Patent Publication Number: US-7906360-B2

Title: Manufacturing process for a photodetector

Description:
FIELD OF THE INVENTION 
     The present invention relates to a manufacturing process for a photo-detector, and more particularly to a manufacturing process for a photo-detector with a metal-insulator-semiconductor structure, a P-type-intrinsic-N-type structure or a metal-semiconductor-metal structure formed on a carrier substrate having a germanium layer. 
     BACKGROUND OF THE INVENTION 
     Taiwan Patent No. 414930 taught that a MOS tunneling diode has been used in the photo-detector, but the detectable wavelength is restrained to the energy gap of the semiconductor material since the additional pairs of electrons and electrical holes are only produced when the energy of photons is higher than the energy gap of material. If a Si substrate is used, the cutoff wavelength thereof is 1.1 μm; and if a Ge substrate is used, the cutoff wavelength thereof is 1.85 μm. Therefore, the Ge material is preferable if 1.3˜1.55 μm of the photo-detection is desired. However, the application of a whole piece of Ge as substrates is highly expensive. 
     In addition, U.S. Pat. No. 5,374,564 disclosed a smart-cut process, which utilizes a hydrogen implanting technique to implant the hydrogen ions into the inner layers of the wafer, wherein the implanted depth is controlled by the implanting energy. Then, cut the wafer by means of the cleft characteristic of the implanted hydrogen ions being processed during high temperature. This kind of technique can cut the expensive and whole piece of Ge substrate into a plurality of thin Ge wafers. 
     Furthermore, in U.S. Pat. No. 6,833,195B1, Intel Corp. first implanted ions into the Ge substrates and activates the surfaces of the Ge and Si substrates, followed by bonding the Ge substrate and the Si substrate. As disclosed in the previous smart-cut process, a Si substrate including a thin Ge layer thereon is obtained. 
     In Applied Physics Letters 89, 101913 (2006), entitled “Low-temperature fabrication and characterization of Ge-on-insulator structures”, a kind of photo-detector is disclosed. The aim of the invention is to provide a photo-detector structure with a metal-insulator-semiconductor formed on a carrier substrate having a Ge layer and a manufacturing process thereof to lower down the expensive cost of using the Ge material. The above invention provides a photo-detector comprising a carrier substrate, a Ge layer, a first metal electrode, an insulating layer and a second metal electrode. The Ge layer is formed on the carrier substrate and includes a first area and a second area. The first metal electrode is formed on the first area, the insulating layer is formed on the second area, and the second metal electrode is formed on the insulating layer which is formed by means of a low temperature liquid phase deposit technique. Despite the invention utilizes the cheaper SiO 2 —Si substrate rather than the rare and expensive Ge material, there still exists a problem with a higher amount of dark current. In addition, further cheaper substrates, like glass are more desirable. 
     Moreover, ISDRS (International Semiconductor Device Research Symposium, 2003) published a paper entitled “MOS Ge/Si Quantum Dot Infrared Photodetectors with Quantum Dot and wetting Layer Responses” where the light currents are also reduced while the problem of the dark currents is improved by of the disclosed methods. 
     From the above description, it is known that how to provide a photo-detector capable of simultaneously enhancing the light current and reducing the dark current has become a major problem waited to be solved. In order to overcome the drawbacks in the prior art, a manufacturing process for a photo-detector is provided. The particular design in the present invention not only solves the problems described above, but also is easy to be implemented. Thus, the invention has the utility for the industry. 
     SUMMARY OF THE INVENTION 
     In accordance with one aspect of the present invention, a manufacturing process for a photo-detector is provided to solve the current problem of the higher amount of dark current existing in the photo-detector formed on the carrier substrate having a Ge layer. 
     The present manufacturing process for a photo-detector comprises the following steps: (a) providing a thin-film Ge on a cheap substrate including a first processing area and a second processing area; (b) defect-reduction processing at least one of the first processing area and the second processing area; and (c) forming a photo-detector element on the Ge. 
     Preferably, the present invention could simultaneously raise the light current and lower down the dark current. 
     Moreover, the present invention could perform a defect-reduction processing to one of the first processing area and the second processing area, and thus the preferable light response of the visible light and the invisible light. Multi-wavelength photo-detection can be also achieved on the same chip. 
     The above aspects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed descriptions and accompanying drawings, in which: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIGS. 1(   a )˜( g ) are section diagrams of the photo-detector according to a first preferable embodiment of the present invention; 
         FIG. 2  is a section diagram of the photo-detector according to a second preferred embodiment of the present invention; 
         FIG. 3  is a section diagram of the photo-detector according a third preferred embodiment of the present invention; 
         FIGS. 4(   a )˜( b ) are atomic force microscopy diagrams of the photo-detector processed with and without a SC-1 etching according to a first preferred embodiment of the present invention; 
         FIG. 5  is a variation diagram of the dark current and green light current versus the voltage of the insulator of the Ge layer  131  before and after etched; 
         FIG. 6  is a variation diagram of the wavelength versus the light response of the photo-detectors with and without etching; and 
         FIG. 7  is a diagram of the photo-detector where the first processing area and the second processing area are etched according to a fourth preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for the purposes of illustration and description only; it is not intended to be exhaustive or to be limited to the precise form disclosed. 
     Please refer to  FIGS. 1(   a )˜( g ), which show section diagrams of the photo-detector according to a first preferable embodiment of the present invention. 
     Please refer to  FIG. 1(   a ), the smart-cut process taught by U.S. Pat. No. 5,374,564 is adopted in the present invention. By means of the ion implanting technique, implant hydrogen ions  113  into an interface  112  of an N-type Ge substrate  111  under the condition of the energy 150 keV and the implanting quantity 1.5E17 (cm −2 ), wherein the implanting depth is related to the implanting energy and the implanting concentration is related to the processing temperature and period of the split wafer. 
     Please refer to  FIG. 1(   b ), which shows a glass substrate  910  provided as a carrier substrate  123 . 
     Then, the technique taught by U.S. Pat. No. 6,833,195B1 will be used in the following steps of the preferable embodiments of the present invention. 
     Sonicate the Ge substrate  111  and the glass substrate  910  with deionized water for 5 mins to remove the powder existing on the surface of the wafer. Immerse the glass substrate  910  in Standard Clean 1 (SC-1, NH 4 OH:H 2 O 2 :H 2 O˜0.5:1:5) at 80° C. for 15 mins and immerse the Ge substrate  111  in potassium hydroxide-deionized and purified water (KOH:H 2 O˜1:5) at 80° C. for 15 mins. Then, wash the respective Ge substrate  111  and the glass substrate  910  with deionized water for 5 mins, followed by performing a blowing therefor with high pressure and pure nitrogen gas, where the surface thereof become the hydrophilic surfaces filled with OH −  bonding. 
     Please refer to  FIG. 1(   c ), line up the bonding interface of the respective Ge substrate  111  and the glass substrate  910  and then directly bond the wafers at room temperature in the clean room. In other embodiments, the bonding of the wafers could be assisted by a viscous layer. 
     Please refer to  FIG. 1(   d ), place the bonded Ge substrate  111  and the glass substrate  910  in a purge environment under the atmosphere, followed by raising the temperature to 150° C. for 6 hours to enhance the strength of the wafer bonding. Then, raise the temperature to 200° C. for 40 mins to produce a wafer separation in the interface  112  that hydrogen ions implanted. The separated carrier substrate  123  includes a thin Ge layer  131  that has been successfully transferred and the separated Ge substrate  111  becomes a remaining Ge substrate  900  thinner than the original Ge substrate  111 . 
     In other embodiments, the Ge substrate  111  and the glass substrate  910  could be separated by heating for a period ranged from couple minutes to couple hours at a temperature ranged from 100° C. to 600° C. 
     Please refer to  FIG. 1(   e ). The Ge layer  131  could be divided into a first area  132 , a second area  133  and a third area  134  along the x-axis direction. Besides, the Ge layer  131  could be divided into a first processing area  135  and a second processing area  136  along the y-axis direction. Then, perform a defect-reduction processing to the first processing area  135  to reduce the surface defect thereof. In the embodiment, the surface defect of the first processing area  135  of the Ge layer  131  is processed by a SC-1 etching among the wet etching methods. In other embodiments the defect-reduction processing could be performed by other kinds of common wet etching methods or one of the following methods including a dry etching, a chemical mechanical polishing and a low-defect Ge layer re-growth. 
     In addition, only the first processing area  135  is processed to reduce the defect in the embodiment. However, the whole surface of the Ge layer could be processed to reduce both of the surface of the first and second processing area  135  and  136  in other embodiments. Next, dispose a photo-detector element onto the first processing area  135  and dispose another photo-detector element onto the second processing area  136 . In the first preferred embodiment, both of the photo-detectors formed on the Ge layer  131  are metal-insulator-semiconductor (MIS) elements and in other embodiments both of the photo-detectors could be P-type-intrinsic type-N-type (PIN) elements or metal-semiconductor-metal (MSM) elements. In addition to the mentioned photo-detector elements, any kinds of photo-detector elements could be formed on the Ge layer  131  for those skilled in the art. 
     Please refer to  FIG. 1(   f ). To form the MIS element of the first preferred embodiment on the Ge layer  131 , deposit the insulating layer  141  of 1.6 nm silicon dioxide as a tunneling gate insulator by means of a low temperature liquid phase deposition technique. Then, form a second metal electrode  142  on the insulating layer  141  by means of lithography, remove the insulating layer  141  on the first area  132  and the third area  134  by means of lithography and an etching technique, followed by forming a first metal electrode  143  on the mentioned removed area by coating aluminum thereon as an ohm contact point to form a photo-detector  140  of the present embodiment. Furthermore, the prior MIS photo-detector has different kinds of MIS structures and thus any kinds of MIS photo-detectors can be formed on the Ge layer  131  by a defect-reduction processing for those skilled in the art. 
     In the embodiment, the insulating layer  141  is formed through a low temperature liquid phase deposition technique. However, in other embodiments the present insulating layer  141  could be formed through a chemical vapor deposition or a physical vapor deposition. 
     Moreover, in this embodiment the insulating layer  141  is made of silicon dioxide. However, in other embodiments the insulating layer  141  could be made of high-dielectric materials. 
     Please refer to  FIG. 1(   g ). A power voltage  210  is provided. The power source  210  comprises a first electrode  211  and a second electrode  212 . The first electrode  211  is connected to the first metal electrode  143  and the second electrode is a switching circuit and connected to the second metal electrode  142  so that the second electrode  212  is optionally switched into connecting the second metal electrode  142  disposed on the first processing area  135  or into connecting the second metal electrode  142  disposed on the second processing area  136 . Further, the power source  210  provides a bias to generate a quantum tunneling effect so that the photo-detector  140  generates a photo-current upon being illuminated. 
     Please refer to  FIG. 2 , which shows a section diagram of the photo-detector according to a second preferred embodiment of the present invention. After finishing the defect-reduction processing to the Ge layer  131  (as illustrated in  FIG. 1(   e )), perform an ion implantation and bombard arsenic ions (As + ) into the first area  132  to form an N-type contact, followed by performing an ion implantation and bombarding boron ions (B − ) into the third area  134  to form a P-type contact. Then, dispose the first metal electrode  143  onto the first area  132  and dispose the second metal electrode  142  onto the third area  134  to form the P-type-intrinsic-N-type (PIN) photo-detector element of the second preferred embodiment. Furthermore, the prior PIN photo-detector has different kinds of PIN structures and thus any kinds of PIN photo-detectors can be formed on the Ge layer  131  by a defect-reduction processing for those skilled in the art. 
     Please refer to  FIG. 3 , which shows a section of the photo-detector according a third preferred embodiment of the present invention. After finishing the defect-reduction processing on the Ge layer  131  (as illustrated in  FIG. 1(   e )), dispose the first metal electrode  143  onto the first area  132  and dispose the second metal electrode  142  onto the third area  134  so that the metal-semiconductor-metal (MSM) photo-detector element of the third preferred embodiment is formed. Furthermore, the prior MSM photo-detector has different kinds of MSM structures and thus any kinds of PIN photo-detectors can be formed on the Ge layer  131  by a defect-reduction processing for those skilled in the art. 
     Please refer to  FIGS. 4(   a )˜( b ), which show atomic force microscope diagrams of the photo-detector processed with and without a SC-1 etching according to a first preferred embodiment of the present invention. 
     Please refer to  FIG. 4(   a ), which shows an atomic force microscope diagram of the second processing area  136  of the Ge layer  131  without a SC-1 etching. 
     Please refer to  FIG. 4(   b ), which shows an atomic force microscope diagram of the first processing area  135  of the Ge layer  131  processed with a SC-1 etching. As compared  FIG. 4(   a ) and  FIG. 4(   b ), it is known that the etched Ge layer  131  has a lower surface roughness. 
     Please refer to  FIG. 5 , which shows a variation diagram of the dark current and green light current versus the voltage of the photo-detectors with and without etching. It is known from the result of  FIG. 5  that the dark current after etched is highly reduced, whereas the green light current, whose wavelength is 532 nm, after etched is increased. 
     Please refer to  FIG. 6 , which shows a variation diagram of the wavelength versus the light response of the photo-detectors with and without etching. It is known that the light response being etched at the visible wavelength is increased. In addition, the light response being etched at the invisible wavelength turns out to be reduced. 
     Therefore, in the first preferred embodiment only parts of the Ge layer  131  is processed with a SC-1 etching. That is, only the first processing area  135  is processed with a SC-1 etching, which removes the defects formed during the smart-cut process and reduces the surface peaks of the Ge layer so that the response of the visible light can be highly enhanced since the probability of the recombination of the electron hole pairs can be reduced regarding the visible light with a less absorbing depth. In addition, the second processing area  136  retains a higher response of the infrared light due to a deeper active area since it does not process with a SC-1 etching. Therefore, in the first preferred embodiment only the first processing area  135  is processed with an etching and the photo-detector is selected from one disposed onto the first processing area  135  and the second processing area  136  by means of the second metal electrode  212  so as to achieve the multi-wavelength photo-detection. 
     Please refer to  FIG. 7 , which shows a diagram of the photo-detector where the first processing area and the second processing area are etched according to a fourth embodiment of the present invention. In the fourth preferred embodiment, the first processing area  135  and the second processing area  136  are both processed with a SC-1 etching to obtain a less dark current and acquire a better light current. After an etching process, form the insulating layer  141 , the second metal electrode  142  and the metal electrode  143  to form the MIS photo-detection element. The difference between the first embodiment and the fourth embodiment lies in the fourth embodiment has only one second metal electrode  142  rather than two of the first embodiment. 
     Furthermore, in the other embodiments PIN, MSM or other kinds of photo-detector elements can be disposed onto the Ge layer where the first processing area  135  and the second processing area  136  are processed with a SC-1 etching so that a smaller dark current is obtained and a better light current is also obtained. 
     In the present embodiments, the carrier substrate  123  can be a glass substrate, a Si wafer with silicon dioxide, a plastic substrate or other kinds of substrates capable of transferring thin Ge layer. 
     In the present embodiments, the Ge layer  131  is cleaved by n-type Ge substrate  111 . However, the Ge layer  131  can be cleaved by P-type or undoping substrate  111 , wherein the doping concentration can be adjusted depending on the necessity. The Ge substrate  111  can be deemed as a mono-crystalline substrate, a poly-crystalline substrate or a non-crystalline substrate and can be a [100] substrate, a [110] substrate or a [111] substrate. 
     In the present embodiments, the first and second metal electrodes  143  and  142  are made of aluminum and it can also be made of any metals. 
     While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiments. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.