Monolithic X-Ray Image Detector and Method of Manufacturing

1. Field of the Invention

This invention relates to x-ray imaging technology, and more particularly relates to a monolithic x-ray image detector for use with digital signal image processing.

2. Background Information

Recently there has been great interest in improving diagnostic techniques from x-rays. These efforts include sophisticated image signal processing to improve detection and diagnosis of cancer in patients. In this process, an x-ray image is digitized for processing by computer software. This technology has been developed because the resolution achieved with the present x-ray techniques, is only in the range of 100 to 200 dots per inch (dpi), and is dependent upon exposure and the development process. With digital image signal processing, significant improvements in the early detection diagnosis of cancer has been achieved.

In one application described in Physics Today, June 1995, Pages 21 and 22, digital image signal processing has revealed potentially cancerous calcifications that would probably be missed with the usual x-ray techniques. For example, current mammogram techniques only show micro calcifications in the range of 250 microns or greater; smaller tell-tale micro calcifications would be missed or require additional diagnostic procedures to verify. Digital image signal processing combined with current film techniques to digitize a mammogram, has shown promise of detecting micro-calcifications in the range of 50 to 100 microns. Digitizing an image in this matter, to improve detection of a weak signal in a complex background, resulted from signal processing to detect a faint star in a blurry image. This clearly suggests that an enhanced image can greatly improve the results of current imaging techniques.

Present x-ray techniques are also limited by the film to create the images. Not only is the image resolution limited, but large object images are not generally possible. Large scale imaging or full torso x-rays in medical applications as well as large object imaging for industrial applications, is needed.

Another disadvantage of present x-ray techniques is that the application of x-ray depends upon the film sensitivity. Thus, the dosage of x-rays has to be sufficiently high to produce a good,useful image even with currently developed digital image signal processing. Further, digitization of an x-ray image is a difficult and time-consuming process. A higher resolution image than those presently available from x-ray film, that is directly available for digital image signal processing or image construction would be a great advantage.

It is thus, one object of the present invention, to provide an x-ray image detector that will provide higher resolution than x-ray film.

Another object of the present invention is to provide an x-ray image detector that has higher sensitivity than the usual x-ray film permitting a lower dosage of x-rays.

Yet another object of the present invention is to provide an x-ray image detector that includes integrated read-out electronics so that an image is directly available for digital image signal processing.

Still another object of the present invention is to provide a monolithic x-ray image detector that is rugged and can be built into x-ray devices used on-site with portable x-ray equipment.

Still another object of the present invention is to provide a monolithic x-ray image detector comprised of P-type/intrinsic/N-type (PIN) diodes and read-out electronics produced on a high resistance silicon substrate which incorporates dielectric isolation providing three-dimensional integration.

Yet another object of the present invention is to provide x-ray imaging sensors that are constructed from detector elements that allow the die to be very closely abutted to provide edge-to-edge detection capabilities.

Still another object of the present invention is to provide an x-ray image detector having integrated read-out electronics and PIN diodes that permit die to be formed into panels for use in large object imaging.

The purpose of the present invention is to provide an x-ray imaging detector that has improved resolution and allows direct access to digital image signal processing technology.

The invention consists of a multi-layer silicon-based structure in which x-ray detectors are fabricated within the same substrate as the read-out electronics, resulting in a monolithic detector structure capable of supporting two dimensional imaging. With the x-ray imaging detectors fabricated according to the present invention, pixel density can be made very high--up to 600 dpi. This pixel density is considerably higher than other approaches which are currently in use or in development, and allow much greater resolution than with current x-ray films. Further, since the detector is monolithic (unlike all other approaches currently in use or development) the device can be easily manufactured in a cost effective manner.

The integration of read-out electronics into the die forming the x-ray imaging detectors, also allows pixel placement out to the die edge unlike chip designs in which input/output (I/O) pads are placed around the die periphery. Since the I/O connection is made on the other side of the die from detector elements, die can be placed very close to one another, allowing the construction of large area x-ray detecting panels. X-ray imaging die, constructed according to the invention, will be approximately 550 mils.times.550 mils and would contain roughly 100,000 pixels. This corresponds to about 600 dpi, which can be easily mapped into commercial laser printers in order to provide transparent or opaque hard copy output. The individual die can then be tiled into a panel assembly of sensors with adjacent die being very closely abutted by using multi-chip module technology. In this manner, large panels of sensors can be assembled that will be suitable for full torso x-ray imaging in medical applications, and large object imaging in industrial applications.

To produce the large panels, the x-ray image detector die are "flipped" upside down on a layer of Kapton and embedded in plastic. This arrangement will permit very close placement of adjacent die. The plastic is then cured and interconnect metalization is patterned on the read-out side of the x-ray image detector and outputs brought out to metal conductive pads. The multi-chip modules are then assembled onto printed circuit boards using a ball grid array (BGA) technology that allows close positioning and alignment of adjacent multi-chip modules. The resulting panel would be connected to processing electronics through a standard edge connector. The entire large panel assembly can be produced economically and could provide a form, fit and function replacement for film cassettes in existing x-ray equipment.

With the x-ray imaging detector die produced according to the invention, a detector that is highly sensitive, resulting in significant reductions in radiation exposure to people and equipment, can be produced. The x-ray imaging detector has wide application in areas such as: Digital mammography, high resolution chest x-rays, emergency room trauma assessment, intensive care units (ICUs), real-time imaging, materials analysis and evaluation, x-ray astronomy and real-time continuous surgical monitoring applications. The image x-ray detector represents an enabling technology for applications in real-time emergency trauma assessment, three-dimensional breast and tissue imaging (mammotomography), battle field and paramedic applications and large area biomedical imaging such as full body imaging, not presently supported by current film sizes and current film processing requirements. Spectrometry, based upon the energy of incident protons is also possible, thereby enabling accumulation of information not presently possible using film.

The x-ray imaging detectors are produced by simple cost effective manufacturing methods that are fully compatible with industry standard bulk silicon production tools. The detectors use silicon-on-insulator (SOI) substrate fabrication methods to fabricate unique high resistivity substrates in one of two possible ways. The first, and preferred method is a substrate created by using a Bond and Etch-Back Silicon-On-Insulator (BESOI) method. In this method, a first wafer is produced starting with a n+ wafer having a special high resistivity epitaxial silicon layer deposited onto it. The n+ wafer, with the high resistivity epitaxial silicon is then oxidized. A second p- wafer is then oxidized and placed with the oxide surfaces together on the first wafer having the special high resistivity epitaxial silicon. The wafers are then bonded by an oxidation process.

If desired, the p- silicon wafer thickness can be thinned down. Nominal thicknesses range is from 3,000 to 5,000 angstroms, but thicknesses from 500 to 30,000 angstroms can be used. If necessary, the n+ silicon wafer may be thinned to make the wafer conform to conventional wafter thickness or increase the detector's sensitivity to low energy photons if desired. This can be accomplished by one of many techniques, including plasma, chemical etching or mechanical polishing.

Another method of producing a substrate is by Separation by Implantation of Oxygen (SIMOX) in which an n+ wafer first receives a special high resistivity epitaxial silicon as before. An oxygen implant is then performed, typically about 1.6 to 1.8.times.10.sup.18 /cm.sup.2 at about 180 KeV. The oxygen implanted high resistivity epitaxial silicon layer on the n+ wafer is then annealed at about 1,300 to 1,350.degree. C. for six to eight hours. Again, it may be necessary to thin down the total wafer thickness by etching away some of the n+ layer. This can be accomplished by the techniques well known in the art described above.

The methods described above produce a substrate consisting of a highly doped n+ region bottom layer on top of which is formed a high resistivity silicon film. Above this high resistivity film, a layer of buried oxide lies beneath the thin film of silicon on the top surface in which the micro-electronics will ultimately be fabricated. The processing then utilizes conventional semiconductor device processing steps and tools to fabricate read-out electronics within the thin film of silicon.

Apertures are then etched through selected areas of the superficial silicon film and the underlying buried oxide barrier layer thereby exposing the top of the high resistivity silicon film. Anodes of the x-ray imaging detectors are then formed by implanting heavily doped p+ regions into the apertures exposing the portion of the high resistivity silicon to form the top of the vertical P-type/intrinsic/N-type (PIN) diode detectors. Standard back end processing is then used to complete the metalization and passivation of the structure. At the conclusion of this processing, the back of the wafer is ground smooth and a thin layer of metal is deposited to complete the electrical connections to the n+ layer of the vertical detectors. This method provides an inexpensive way to mass produce monolithic x-ray imaging detector chips with commercial semiconductor manufacturing techniques and environment.

The x-ray imaging detectors, produced by the method described above, include read-out electronics fabricated in the top thin silicon layer of the SOI substrate. The read-out electronics produce circuits, such as integrators and transimpedance amplifiers, which are required to transform the charge collected from the PIN diode sensors into an analog voltage. The circuitry is fabricated on a pitch of 20 to 40 microns. This would provide an x-ray image detector with considerably greater resolution than the present 100 to 200 dpi resolution of x-ray film, and which depends strongly upon the exposure and developing processes. The x-ray imaging detecting concept of the present invention will produce a three to six times better (9 to 36 times of the area) resolution than with film technology, and will provide data immediately available for image construction, transmission or digital image signal processing. The integrated x-ray image detector/read-out is also considerably more sensitive than present x-ray films, which would result in x-ray doses to the patient being significant reduced.

Another advantage of x-ray image detectors constructed according to the invention, is that the resulting data can be digitized to 12 bits (4096 levels) versus 8 bits (256 levels) available from post processing digitization of x-ray images. Thus the overall x-ray image detector constructed according to the invention can provide significant improvements in x-ray imaging.

The above and other novel features of the invention will be more fully understood from the following detailed description and the accompanying drawings, in which:

FIG. 1 is a partial cross section illustrating x-ray imaging detector device constructed according to the invention.

FIG. 2 is a diagram illustrating the fabrication of the novel SOI substrate by a Bond and Etch-Back Silicon-On-Insulator (BESOI) process.

FIGS. 3 and 3a illustrate a method of producing the novel SOI substrate by a Separation by Implantation of Oxygen (SIMOX) fabrication process.

FIGS. 4 through 4a illustrate the method of fabricating detector cells of the integrated x-ray image detector chip of FIG. 1.

FIGS. 5 and 5a and 5b illustrate the x-ray image detector die of the present invention being used to produce a large object x-ray imaging detector panel.

An x-ray image detector, constructed according to the invention, is shown in FIG. 1. In the detector shown, the read-out electronics are fabricated within the thin top silicon layer of the SOI substrate. These electronics are configured into circuits such as amplifiers which are required to transform the charge collected from a PIN diode sensor into an analog voltage. X-ray image detectors, constructed according to the invention, can be fabricated on a pitch of 20 to 40 microns. As shown in FIG. 1, an x-ray image detector 10 is fabricated on a substrate 12 which contains a buried dielectric layer 18 such as oxide as will be described in greater detail hereinafter. Substrate 12 is comprised of an intrinsic silicon layer 14 and thin etched silicon layer 16 and an oxide boundary layer or barrier 18.

The vertical diode structure is formed by heavily doped n+ layer 22 on one side of substrate 14, and p+ anode 24 in apertures or cavities 26 etched through oxide barrier 18. Silicided poly or metal contacts 26 are formed on anode 24 of the PIN diode to complete the read-out electronics. Anodes 24 of PIN diodes are formed of heavily doped p+ type silicon regions formed by implantation methods in apertures 26. The x-ray imaging detector dies can be formed to produce as many as 100,000 pixels on a die approximately 550 mils.times.550 mils. This corresponds to a resolution of about 600 dpi, which is advantageous because it can be easily mapped into commercial laser printers.

The preferred method of fabricating substrate 12 for the x-ray imaging detector is illustrated in FIG. 2. This method is called a Bond and Etch-back Silicon-On-Insulator method (BESOI). The process begins with an n+ wafer 28, which receives a deposition of special high resistivity epitaxial silicon 30. n+ wafer 28, with the special high resistivity epitaxial silicon 30 is then oxidized producing an oxide layer 34. A second p- wafer 32 is oxidized to also produce an oxide layer 34. The oxide layers 34 on n+ wafer 28 and p- wafer 32 are placed together. The oxide layers 34 on n+ wafer 28 and p- wafer are then bonded by an oxidation process to produce bonded oxide layer 34'. A p- silicon wafer 32 is then thinned down to a thickness of the desired value. This can be accomplished in several methods known in the art. The best method available is known as "Hughes Accuthin Technique." A nominal thickness is in the range of 3,000 to 5,000 angstroms, but thicknesses of 500 to 30,000 angstroms may be used. It may also be necessary to thin down n+ layer 28 if the total thickness is too great. Thinning of n+ layer 28 can be achieved by many techniques including plasma or chemical etching, or mechanical polishing.

Another method of creating substrate 12 called Separation by Implantation of Oxygen (SIMOX) is illustrated in FIGS. 3 and 3a. This method starts as before, with an n+ wafer 36 receiving a deposition of a special high resistivity epitaxial silicon layer 38. An oxygen implant is then performed; typically 1.6 to 1.8.times.10.sup.18 /cm.sup.2 at about 100 to 180 KeV, forming oxide layer 40. This will result in a construction illustrated in FIG. 3a comprised of an n+ layer 36, high resistivity layer 38 and thin silicon layer 40 separated by oxide barrier layer 41.

A substrate fabricated according to the methods shown in either FIG. 2 or FIG. 3 is then processed to produce a structure of the type having integrated x-ray imaging detectors or cells, as shown in FIGS. 4 and 4a. The steps listed in the table provided below, describe conventional semiconductor device fabrication plus the steps unique to the invention disclosed herein for forming the PIN diode detectors and mesa isolation.

* 1. Initial oxidation (200-400.ANG.)

2. Nitride deposition (1000-2000.ANG.)

* 3. Mesa pattern

* 4. Mesa etch (etch nitride/oxide/silicon stack down to buried oxide)

5. Strip photoresist

6. Sidewall oxidation (100-400.ANG.)

7. Strip nitride

8. Threshold adjust n-channel pattern

* 9. p-well implant

10. Vtn adjust implant

11. Strip photoresist

12. Threshold adjust p-channel pattern

*13. n-well implant

14. Vtp adjust implant

15. Strip Photoresist

16. Polysilicon-1 deposition (4000-5000.ANG.) (needed for capacitors)

17. Dope poly (POC13 diffusion or n+ implant/anneal)

18. Deposit capacitor dielectric (re-oxidized nitride or oxide typical) (300-500.ANG.)

19. Polysilicon-2 deposition (4000-5000.ANG.)

20. Dope poly (POC13 diffusion or n+ implant/anneal)

21. Capacitor (polysilicon-2) pattern

22. Etch polysilicon-2

23. Deposit capacitor dielectric (re-oxidized nitride or oxide typical (300-500.ANG.)

24. Polysilicon-1 pattern

25. Etch capacitor dielectric

26. Etch Polysilicon-1

27. Strip photoresist

*28. Strip oxide

*29. Gate oxidation (50-250.ANG.)

*30. Polysilicon-3 deposition (4000-5000.ANG.)

*31. Dope poly (POC13 diffusion or n+ implant/anneal)

*32. Polysilicon-3 pattern

33. Polysilicon-3 etch

34. Strip photoresist

35. Re-oxidation of polysilicon and source/drain regions

36. Detector pattern

37. Detector etch (etch through buried oxide to underlying silicon)

38. Strip photoresist

39. pFET source/drain and detector pattern

40. p-LLD implant (boron, 10.sup.13 /cm.sup.2)

41. Strip photoresist

42. nFET pattern

43. n-LDD implant (phosphorus or arsenic, 10.sup.13 /cm.sup.2)

44. Spacer deposition (oxide or nitride, 1500-3000.ANG.)

45. Spacer etch (RIE anisotropic etch)

*46. pFET source/drain and detector pattern

*47. p+ SD implant (boron, 10.sup.15-16 /cm.sup.2) and detector implant)

48. strip photoresist

*49. nFET pattern

*50. n+ SD implant (phosphorus or arsenic, 10.sup.15-16 /cm.sup.2)

51. Implant activation/anneal (850.degree. C., inert, 30 minutes)

52. Oxide strip

53. Titanium deposition (200-1000.ANG.)

54. Silicide formation (rapid thermal anneal or furnace, 600-800.degree. C.) to reduce polysilicon and diffusion sheet resistances

55. Interlevel dielectric deposition (may consist of undoped oxide, PSG, BSG, BPSG, TEOS, nitride)

56. Denisification/reflow (diffusion, oxidizing or steam, 800-1000.degree. C.)

57. Contact pattern and etch

58. Strip Photoresist

59. Metal-1 deposition

60. Metal-1 pattern and etch

61. Strip photoresist

62. Via pattern and etch

63. Strip photoresist

64. Repeat 59-63 for metal-2 and metal-3, if applicable

65. Final passivation

66. Bond pad pattern and etch

67. Strip photoresist

The steps above, indicated by an asterisk (*) are unique to forming the x-ray image detector and PIN detectors according to the invention disclosed herein.

The formation of the image detector is briefly shown in the diagrams of FIGS. 4 and 4a. A plurality of mesas 42 and 44, are patterned and etched (steps 3 and 4) and then receive p- well and n+ well implants respectively (steps 9 and 13). Alternatively, mesa pattern and etch steps can be replaced by lateral isolation approach which uses Local Oxidation of Silicon (LOCOS) instead. After annealing, patterning and forming polysilicon gates, detector apertures 46 are then etched through oxide layer or barrier 18 (FIG. 1). The p+ SD (source/drain) and n+ SD implants are then patterned and performed. Contacts are then provided by patterning, etching and deposition to produce metal contacts 26 connected to PIN diode detectors. Standard back end processing and deposition of a thin layer of metal 48 on the back surface completes the processing.

The pattern, etch and implant steps 35 through 45 of p- LDD (lightly doped drain) and n- LLD, and formation of spacers are optional, but preferred steps in the fabrication of the x-ray imaging detector 10. In the approach described above, the p+ region of the PIN detector is doped by steps 40 and 47. However, the p+ region of the detectors may be doped by a separate and additional p+ implant if desired. This pattern and implant would be best performed after completion of the p+ SD implant, but could be implemented at various points within the process. The above process flow/description creates triple-plate capacitors out of polysilicon layers. There are several other methods for integrating capacitors into the circuit. The description above represents one possible, but preferred approach.

At the conclusion of processing, the back of the x-ray imaging detector fabricator is ground smooth and a thin layer of metal 48 is deposited to complete the electrical connection to the vertical detectors. The method described above provides an inexpensive way to mass produce monolithic x-ray imaging detectors using commercial semiconductor manufacturing tools and environments that produce cell pitches as small as 20 microns. Thus, the x-ray imaging detector can be used in applications that will benefit from fine pitch read-out electronics and pixel densities.

In operation, the x-ray imaging detector absorbs x-rays in intrinsic silicon region 14 (FIG. 1) generating hole-electron pairs which drift and diffuse to PIN diode anodes 24 and cathodes 22 (FIG. 1) respectively. The resulting electrical currents are transformed to analog voltage levels by the read-out cell associated with each anode and routed off the die for analog-to-digital conversion and subsequent signal processing. In essence, the detector performs a real-time conversion of an x-ray exposure into electronic signals that can be addressed pixel-by-pixel. By converting signals to a digital format, they can be read and subjected to sophisticated data analysis and visualization techniques immediately upon exposure to x-rays.

One such application would be the production of an x-ray imaging detector panel assembly that could be used in large object imaging. An x-ray imaging detector panel, constructed in this manner, is illustrated in FIGS. 5 and 5a. An x-ray imaging panel 50 is comprised of an array of x-ray imaging detecting modules 52 mounted on a printed circuit board 54, having a standard edge connector 56. X-ray imaging detecting modules 52 would be constructed using multi-chip module technology and advanced assembly techniques allowing individual x-ray imaging detector die 10 (FIG. 1) to be tiled into panel assembly 50 of detectors with adjacent die very closely abutted. Thus, large panels of x-ray image detecting die 10 can be assembled to permit full torso x-ray imaging in medical applications and large field imaging in industrial applications.

Multi-chip modules 52 will be built using high density interconnect technology such as that available using General Electric (GE) technology. X-ray imaging detector die 10 (FIG. 1) would be "flipped" upside down with the read-out side of the detector on a layer of Kapton and metal die interconnect 56 and embedded in plastic substrate layer 58. This arrangement will permit very close placement of adjacent image detector die 10. Multi-chip modules 52 would be assembled into printed circuit boards with Ball Grid Array (BGA) technology that allows close positioning and alignment of adjacent modules connected by solder ball interfaces 55 to printed circuit board 54. Ball grid array (BGA) assembly is readily available commercially. The resulting panel 50 can be connected to processing electronics through standard edge connector 56. A particular advantage of this system is that it can be produced very economically and could be used to provide a form, fit and function replacement of film cassettes in existing x-ray equipment.

Thus, there has been disclosed a novel x-ray image detector comprised of a high resistivity SOI substrate with integrated read-out electronics and PIN diode detectors. The monolithic detector array disclosed and the method of manufacture allow die to be produced with an edge-to-edge construction so they can be arranged in sensor modules. The x-ray detector or image sensor provides substantial increase in resolution and sensitivity allowing low dosages and the use of digital imaging analysis.

This invention is not to be limited by the embodiment shown in the drawings and described in the description which is given by way of example and not of limitation, but only in accordance with the scope of the appended claims.