easy to add spatial resolution to the detector of  FIG. 1  by patterning the metal contact layer  103  with e.g. photolithographic methods. However, using the front surface for patterning would mean that also the readout electronics should be located on the front surface of the detector, where they may interfere with the incoming radiation. The diffused indium layer  102  is typically too thick (in the order of some tens of micrometers) to be patterned with e.g. photolithography. 
         [0007]    Basically it would be possible to replace the thick diffused indium layer  102  with a thin metal contact also on the back surface, which thin metal contact could then be patterned. However, this would easily lead to an unacceptably high leakage current in the detector. The PN junction at the interface between cadmium telluride and indium is vital in curbing the leakage current. 
       SUMMARY OF THE INVENTION 
       [0008]    An objective of the present invention is to present a method for patterning a solid-state semiconductor detector. A further objective of the present invention is to present a patterning method that is applicable to detector crystals having thick diffused layers. Yet another objective of the invention is to present a patterned semiconductor detector with low leakage current and small number of lattice faults. 
         [0009]    The objectives of the invention are achieved by selectively removing material from the surface of a layered structure where a diffused layer covers a semiconductor crystal, so that grooves thus produced penetrate deeper into said layered structure than the thickness of said diffused layer. 
         [0010]    A method according to the invention is characterized in that it comprises: 
         [0011]    selectively removing material from a surface of said semiconductor radiation detector crystal, thus producing a groove in said surface that penetrates deeper than the thickness of a diffused layer on said surface. 
         [0012]    A semiconductor detector according to the invention is characterized in that it comprises: 
         [0013]    a detector crystal made of a semiconductor material, 
         [0014]    a diffused layer of a substance on a back surface of said detector crystal, said diffused layer giving rise to a PN junction between a front surface and said back surface of the detector crystal, and 
         [0015]    a groove in said back surface that penetrates deeper into the detector crystal than the thickness of said diffused layer. 
         [0016]    The thickness of the diffused layer means that selectively removing material from that side of the detector crystal must be accomplished with a method that will reach deep enough into the crystal without causing excessive damage to the lattice structure. Suitable methods include at least the use of a spatially controllable Q-switched laser, sawing, and plasma etching. 
         [0017]    A Q-switched laser is a device capable of creating an exceptionally high power density at a very small focal point area. If focused on a surface, the output pulse of a Q-switched laser will convert a part of the surface material into a state of plasma, so that a certain amount of the material is ejected leaving an empty cavity on the surface. Repeated application of Q-switched laser pulses at the same location will make the cavity deeper, and moving the focal point across the surface will cause a groove to be created. 
         [0018]    A detector crystal that has a relatively thick diffused layer on its back surface can be patterned by using a Q-switched laser to “saw” grooves in the back surface. If the grooves reach deeper than the thickness of the diffused layer, and if a groove or a number of intersecting grooves form a closed contour, they effectively isolate an island or a number of islands of the diffused material from the remaining parts of the original crystal surface. If independent electric contacts are made to a number of islands so isolated, the charge collected by each island can be separately measured, which means that the detector has spatial resolution. 
         [0019]    Using a Q-switched laser to produce said grooves has the advantage that the amount of excess energy that is distributed to the remaining atoms of the crystal lattice is relatively low, resulting in a correspondingly low number of permanent faults introduced into the crystal lattice. If cleaning of the grooves is nevertheless required, it can be done by e.g. wet etching. Since the Q-switched laser eats its way easily through a photoresist, it is not necessary to preproduce any patterns into the resist layer before the application of the Q-switched laser. 
         [0020]    Other viable methods for producing the grooves include but are not limited to mechanical sawing and plasma etching. 
         [0021]    The exemplary embodiments of the invention presented in this patent application are not to be interpreted to pose limitations to the applicability of the appended claims. The verb “to comprise” is used in this patent application as an open limitation that does not exclude the existence of also unrecited features. The features recited in depending claims are mutually freely combinable unless otherwise explicitly stated. 
         [0022]    The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The invention itself, however, both as to its construction and its method of operation, together with additional objects and advantages thereof, will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  illustrates a known, unpatterned CdTe semiconductor detector, 
           [0024]      FIG. 2  illustrates the preparation of a patterned detector crystal, 
           [0025]      FIG. 3  illustrates a patterned detector crystal with pixels, 
           [0026]      FIG. 4  illustrates a patterned detector crystal with a guard ring, and 
           [0027]      FIG. 5  illustrates a method for patterning a detector crystal. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0028]    Q-switched lasers represent known technology, but for clarity some of their features are summarized here. Passive Q-switching involves using a piece of optical gain medium in connection with a saturable absorber, also known as the passive Q-switch. Optical energy is pumped into the gain medium using e.g. a so-called pump laser. A saturation effect in the absorber leads to a rapid reduction of resonator loss, so that energy temporarily stored in the gain medium is instantaneously extracted in the form of a laser pulse. The cycle of storing and releasing energy is repeated at a rate determined by the pumping power and the characteristics of the saturable absorber. As a difference to passive Q-switching, active Q-switching involves using a medium with actively controllable optical characteristics in place of the saturable absorber. 
         [0029]    A laser machining apparatus that employs a Q-switched laser typically includes controllable focusing means, with which it is possible to choose very exactly the location at which the laser pulses will hit the target surface, and also to move the focal point of the laser pulses very exactly across the target surface along a predefined route. The controllable focusing means may include e.g. piezoelectrically controlled mirrors, lenses and so forth. The predefined route, along which the focal point should be moved, may include straight sections, curved sections and discontinuities (which are implemented by switching the laser on and off according to need). The speed at which the focal point will be moved, as well as the output power of the Q-switched laser system can be controlled exactly. This means that when the resistance of the target material to laser machining is known, it is possible to predict relatively exactly, how deep a groove will result from a certain movement of the focal point across the target surface. 
         [0030]    In  FIG. 2  the topmost cross-section represents a detector crystal  101  that comprises a relatively thick diffused layer  102  on its back surface and a thin contact layer  103  on its front surface. Actually it is quite irrelevant whether the detector crystal comprises anything on its front surface at this moment; the thin contact layer  103  is there merely to underline that prepared crystals of this kind are easily obtained e.g. from production lines that conventionally have produced non-patterned detectors of the type illustrated in  FIG. 1 . Similarly to  FIG. 1  also here the layer thicknesses are not to scale. 
         [0031]    In  FIG. 2  we assume that some further processing steps will benefit from the application of a photoresist layer  201  onto the back surface of the crystal. After applying the photoresist a Q-switched laser is used to produce initial grooves  202  that penetrate from the back surface deep enough into the crystal  101  to go through the diffused layer  102 . Depending on the accuracy of controlling the Q-switched laser apparatus and other factors, the initial grooves  202  may first have somewhat rough edges. Wet etching is used to clean and smooth the grooves, so that nice clean final grooves  203  are produced. Assuming that the photoresist layer  201  is not needed for anything else, it can be removed. In order to utilize the thus accomplished patterning there are formed some kinds of contacts  204  and possibly readout electronics onto the exposed surfaces of the patterns, using known methods of semiconductor circuit production. 
         [0032]    Some possible results of patterning are shown in  FIGS. 3 and 4 .  FIG. 3  illustrates schematically a patterned detector crystal  301 , the back surface of which comprises pixels  302  separated from each other by laser-machined grooves. One portion  303  of the back surface has been left without grooves to enable using it the substrate for readout electronics.  FIG. 4  illustrates schematically a patterned detector crystal which does not need to have spatial resolution, but in which a guard ring consisting of a groove in the form of a closed curve separates a central part  402  from an edge portion  403 . Many other ways of patterning, including but not being limited to various combinations of the principles shown in  FIGS. 3 and 4 , are possible. 
         [0033]      FIG. 5  illustrates a method according to an embodiment of the invention. At some stage of manufacturing a semiconductor radiation detector, there is prepared a crystal with a relatively thick diffused layer on its back side according to step  501 . Whether or not the process involves applying a resist on the surface of the diffused layer is not important to the invention, which in  FIG. 5  is illustrated as a possible straight transition from step  501  to step  503  as an alternative to the route going through the step  502  of applying a resist. Even if a resist is used to protect those portions of the diffused surface against a later etching step that should remain intact, it is interesting to note that step  502  does not need to include using any masks or other ways to pattern the resist, because the laser will easily go through the resist also. Thus it is sufficient to apply a smooth, even layer of resist in step  502  and expose and develop it all, if a resist is needed. 
         [0034]    Using a Q-switched laser to machine grooves through the diffused layer is illustrated relatively simply as switching on the laser in step  503 , focusing the Q-switched laser into a focal point on the surface to be machined and moving the focal point across the surface in step  504 . In practice, especially if the groove patterns to be produced are complicated, the machining phase may include numerous steps of controlling the use of the laser in various ways. Moving the focal point may include moving the laser apparatus, otherwise controlling the aim of the laser apparatus, moving the crystal in relation with the laser apparatus, or a combination of any of these. 
         [0035]    If the laser machining leaves the grooves clean enough, it is possible that no etching is required, but for the sake of completeness a possible etching step is illustrated as step  505 . Here we also assume that the resist that was possibly applied in step  502  was there indeed for the purpose of protecting the desired remaining portions of the diffused surface, so the remaining resist can be removed in step  506 . Step  507  involves producing the required contacts and possibly other circuit elements on the patterned surface according to need, using techniques known as such. 
         [0036]    If mechanical sawing or plasma etching is used to produce the grooves, steps  503  and  504  are replaced by switching on the appropriate tooling apparatus and moving the tooling blade or plasma cannon in relation to the semiconductor crystal (again, all possible ways of causing relative movement are possible). 
         [0037]    It should be noted that even if we speak above about producing a single detector, it is in many cases most advantageous to handle an array of detector preforms simultaneously, so that they are all produced from a single cadmium telluride wafer that goes through the process illustrated in  FIG. 5 .