Abstract:
Neutron detectors, advanced detector process techniques and advanced compound film designs have greatly increased neutron-detection efficiency. One embodiment of the detectors utilizes a semiconductor wafer with a matrix of spaced cavities filled with one or more types of neutron reactive material such as  10 B or  6 LiF. The cavities are etched into both the front and back surfaces of the device such that the cavities from one side surround the cavities from the other side. The cavities may be etched via holes or etched slots or trenches. In another embodiment, the cavities are different-sized and the smaller cavities extend into the wafer from the lower surfaces of the larger cavities. In a third embodiment, multiple layers of different neutron-responsive material are formed on one or more sides of the wafer. The new devices operate at room temperature, are compact, rugged, and reliable in design.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims the benefit of U.S. provisional application Ser. No. 60/422,148, filed Oct. 29, 2002 and entitled “High Efficiency Neutron Detectors.” 

   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   This invention was made with Government support under Contract No. W-31-109-ENG-38 from the U.S. Department of Energy (DOE). The Government has certain rights in this invention. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates to neutron detectors, methods of making same, and in particular, to high-efficiency neutron detectors and methods of making same. 
   2. Background Art 
   Semiconductor detectors coated with neutron reactive materials offer an alternative approach to scintillator-based neutron imaging devices for neutron radiography (normally scintillating screens coupled to photographic film or to other photorecording devices). The detectors also offer an alternative to He-3 proportional counters, BF3 proportional counters, Li-loaded glasses, and other scintillator-based systems for neutron detection. Neutron reactive film-coated devices investigated in previous works include Si, SiC, GaAs, and diamond detectors, all of which have advantages and disadvantages as described in references 1–6 noted at the end of this section. 
   The converter films attached to semiconductor devices most often used for neutron-detection utilize either the  6 Li(n,α) 3 H reaction or the  10 B(n,α) 7 Li reaction. Due to low chemical reactivity, the most common materials used are pure  10 B and  6 LiF. Neutron reactive films based on the  157 Gd(n,γ) 158 Gd reaction show a higher neutron absorption efficiency than  10 B(n,α) 7 Li and  6 Li(n,α) 3 H-based films, however the combined emission of low energy gamma rays and conversion electrons from  157 Gd(n,γ) 158 Gd reactions make neutron-induced events difficult to discriminate from background gamma-ray events. As a result, Gd-based films are less attractive for devices where background gamma ray contamination is a problem. Alternatively, the particle energies emitted from the  6 Li(n,α) 3 H and the  10 B(n,α) 7 Li reactions are relatively large and produce signals easily discernable from background gamma ray noise. Thus far, thermal neutron-detection efficiencies have been limited to only 4% for  6 LiF and  10 B single-coated devices. 
   Expected Efficiency of Conventional  10 B and  6 Li Coated Detectors 
   The  10 B(n,α) 7 Li reaction leads to the following reaction products, as described in reference 7 noted at the end of this section: 
                     10     ⁢   B     +           0   1     ⁢   n       -&gt;     
     ⁢                         Reaction   ⁢           ⁢   Q   ⁢     -     ⁢   Value     _               {                       7     ⁢   Li     ⁡     (     at   ⁢           ⁢   1.015   ⁢           ⁢   MeV     )       +     α   ⁡     (     at   ⁢           ⁢   1.777   ⁢           ⁢   MeV     )         ,                     Li       *           7     ⁡     (     at   ⁢           ⁢   0.840   ⁢           ⁢   MeV     )       +     α   ⁡     (     at   ⁢           ⁢   1.470   ⁢           ⁢   MeV     )         ,                           2.792   ⁢           ⁢     MeV   ⁡     (     to   ⁢           ⁢   ground   ⁢           ⁢   state     )                             2.310   ⁢           ⁢     MeV   ⁡     (     1   ⁢   st   ⁢           ⁢   excited   ⁢           ⁢   state     )                           
which are released in opposite directions when thermal neutrons (0.0259 eV) are absorbed by  10 B. After absorption, 94% of the reactions leave the  7 Li ion in its first excited state, which rapidly de-excites to the ground state (˜10 −13  seconds) by releasing a 480 keV gamma ray. The remaining 6% of the reactions result in the  7 Li ion dropping directly to its ground state. The microscopic thermal neutron absorption cross section is 3840 barns. Additionally, the microscopic thermal neutron absorption cross section decreases with increasing neutron energy, with a dependence proportional to the inverse of the neutron velocity (1/v ) over much of the energy range, as described in references 8 and 9.
 
   The  6 Li(n,α) 3 H reaction leads to the following products: 
                               Reaction   ⁢           ⁢   Q   ⁢     -     ⁢   Value     _                                 6     ⁢   Li       +           0   1     ⁢   n       ⁢     -&gt;   3     ⁢       H   ⁡     (     at   ⁢           ⁢   2.73   ⁢           ⁢   MeV     )       +     α   ⁡     (     at   ⁢           ⁢   2.05   ⁢           ⁢   MeV     )           ,             4.78   ⁢           ⁢   MeV                         
which again are oppositely directed if the neutron energy is sufficiently small. The microscopic thermal neutron (0.0259 eV) absorption cross section is 940 barns. The thermal neutron absorption cross section also demonstrates a 1/v dependence, except at a salient resonance above 100 keV, in which the absorption cross section surpasses that of  10 B for energies between approximately 150 keV to 300 keV, as described in references 8 and 9. Additional resonances characteristic to either isotope cause the absorption cross section to surpass one or the other as the neutron energy increases. Due to its higher absorption cross section, the  10 B(n,α) 7 Li reaction leads to a generally higher reaction probability than the  6 Li(n,α) 3 H reaction for neutron energies below 100 keV. However, the higher energy reaction products emitted from the  6 Li(n,α) 3 H reaction lead to greater ease of detection than the particles emitted from the  10 B(n,α) 7 Li reaction.
 
   The term “effective range” (denoted L) is the distance through which a particle may travel within the neutron reactive film before its energy decreases below the set minimum detectable threshold, or rather, before its energy decreases below the electronic lower level discriminator (LLD) setting. The term does not take into account additional energy losses from contact “dead regions.” The neutron reaction products released do not have equal masses, and therefore do not have equal energies or effective ranges. Neutrons may interact anywhere within the reactive film, and the reaction products lose energy as they move through the neutron reactive film. Reaction product self-absorption reduces the energy transferred to the semiconductor detector, and ultimately limits the maximum film thickness that can be deposited over the semiconductor device. The measured voltage signal is directly proportional to the number of electron-hole pairs excited within the semiconductor. Reaction products that deposit most or all of their energy in the detector will produce much larger voltage signals than those reaction products that lose most of their energy before reaching the detector. 
   The energy absorbed in the detector is simply the original particle energy minus the combined energy lost in the boron film and the detector contact during transit. At any reaction location within the reactive film, a reduced energy will be retained by either particle that should enter the detector, being the maximum possible if the trajectory is orthogonal to the device contact. Hence, if the interaction occurs in the  10 B film at a distance of 0.5 μm away from the detector, the maximum energy retained by the  7 Li ion when it enters the detector will be 430 keV, and the maximum energy retained by the alpha particle will be 1250 keV, as described in references 10 and 11. For the same interaction distance of 0.5 μm from the detector, the energy retained by the particle when it reaches the detector decreases as the angle increases from orthogonal (&gt;0°). Given a predetermined minimum detection threshold (or LLD setting), the effective range (L) for either particle can be determined For instance, an LLD setting of 300 keV yields L Li  as 0.810 microns and L α  as 2.648 microns, as described in references 10 and 11. Similar conditions exist for  6 LiF and  6 Li films. 
   A commonly used geometry involves the use of a planar semiconductor detector, generally indicated at  10 , over which a neutron reactive film  11  has been deposited, as shown in  FIG. 1 . Upon a surface of the semiconductor detector is attached a coating that releases ionizing radiation reaction products  12  upon the interaction with a neutron  13 . The ionizing radiation reaction products  12  can then enter into the semiconductor material  14  of the detector  10 , thereby creating a charge cloud  15  of electrons and “holes,” which can be sensed to indicate the occurrence of a neutron interaction within the neutron sensitive film. The charges  15  are swept through the detector through the application of a voltage, which is applied through the use of conductive contacts  16  and  17  upon the surfaces of the semiconductor detector, where the surfaces are generally parallel to each other. 
   Assuming that the neutron beam is perpendicular to the detector front contact  16 , the sensitivity contribution for a reaction product species can be found by integrating the product of the neutron interaction probability and the fractional solid angle, defined by the reaction product effective ranges subtending the device interface, as described in references 10 and 11, which yields: 
                       S   p     ⁡     (     D   F     )       =     0.5   ⁢     F   p     ⁢     {         (     1   +     1       ∑   F     ⁢   L         )     ⁢     (     1   -     ⅇ     -       ∑   F     ⁢     D   F             )       -       D   F     L       }         ⁢     
     ⁢         for   ⁢           ⁢     D   F       ≤   L     ,   and             (     1   ⁢   a     )                     S   p     ⁡     (     D   F     )       =     0.5   ⁢     F   p     ⁢     ⅇ     -       ∑   F     ⁢     (       D   F     -   L     )           ⁢     {         (     1   +     1       ∑   F     ⁢   L         )     ⁢     (     1   -     ⅇ     -       ∑   F     ⁢   L           )       -   1     }         ⁢     
     ⁢         for   ⁢           ⁢     D   F       &gt;   L     ,             (     1   ⁢   b     )               
where Σ F  is the macroscopic neutron absorption cross section, D F  is the film thickness, and F P  is the branching ratio of the reaction product emissions. The total sensitivity accordingly can be found by adding all of the reaction product sensitivities:
 
                       S   ⁡     (     D   F     )       ⁢     ❘   Total       =       ∑     p   =   1     N     ⁢       S   p     ⁡     (     D   F     )           ,           (   2   )               
where N is the number of different reaction product emissions. In the case of  10 B-based films, N equals 4. Notice from equation 1b that the value of S P  reduces as D F  becomes larger than the value of L. As a result of this, there will be an optimum neutron reactive film thickness for front-irradiated detectors. Since the minimum particle detection threshold determines the effective range (L), the optimum film thickness is also a function of the LLD setting. For example, with the LLD set at 300 keV, the maximum achievable thermal neutron-detection efficiency is 3.95%. The thermal neutron-detection efficiency can be increased to 4.8% by lowering the LLD setting, but only at the expense of accepting more system noise and gamma-ray background interference, as described in references 1, 10 and 11. Similar cases exist for  6 LiF and pure  6 Li films. Using an LLD setting of 300 keV, obverse detector irradiation yields maximum thermal neutron-detection efficiencies of 4.3% for  6 LiF-coated devices and 11.6% for pure  6 Li-coated devices.
 
   Increasing the efficiency can be achieved by intimately attaching two coated devices such that they are facing each other, as shown in  FIG. 2 . Between the two semiconductor detectors is placed a coating  20  that releases ionizing radiation reaction products  21  upon the interaction with a neutron  22 . The ionizing radiation reaction products  21  can then enter into the semiconductor material  23  of either or both detectors, thereby creating a charge cloud  24  of electrons and “holes,” which can be sensed to indicate the occurrence of a neutron interaction within the neutron sensitive film  20 . The charges are swept through the detector through the application of a voltage, much like the case shown in  FIG. 1 , which is applied through the use of conductive contacts  25  upon the surfaces of the semiconductor detector, where the surfaces are generally parallel to each other. 
   The design does not rely on the full depletion of the detectors and can be operated with modest operating voltages. The most straightforward method for producing such a device is to simply fasten two front-coated devices together. If the neutron reaction film thickness is thin, coincident charged particle emissions from a single neutron absorption event can be measured simultaneously by both detectors if operated individually, thus giving rise to the erroneous recording of two neutron interaction events when only one actually occurred. Erroneous “double counts” can be eliminated by connecting both devices to a single preamplifier, in which a single event always registers as only one count on the preamplifier circuit. Thermal neutron-detection efficiencies of 24% can be reached for pure  6 Li-coated sandwich devices. 
   Morphological Improvements for Improved Efficiency 
   Neutron-detection efficiency is limited for single coated devices since charged particle reaction products have limited ranges in the neutron reactive thin film. The detector design of  FIG. 3  and U.S. Pat. No. 6,545,281 addresses two methods to improve neutron-detection efficiency, both using morphological alterations: (1) to increase the overall surface area of the device, and (2) to increase the statistical probability that the charged particle reaction product will enter the detector sensitive region. 
   Tiny holes (only one of which is shown in  FIG. 3  at  30 ) may be etched into the top substrate surface, as described in U.S. Pat. No. 6,545,281, using reactive etching techniques, a method that allows for very precise and accurate control of miniature dimensions, as also described in reference 12. Holes etched into the top surface of a material are filled with the neutron reactive material  32 , which serves to increase the probability that charged particle reaction products will enter the active region of the detector  34 . 
   From the previous discussion, the maximum probability that a single charge particle product can enter the detector active region is 50%, which corresponds to neutron absorption events that occur at the  10 B film/detector interface. By etching a trench into the substrate, the charged particle entrance probability is increased for absorption events that occur in the trench region. Hence, by simply etching trenches into the substrate material before administering the metal contacts  36  and the neutron-sensitive films, the overall detection efficiency of the device can be increased. Yet, charged particle reaction products can still be emitted in trajectories parallel or nearly parallel to the trenches, thereby never coming into contact with the active semiconductor detector  34 . The difficulty is resolved by making the trenches circular in shape, or rather, tiny holes that are etched into the device surface. Holes etched into the surface of a material can be filled with the neutron reactive material, which serves to increase the probability that charged particle reaction products will enter the active region of the detector. 
   Tiny holes can be precisely etched into semiconductors with very high aspect ratios (exceeding 10:1) with various dry etching techniques such as reactive ion etching (RIE), as described in reference 12. The processes use precision photolithography and VLSI thin film techniques, hence the placement of tiny holes is straightforward. Preliminary calculations indicate that the tiny hole diameters should be on the order of the total added charged particle range. The hole sizes are theoretically optimized by making their working diameters exactly the same total effective length L for both charged particle reaction products. Using  10 B as an example, the hole diameters should be approximately
 
 D=L   Li   +L   α ,
 
such that no matter what lateral direction that the charged particles are emitted, one or the other particle will enter the detector. For instance, the value of L Li +L α =3.458 microns in pure  10 B, hence the optimized hole diameters should be approximately 3.5 microns for  10 B coated devices. It follows that the hole diameters should be approximately 30 microns for  6 LiF films and 100 microns for  6 Li films. Etching tiny holes with depths up to 12 microns can be done simply for hole diameters of only 3.5 microns, an aspect ratio of less than 3.5:1. The vertical direction can be optimized from equations 1a and 1b. Still, charged particles can escape detection under some circumstances, but, as depicted in  FIG. 3 , the probability of one or the other charged particle reaction product entering the detector is tremendously increased. The entire upper surface may be processed such that an optimized matrix of holes covers the entire device upper surface, as shown in U.S. Pat. No. 6,545,281.
 
   The actual contact on the semiconductor devices can be produced by various means, including implantation as described in reference 13, epitaxial growth as described in references 14 and 15, and evaporation or sputtering as described in references 13–15. All of these methods have been explored and developed. The contacts can be made with the low voltage self-biased design, as described in reference 16, or the highly radiation hard Schottky barrier design, as described in references 1 and 17. 
   REFERENCES 
   1. D. S. McGregor et al., “Semi-insulating Bulk GaAs Thermal Neutron Imaging Arrays,” IEEE TRANS. NUCL. SCI., NS-43 (1996), p. 1357. 
   2. A. Rose, “Sputtered Boron Films on Silicon Surface Barrier Detectors,” NUCL. INSTR. AND METH., 52 (1967), p. 166. 
   3. B. Feigl et al., “Der Gd-Neutronenzähler,” NUCL. INSTRL. AND METH., 61 (1968), p. 349. 
   4. A. Mireshghi et al., “High Efficiency Neutron Sensitive Amorphous Silicon Pixel Detectors,” IEEE TRANS. NUCL. SCI., NS-41 (1994), p. 915. 
   5. F. Foulon et al., “Neutron Detectors Made from Chemically Vapour Deposited Semiconductors,” PROC. MRS, 487 (1998), p. 591. 
   6. A. R. Dulloo et al., “Radiation Response Testing of Silicon Carbide Semiconductor Neutron Detectors for Monitoring Thermal Neutron Flux,” REPORT 97-9TK1-NUSIC-R1, Westinghouse STC, Pittsburgh, Pa. (Nov. 18, 1997). 
   7. G. F. Knoll, “Radiation Detection and Measurement,” 3rd Ed. (Wiley, New York, 2000). 
   8. D. I. Garber et al., “BNL 325: Neutron Cross Sections,” 3rd Ed., Vol. 2, Curves (Brookhaven National Laboratory, Upton, 1976). 
   9. V. McLane et al., “Neutron Cross Sections,” Vol. 2, (Academic Press, San Diego, 1988). 
   10. D. S. McGregor et al., “Thin-Film-Coated Bulk GaAs Detectors for Thermal and Fast Neutron Measurements,” NUCLEAR INSTRUMENTS AND METHODS, A466 (2001), pp. 126–141. 
   11. D. S. McGregor et al., “Design Considerations for Thin Film Coated Semiconductor Thermal Neutron Detectors; Part I: Basics Regarding Alpha Particle Emitting Neutron Reactive Films,” NUCL. INSTRUM. AND METH., A 500 (2003), pp. 272–308. 
   12. P. R. Puckett et al., “Thin Film Processes II,” Chapter V-2, J. L. Vossen and W. Kern, Eds., (Academic Press, Boston, 1991), p. 749. 
   13. S. M. Sze, “VLSI Technology,” (McGraw-Hill, New York, 1983). 
   14. W. S. Ruska, “Microelectronic Processing,” (McGraw-Hill, New York, 1987). 
   15. S. Wolf et al., “Silicon Processing for the VLSI Era,” (Lattice Press, Sunset Beach, 1986). 
   16. D. S. McGregor et al., “Self-Biased Boron-10 Coated High Purity Epitaxial GaAs Thermal Neutron Detectors,” IEEE TRANS. NUCLEAR SCIENCE, 47 (2000), pp. 1364–1370. 
   17. R. T. Klann et al., “Development of Coated GaAs Neutron Detectors,” CONFERENCE RECORD OF ICONE-8, 8th International Conference on Nuclear Engineering, Apr. 2–6, 2000, Baltimore, Md. USA. 
   18. D. S. McGregor et al., “New Surface Morphology for Low Stress Thin-Film-Coated Thermal Neutron Detectors,” IEEE TRANS. NUCLEAR SCIENCE, 49 (2002), pp. 1999–2004. 
   19. See the MEMS Exchange website at:
         http://www.mems-exchange.org/.       

   20. See the NIST website for the specifications on the inductively coupled plasma reference cell at:
         http://physics.nist.gov/MajResProj/rfcell/drawings.html.       

   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide neutron detectors that have detection efficiencies many times greater than present neutron detectors while remaining relatively thin, compact and rugged. The detectors detect neutrons of any energy including thermal neutrons. 
   Another object is to provide improved methods of making such neutron detectors. 
   In carrying out the above objects and other objects of the present invention, an apparatus for efficiently detecting neutrons is provided. The apparatus includes a particle-detecting first substrate having first and second surfaces spaced apart by a region of the substrate and a plurality of cavities extending into the substrate from the first and second surfaces. Neutron-responsive material is disposed in the plurality of cavities. The material is responsive to neutrons absorbed thereby for releasing ionizing radiation reaction products. The neutron-responsive material disposed in the cavities at the first and second surfaces increases neutron-detection efficiency by increasing the likelihood that the reaction products will be directed into the substrate for increased neutron-detection efficiency. 
   The apparatus may further include a first contact layer disposed on the first surface, and a second contact layer disposed on the second surface. 
   The first and second contact layers may be disposed in the plurality of cavities. 
   The plurality of cavities may include etched via holes, trenches or other types of cavities. 
   The neutron-responsive material may be disposed as at least one layer on the first and second surfaces. 
   The neutron-responsive material disposed in the cavities at the first surface may be of a first type different from a second type of neutron-responsive material disposed in the cavities at the second surface. 
   The neutrons may be thermal neutrons. 
   The cavities extending into the substrate from one of the surfaces may be surrounded by cavities extending into the substrate from the other of the surfaces. 
   The first substrate may be composed primarily of a semiconductor material. 
   The semiconductor material may be silicon, silicon carbide, gallium arsenide, gallium nitride, indium phosphide, cadmium telluride, cadmium-zinc-telluride, gallium phosphide, mercuric iodide, or lead iodide. 
   The apparatus may further include a particle-detecting second substrate having neutron-responsive material disposed in cavities of the second substrate and stacked on the first substrate. 
   The neutron-responsive material may be disposed as layers between the substrates. 
   Different types of neutron-responsive material may be disposed in each of the plurality of cavities. 
   The neutron-responsive material may be disposed as a plurality of layers of different types of neutron-responsive material on both the first and second surfaces. 
   Further in carrying out the above objects and other objects of the present invention, an apparatus for efficiently detecting neutrons is provided. The apparatus includes a particle-detecting first substrate having first and second surfaces spaced apart by a region of the substrate. A first contact layer is disposed on the first surface of the substrate. A second contact layer is disposed on the second surface of the substrate. A first stack of neutron-responsive layers includes a first layer of neutron-responsive material of a first type disposed on the first contact layer and a second layer of neutron-responsive material of a second type different from the first type disposed on the first layer. Both of the materials are responsive to neutrons absorbed thereby for releasing ionizing radiation reactive products. 
   The first layer may have a higher neutron interaction cross section than the second layer. 
   The first and second types of material may include elemental or compound forms of lithium, lithium fluoride, boron, gadolinium, cadmium, U 235 , Pu and Th. However, it is to be understood that any coating that produces a charged particle, light or photon will work as a coating as long as the product interacts with the substrate. 
   The first substrate may be composed primarily of a semiconductor material. 
   The semiconductor material may be silicon, silicon carbide, gallium arsenide, indium phosphide, cadmium telluride, cadmium-zinc-telluride, gallium phosphide, mercuric iodide, or lead iodide. 
   The apparatus may further include a particle-detecting second substrate having at least one neutron-responsive layer disposed on a contact layer of the second substrate and stacked on the first substrate. 
   The layers of neutron-responsive material may be disposed between the substrates. 
   The apparatus may further include a stack of neutron-responsive layers disposed on the second contact layer. 
   Still further in carrying out the above objects and other objects of the present invention, an apparatus for efficiently detecting neutrons is provided. The apparatus includes a particle-detecting first substrate having spaced first and second surfaces and a plurality of different-sized cavities extending into the substrate from the first and second surfaces. Neutron- responsive material is disposed in the plurality of different-sized cavities. The material is responsive to neutrons absorbed thereby for releasing ionizing radiation reaction products. The neutron-responsive material disposed in the different-sized cavities at the first and second surfaces increases neutron-detection efficiency by increasing the likelihood that the reaction products will be directed into the substrate for increased neutron-detection efficiency. 
   The apparatus may further include a first contact layer disposed on the first and second surfaces, and a second contact layer disposed on a third surface of the substrate spaced apart from the first and second surfaces by a region of the substrate. 
   The first contact layer may be disposed in the plurality of cavities. 
   The plurality of cavities may include etched large and small via holes, large and small trenches or other types of large and small cavities. 
   The neutron-responsive material may be disposed as a layer on the first and second surfaces. 
   The neutron-responsive material disposed in the cavities at the first surface may be of a first type different from a second type of neutron-responsive material disposed in the cavities at the second surface. 
   The neutrons may be thermal neutrons. 
   The holes may be generally circular in cross section. 
   The neutron-responsive material may be elemental or compound forms of lithium, lithium fluoride, boron, gadolinium, cadmium, any form of plastic, U 235 , Pu or Th. 
   The first substrate may be composed primarily of a semiconductor material. 
   The semiconductor material may be silicon, silicon carbide, gallium arsenide, gallium nitride, indium phosphide, cadmium telluride, cadmium-zinc-telluride, gallium phosphide, mercuric iodide, or lead iodide. 
   Relatively small cavities may extend into the first substrate from one of the surfaces and may be disposed within relatively large cavities extending into the first substrate from the other surface. 
   The apparatus may further include a particle-detecting second substrate having neutron-responsive material disposed in a plurality of different-sized cavities in the second substrate and stacked on the first substrate. 
   The neutron-responsive material may be disposed as layers between the substrates. 
   At least two of the layers may be of different neutron-responsive material. 
   Yet still further in carrying out the above object and other objects of the present invention, an apparatus for efficiently detecting neutrons is provided. The apparatus includes a particle-detecting first substrate having first and second surfaces spaced apart by a first region of the first substrate and a first set of cavities extending into the first substrate from the first surface. A particle-detecting second substrate is stacked on the first substrate and has first and second surfaces spaced apart by a second region of the second substrate and a second set of cavities extends into the second substrate from the first surface of the second substrate. The apparatus further includes neutron-responsive material disposed in the first and second sets of cavities and on the first surfaces of the substrates. The material is responsive to neutrons absorbed thereby for releasing ionizing radiation reaction products. The neutron-responsive material disposed in the first and second sets of cavities increases neutron-detection efficiency by increasing the likelihood that the reaction products will be directed into the first and second substrates, respectively, for increased neutron-detection efficiency. 
   The first and second sets of cavities may not be aligned to further optimize neutron absorption. 
   The apparatus may further include a first contact layer disposed on the second surface of the first substrate and a second contact layer disposed on the second surface of the second substrate. 
   Yet still further in carrying out the above object and other objects of the present invention, a method of making a high-efficiency neutron detector is provided. The method includes providing a particle-detecting substrate having a first surface and a plurality of cavities extending into the substrate from the first surface. The plurality of cavities is filled with a neutron-responsive material. The method further includes forming a thick film of the neutron-responsive material over the first surface including the plurality of cavities wherein the cavities relieve stress in the thick film to prevent delamination of the thick film from the first surface. The material is responsive to neutrons absorbed thereby for releasing ionizing radiation reaction products. The neutron-responsive material disposed in the cavities at the first surface increases neutron-detection efficiency by increasing the likelihood that the reaction products will be directed into the substrate for increased neutron-detection efficiency. 
   The thick film may exceed one micron in thickness. 
   Further in carrying out the above object and other objects of the present invention, a method of making a high-efficiency neutron detector is provided. The method includes providing a particle-detecting first substrate having a first surface and a plurality of cavities extending into the substrate from the first surface, and filling the plurality of cavities with a neutron-responsive material. The material is responsive to neutrons absorbed thereby for releasing ionizing radiation reaction products. The neutron-responsive material disposed in the cavities at the first surface increases neutron-detection efficiency by increasing the likelihood that the reaction products will be directed into the substrate for increased neutron-detection efficiency. The step of filling includes the step of spreading the material on the first surface and vibrating the material into the plurality of cavities. 
   The material may be submicron powder. 
   The above object and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a side, sectional, schematic view showing the basic construction of a coated semiconductor neutron detector from the prior art; neutrons interact in the coating, thereby releasing detectable charged particles; for  10 B and  6 Li-based films, only one particle from the interaction can enter the detector; 
       FIG. 2  is a side, sectional, schematic view showing a double-inward facing “sandwich” design of the prior art; a single neutron sensitive film is placed between the active regions of two semiconductor diode detectors; 
       FIG. 3  is a perspective schematic view of a prior art detector which uses a depression filled with neutron reactive material in a semiconductor substrate to increase the neutron sensitivity of the device; 
       FIG. 4  is a side, sectional, schematic view of a stacked film semiconductor neutron detector of the present invention; the device incorporates multiple neutron sensitive films in order to increase the device sensitivity to neutrons of various energies; the films may consist of, but are not restricted to, boron, lithium, lithium fluoride, gadolinium and cadmium; 
       FIG. 5  is a side, sectional, schematic view of a double-inward stacked film detector of the invention showing the use of two different neutron-sensitive film coatings between two detectors; 
       FIG. 6  is a side, sectional, schematic view of a double-inward multi-stacked detector of the invention showing that a series of various neutron-sensitive film coatings can be inserted between two detectors; 
       FIG. 7  is a perspective schematic view, partially broken away, of the double-sided porous design showing that deep holes can be etched into a semiconductor; the holes can be fashioned in one side of the substrate such that they are surrounded by holes from the opposite side of the substrate material; afterward, the holes may be filled with neutron reactive material; 
       FIG. 8  is a perspective schematic view, partially broken away and in cross section, showing a double-sided porous design with deep holes etched from both sides; from the cut-away view, the tiny holes are filled with neutron reactive materials such as  10 B or  6 LiF; the holes may be filled with different materials, such as  10 B on one side and  6 LiF on the other side; 
       FIG. 9  is a side, sectional, schematic view of the double-sided porous design showing a possible configuration for the conductive electrode placement and the voltage application; 
       FIG. 10  is a side, sectional, schematic view of the double-sided porous design showing the direction of the electric field lines for the electrode configuration of  FIG. 9 ; a conductive layer is applied to both sides, such as through doping or evaporation, such that it forms a diode; an applied bias causes the field lines to run perpendicular to the holes thereby increasing the active surface adjacent to the reactive film; 
       FIG. 11   a  is a side, sectional, schematic view of the double-sided porous design with a multi-stacked compound film; 
       FIG. 11   b  is an enlarged view of a circled portion of  FIG. 11   a;    
       FIG. 12  is a side, sectional, schematic view illustrating a compound detector design composed of two double-sided porous design detectors fastened in a sandwich configuration; 
       FIG. 13  is a side sectional view of an alternative double-sided porous design showing another possible electrode configuration; 
       FIG. 14  is a side sectional view of the alternative double-sided porous design from  FIG. 13  showing the direction of the electric field lines; a blocking contact is applied as an anode such that the diode interface forms only at the top plane of the substrate; an applied bias causes the field lines to run parallel to the holes, thereby decreasing the capacitance while retaining the high-efficiency of the unique porous design; 
       FIG. 15  is a conceptual side sectional view of an integrated sandwich/double-layered/perforated neutron detector; shown is a composite utilizing multi-layered neutron reactive films, filled holes of various sizes, attached together in the sandwich design; 
       FIG. 16  is a perspective schematic view, partially broken away and in cross-section, of a detector in which deep holes are etched and filled with a neutron reactive material; 
       FIG. 17  is a side view of a unit cell of  FIG. 16 ; 
       FIG. 18  is a perspective schematic view of a semiconductor having etched slots or trenches; 
       FIG. 19  is a perspective schematic view of a trenched detector having deep trenches filled with a neutron reactive material; 
       FIG. 20  is a side view of a unit cell of  FIG. 19 ; 
       FIG. 21  is a perspective schematic view showing the fastening of two trenched devices to obtain a sandwich design of  FIG. 22 ; 
       FIG. 22  is a perspective schematic view of a sandwich design wherein semiconductor walls or “fingers” on a first device are arranged to align with slots of another device facing the first device; 
       FIG. 23  is a side view of the sandwich design of  FIG. 22 ; and 
       FIG. 24  is a perspective schematic view, similar to the double-sided hole design of  FIG. 9 , of a trench design with interwoven trenches. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Multi-Layered Films for Improved Efficiency 
   Referring now to  FIG. 4 , an embodiment of the invention is shown in which the simplistic prior art approach for the coating of a detector has been modified to incorporate multiple films  40  and  41 . The neutron reactive films  40  and  41  may include, but are not limited to, various compounds and concentrations of boron, lithium, lithium fluoride, gadolinium and cadmium. Fissionable materials such as, but not limited to U 235 , Pu, Th are also applicable. The device can have two or more different neutron reactive films  40  and  41  placed upon the detector surface. The semiconductor material  42  may be composed of a variety of materials, including, but not limited to, silicon, silicon carbide, gallium arsenide, gallium nitride, indium phosphide, cadmium telluride, cadmium-zinc-telluride, gallium phosphide, mercuric iodide, lead iodide, and variations of these aforementioned semiconductors. The detector has conductive contacts  43  and  44 , one contact  43  placed upon a first semiconductor surface and another contact  44  placed upon a second semiconductor surface of the same semiconductor material  42  block, the second surface being generally parallel to the first surface. A voltage can be applied across the semiconductor block by means of the conductive contacts  43  and  44 . 
   In particular, the device of  FIG. 4  incorporates a double-layered film, such as with  6 LiF on  10 B or  6 Li on  10 B. The  10 B(n,α) 7 Li reaction products  45  have shorter ranges within  10 B material than do  6 Li(n,α) 3 H reaction products. Hence, it is possible to attach a coating of  10 B followed by a coating of either  6 LiF or  6 Li. The charged particle reaction products emanating from  10 B(n,α) 7 Li reactions can reach the detector as before since the film is applied directly to the detector contact  43 . In addition, the longer range charged particle products emanating from  6 Li(n,α) 3 H reactions can still reach the detector even though they must transit the  10 B film as well. Since the  10 B film has a higher neutron interaction cross section than  6 LiF or  6 Li films, a net gain is realized. Hence, the short product range and high cross section material  41  is deposited closest to the contact  43  while the longest range and lowest cross section material  40  is placed atop the first film  41 . The opposite case renders no improvement, and in fact actually decreases efficiency. Overall, a  6 LiF/ 10 B system yields a maximum efficiency of 6.9% and a  6 Li/ 10 B system yields a maximum efficiency of 12.9%. 
   A comparison of measured results from a conventional boron-coated detector and a double-layered film detector, in which the device had a boron film deposited directly upon the detector, and a LiF film deposited upon the boron film shows that the device neutron-detection efficiency far exceeded that of the simple boron-coated device, and it far exceeded the theoretical maximum values of the simple coated device in  FIG. 1 . 
   The layered film concept can be improved further by combining the double-layered film detectors of  FIG. 4  in a double-inward facing sandwich design, as shown in  FIG. 5  wherein the second detector has a prime designation for its components. Further improvements include multiple layered films upon a single device, again constructed in the double-inward facing sandwich design, as shown in  FIG. 6  wherein the second detector has a double prime designation for its components. A third type of neutron reactive material is indicated by reference number  46 . 
     FIG. 5  illustrates a double-layered, double-inward facing sandwich detector that incorporates the advantages of stacking the neutron-sensitive films. The neutron reactive films may include, but are not limited to, various compounds and concentrations of boron, lithium, lithium fluoride, gadolinium and cadmium. The device can have two or more different neutron reactive films placed between the semiconductor detectors. The semiconductor material may be composed of a variety of materials, including, but not limited to, silicon, silicon carbide, gallium arsenide, gallium nitride, indium phosphide, cadmium telluride, cadmium-zinc-telluride, gallium phosphide, mercuric iodide, lead iodide, and variations of these aforementioned semiconductors. Each semiconductor detector has conductive contacts, one contact placed upon a single semiconductor surface and another contact placed upon a second semiconductor surface of the same semiconductor material block, the second surface being generally parallel to the first surface. A voltage can be applied across each semiconductor block by means of the conductive contacts. A voltage may be applied separately to each semiconductor detector or may be applied together across both semiconductor detectors, as shown in  FIG. 5  in the parallel configuration. 
     FIG. 6  illustrates a multi-layered, double-inward facing sandwich detector that incorporates the advantages of stacking the neutron sensitive films. The neutron reactive films may include, but are not limited to, various compounds and concentrations of boron, lithium, lithium fluoride, gadolinium and cadmium. The device can have two or more different neutron reactive films placed between the semiconductor devices. The semiconductor material may be composed of a variety of materials, including, but not limited to, silicon, silicon carbide, gallium arsenide, indium phosphide, cadmium telluride, cadmium-zinc-telluride, gallium phosphide, mercuric iodide, lead iodide, and variations of these aforementioned semiconductors. Each semiconductor detector has conductive contacts, one contact placed upon a single semiconductor surface and another contact placed upon a second semiconductor surface of the same semiconductor material block, the second surface being generally parallel to the first surface. A voltage can be applied across each semiconductor block by means of the conductive contacts. A voltage may be applied separately to each semiconductor detector or may be applied together across both semiconductor detectors, as shown in  FIG. 6  in the parallel configuration. 
   The Double-Sided Porous Design 
   Improvements in dry etching methods with inductively coupled plasma systems, as described in references 12, 19 and 20, allow for precision holes to be etched completely through Si semiconductor wafers. The standard processes are developed and commercial systems are available for use, including systems within The University of Michigan MEMS Exchange collaboration. 
   In another embodiment of the invention, holes  70  are etched from both the front and the back of semiconductor material  72  to produce an overall matrix or pattern, where holes  70  from one side are located between and are surrounded by holes  70  from the opposite side, as shown in  FIG. 7 . While the matrix may take any form, a hexagonal pattern produces better geometric packing. The holes  70  can be etched such that they do not reach all the way through the device. A p-n junction will be formed at the surface and in the holes of only one side of the device, thereby forming a diode. Afterward, both front and back holes  70  are filled with neutron reactive material  74  followed by a final coat  76  of neutron reactive material over the surfaces, as shown in  FIGS. 8 and 9 .  FIG. 8  illustrates a cut-away view of a planar semiconductor block through which tiny holes have been etched from both sides. Neutron reactive substances have been deposited within the holes from both sides. The neutron reactive materials on each side may be the same or similar. The neutron reactive materials on either side may be of entirely different materials. 
     FIG. 9  illustrates a side view of a planar semiconductor block through which tiny holes have been etched from both sides. Neutron reactive substances have been deposited within the holes from both sides. The neutron reactive materials on either side may be of entirely different materials. The conductive contacts  78  for the device are fabricated on opposite surfaces and within the holes. Voltage can be applied across the opposite conductive contacts. 
     FIG. 10  depicts a cross section view of the concept, in which it is shown that the electric field will extend across the small regions between the holes and not just the parallel faces of the wafer. A single device utilizing either  10 B or  6 LiF can achieve thermal neutron-detection efficiencies greater than 25%, reaching over 50% for a sandwich design. Furthermore, with high purity silicon as the semiconductor base, the voltage (and power) requirement to operate the detector will be under 100 volts, since the active width resides between the holes and not just the front and back of the wafers When using n-type silicon material, a p-n junction contact can be formed by several methods, all of which will be explored herein, thereby establishing a most reliable fabrication process. 
   As previously mentioned,  FIG. 10  illustrates a side view of the planar semiconductor block from  FIG. 9 , through which tiny holes have been etched from both sides. Neutron reactive substances have been deposited within the holes from both sides. The neutron reactive materials on each side may be the same or similar. The neutron reactive materials on either side may be of entirely different materials. The conductive contacts for the device are fabricated on opposite surfaces and within the holes. With voltage applied across the detector, the electric field lines  79  are perpendicular to the etched holes. 
   For instance, methods by which the contacts can be formed include: 
   Traditional Diffusion Method. Hole etched wafers are arranged in a quartz (or SiC) boat on edge with boron nitride solid wafer sources in between each wafer. The boat is then inserted into a high-temperature furnace and heated to 1150° C. A carrier gas (such as nitrogen) flows into the furnace and carries boron atoms to the surface of the silicon wafers, which then diffuses into the semiconductor surface to form a p-n junction. If the holes are very deep, then stagnation with the nitrogen flow may cause some difficulty with doping deep in the holes. 
   Dry Powder and Gas Method. One side of the hole etched wafer is lightly coated with  10 B material with an evaporator as shown in reference 18. The wafer is aligned such that boron enters and coats the hole walls of one side of the wafer only. Afterward,  10 B powder is inserted into the holes as previously described such that they are filled. The wafers are then inserted into a high temperature furnace with typical diffusion gases (such as nitrogen) and heated to 1150° C. for approximately 30 minutes. The thermal treatment causes boron, a p-type dopant in silicon, to diffuse into the n-type material thereby forming a p-n junction. 
   Implantation Method. Hole etched wafers are sent to a typical ion implantation company where boron ions are implanted directly into the holes. The process requires careful alignment of the wafers such that the holes are parallel with the ion beam. Rotation of the wafers during implantation improves uniformity. Afterward, the wafers are annealed such that the implanted boron atoms are activated, which form the p-n junction. 
   One drawback of the design is the possibility of a high capacitance from the increased surface area and decreased effective electrode width. Hence, the spacing between the holes could be optimized not only to increase the neutron-detection efficiency, but to also decrease the device capacitive noise. An alternative approach that will render good neutron-detection efficiency while retaining low capacitance is shown in  FIG. 13 . Here it is shown that holes  80  are etched as depicted in  FIGS. 7 and 8 . 
   As shown in  FIG. 14 , under operation the electric field lines  88  run parallel to the holes  80  and the capacitance is now determined by the substrate width and not the hole spacing. The only drawback is that the depletion region of the device must extend across most of the detector bulk.  FIG. 14  illustrates a side view of the planar semiconductor block from  FIG. 13 , through which tiny holes  80  have been etched from both sides. Neutron reactive substances  86  have been deposited within the holes from both sides. The neutron reactive materials on each side may be the same or similar. The neutron reactive materials on either side may be of entirely different materials. The conductive contacts  82  for the device are fabricated on opposite surfaces and are not fabricated within the tiny holes  80 . With voltage applied across the detector, the electric field lines  88  are perpendicular to the planar semiconductor detector surfaces. 
   Utilization of pure  6 Li material for the neutron converter will yield the highest efficiency results, and estimates indicate that thermal neutron-detection efficiencies for a single device can exceed 30%. A sandwich device should easily exceed 60% thermal neutron-detection efficiency. Pure Li metal is highly reactive and decomposes easily, hence a durable encapsulation method is required. Furthermore, inserting pure Li into the holes is not as easy as with boron or LiF powders. Lithium metal is very malleable with a low melting point (180° C.); hence the material can be applied to the semiconductor surface and melted into the holes. Also highly diffusive, a durable diffusion barrier must be applied to the semiconductor before the lithium treatment. The process can be performed in an inert gas thereby preventing decomposition of the Li during the treatment. Afterward, a thick overcoat of encapsulant, such as zirconium, is evaporated over the entire device (front and back) so as to prevent decomposition of the lithium metal. 
   The High-Efficiency Integrated Designs 
   Integrating the efficiency enhancing features described in the earlier sections results in a remarkable device capable of exceeding 35% thermal neutron-detection efficiency for a device only 1 mm thick.  FIGS. 11   a ,  11   b ,  12  and  15  show the basic concepts. 
     FIGS. 11   a  and  11   b  illustrate side views of a planar semiconductor block  50  through which tiny holes  52  have been etched from both sides.  FIG. 11   b  is an enlarged view of a section of the detector profile. Multi-layered neutron reactive substances have been deposited within the holes  52  from both sides. The neutron reactive materials on each side may consist of two or more layers  53 ,  54 ,  55  and  56 . The neutron reactive film layering on either side may be similar. The neutron reactive film layering on either side may be of different materials or sequence of materials. The conductive contacts  57  and  58  for the device are fabricated on opposite surfaces and within the holes  52 . Voltage can be applied across the opposite conductive contacts  58 . 
     FIG. 12  illustrates a sandwich detector design utilizing two detectors described in  FIG. 9 . The sandwich detectors may, alternatively, consist of detectors as illustrated in  FIGS. 11   a  and 11 b.    
     FIG. 15  illustrates a side view of a compound semiconductor neutron detector, generally indicated at  90 . The compound detector  90  has more than one diameter size of hole, within which the small holes  91  are etched over the entire surface including the large holes  92 . Conductive contacts  93  are placed over the etched surfaces to fill the small and large holes  91  and  92 , respectively, over which multiple layers  94  and  95  of neutron reactive materials are placed to fill the small and large holes  91  and  92 , respectively. The etched and coated semiconductor devices  96  and  97  are arranged in a double-inward sandwich design. 
   In one possible design, as shown in  FIG. 15 , the semiconductor substrate  98 , such as Si, SiC, GaAs or GaN is etched such that miniature holes  91  (approximately 3.5 microns in diameter) cover the devices  96  and  97 . Additionally, larger diameter holes  92  cover the devices  96  and  97  and some of the smaller holes  91  are within the larger holes  92 . The devices  96  and  97  are coated with conductive layers  93  and  99 , such as Au, Pd, Pt, Ti and combinations, mixtures and alloys thereof, easily accomplished with either evaporative or sputter deposition, over which a first layer  94  of  10 B is deposited.  10 B material is subsequently deposited into the small holes with ultrasonic vibration. Afterward, a thicker layer  95  of pure  6 Li is deposited over the devices  96  and  97  such that it fills the larger holes  92 . The devices  96  and  97  are then pressed together such that they face each other. The end result is a remarkably efficient and compact neutron detector  90  that can yield over 35% thermal neutron-detection efficiency for a device no thicker than 1 mm. 
   All of the devices in the previous sections can be formed into thermal neutron imaging arrays. Since VLSI technology is used to form the detectors, it is a straightforward extension to fabricate arrays of any or all of the devices discussed. 
   Preliminary experiments demonstrated a 10% increase in efficiency when 10.6% of a device surface was covered with 5.0-micron deep holes, all filled with  10 B, which increased the thermal neutron-detection efficiency from 3.0% up to 3.3%, as described in reference 18. Based on the results, calculations indicate that 10-micron deep holes covering 40% of the surface area yields a thermal neutron efficiency of 6.4%, yielding a 194% increase in efficiency. Powder filling is a less efficient method of introducing boron into the tiny holes than thin film methods such as sputtering, hence the density of material can be increased which will further increase the detection efficiency. The efficiency can be increased even higher by increasing the hole depth, leading to more neutron absorption interactions in which more charged particle reaction products can enter into the tiny hole walls. It is possible to achieve thermal neutron efficiencies greater than 12% with the configuration, a remarkable efficiency for a radiation hard device that is only a few hundred microns in total thickness. 
   A substrate-chilled evaporation technique has been developed to deposit thin films of  10 B onto devices with successful results. Evaporated boron thin films have high stress, and films exceeding one micron in thickness usually delaminate and peel away from the device. Tiny holes patterned over the semiconductor surface relieve the stress, thereby making very thick films easy to manufacture by eliminating the delamination problem. The holes can be filled by a variety of methods, including evaporation deposition, sputter deposition, and ultrasonic vibration of fine powders into the holes. Two techniques to fill the holes with neutron reactive material have been demonstrated with success. The first method involves the use of electron-beam evaporator in which the boron or LiF material is directly deposited into the holes and over the device surface. For holes deeper than 10 microns, powder filling has proven to be effective. The process involves the use of submicron  10 B powder (particle size distributions ranging from 0.5 microns to 1.8 microns can be purchased through commercial vendors). The boron powder is spread over the device and ultrasonically vibrated into the tiny holes. 
   The technology presently used to etch the GaAs wafers incorporates reactive ion etching (RIE) in which chemical etchant ions are accelerated toward the semiconductor surface. The method uses capacitive coupling and has proven to work, but is a slow process with rates of only one micron per hour for a standard etch process. Capacitive-coupled RIE is slow to etch due to the limitation in power that can be applied. Newer methods incorporating inductively coupled plasmas (ICP) allow for cooler temperatures, higher operating voltages, higher electron densities, and much higher etching rates than capacitive-coupled RIE. 
   Although Si seems an obvious choice for the neutron detectors, other semiconductors have advantages as well. For instance, the “stopping power” of Si is less than GaAs, hence the charged particle reaction products can be absorbed in a smaller region with GaAs than with Si. For instance, a 1.5 MeV alpha particle is fully absorbed within 5.8 microns of Si, but only 4.2 microns of GaAs. Since it is the material between the holes that absorbs the reaction product energy, a much denser matrix of holes can be formed in a GaAs substrate than a Si substrate. The end result is a higher overall neutron-detection efficiency for GaAs than with Si. Yet, the gamma ray background noise will be higher for GaAs than for Si, hence the designs for either Si devices or GaAs devices must be optimized for the detection application. 
   Either Si or GaAs can be used for general-purpose neutron detectors, but both have a radiation hardness limit of 10 12  n/cm 2  before degradation becomes noticeable. SiC, however, has been shown to more radiation had than GaAs or Si, as described in reference 6, and it is far less sensitive to gamma ray background noise. Furthermore, with its band gap energy of 3.0 eV, SiC can be used as a neutron detector in elevated temperature environments, as also described in reference 6. 
     FIG. 16  is a cut-away view of a perforated semiconductor neutron detector in which deep holes  108  are etched into a surface of the semiconductor material  102  and filled with neutron reactive material ( 114  in  FIG. 16 ) such as  10 B or  6 LiF. A back contact is indicated at  104 . The periodic structure of the holes  108  allows for analysis of a single unit cell  106 , as shown by the doted line surrounding a single hole that has been filled with the neutron reactive material  114 . The cell  106  is a square, hence all four sides of the cell  106  as shown have the same length. 
     FIG. 17  is a side view of the unit cell  106  of  FIG. 16  wherein the neutron reactive material  114  fills a hole having a diameter  116  and a depth  112  in the semiconductor  102 . A cap of the material  114  has a depth  110 . Cell length and width is indicated at  118 . For a LiF-filled detector with circular holes 300 microns deep and with a cell of 50 microns×50 microns, a hole diameter of 30 microns and no cap layer, the efficiency is approximately 17.5%. With a 10 or 20 micron cap layer, the efficiency increases to 20.5%. This is for frontal irradiation. 
   For backside irradiation, with a cell of 50 microns×50 microns, a hole diameter of 30 microns and no cap layer, the efficiency is approximately 19.5%. With a 30 micron cap layer, the efficiency increases to 22.5%. 
     FIG. 18  shows slots or trenches  126  etched into the top surface of semiconductor material  124  in the same manner that holes can be etched into the semiconductor surface. Upper and lower contacts  120  and  122 , respectively, are also provided. 
     FIGS. 19 and 20  show a trenched or slotted semiconductor neutron detector, generally indicated at  135 , in which deep trenches  138  are etched into the surface of semiconductor material  130  and filled with neutron reactive material  140  such as  10 B or  6 LiF. The periodic structure of the trenches  138  allows for analysis of a single unit cell  132 , as shown by the dotted line surrounding outlining a single slot  138  that has been filled with the neutron reactive material  140 . The lateral dimension  136  of the cell  132  perpendicular to the slots  138  is used as the reference. A cap layer  134  is formed at the top surface and a contact  131  at the bottom surface.  FIG. 20  is a side view of the unit cell  132  wherein cap depth is indicated at  142 , trench width at  146 , cell width at  136  and trench depth at  144 . For example, for frontal radiation, with a cell of 50 microns wide, a trench width of 25 microns and no cap layer, the efficiency is approximately 24.5%. With a 10 micron cap layer, the efficiency slightly increases to 25%. 
   For backside radiation, with a cell of 50 microns wide, a trench width of 25 microns and no cap layer, the efficiency is approximately 25.5%. With a 20 micron cap layer, the efficiency increases to 27%. Backside irradiation allows for slightly higher efficiency than frontal irradiation. 
     FIGS. 21 and 22  show a sandwich design of two detectors  135  and  135 ′ wherein the detector  135 ′ is substantially identical to the detector  135 , as indicated by the prime designation. The resulting device efficiency is further increased by the sandwich design in which the two trench devices  135  and  135 ′ are fastened facing each other. 
   Semiconductor “fingers” on one device  135 ′ are arranged to align with the slots of the other device  135  facing it, as shown in  FIG. 22 . In other words, to optimize neutron absorption, the semiconductor fingers from one device  135 ′ can be placed over the trenches of the other device 135. 
     FIG. 23  shows oppositely facing devices  135  and  135 ′ in which the fingers of one device align with the trenches of another device. This allows for neutron reactive material to completely obscure the path of neutrons impinging perpendicular to the device planes. The expected thermal neutron intrinsic detection efficiency can be increased above 50% for the sandwich design depicted in  FIG. 23 . 
   Similar to the previously described double-sided hole design, a trench design of  FIG. 24  has interwoven trenches  156  etched into a semiconductor  154  from both top and bottom surfaces. A unit cell  150  is shown as well as a cap layer  152  and a contact  158  as before. 
   While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.