Abstract:
A method of non-thermal annealing of a silicon wafer comprising irradiating a doped silicon wafer with electromagnetic radiation in a wavelength or frequency range coinciding with lattice phonon frequencies of the doped semiconductor material. The wafer is annealed in an apparatus including a cavity and a radiation source of a wavelength ranging from 10-25 μm and more particularly 15-18 μm, or a frequency ranging from 12-30 THz and more particularly 16.5-20 THz.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This is a continuation of patent application Ser. No. 11/112,643 filed Apr. 22, 2005, now U.S. Pat. No. 7,176,112 granted Feb. 13, 2007. 

   FIELD OF THE INVENTION 
   The invention relates to the annealing of crystalline materials, in general, and to a thermal annealing of doped semiconductor materials, in particular. 
   BACKGROUND OF THE INVENTION 
   After ion implantation, semiconductor wafers are annealed to remove any damage including near-surface lattice modification caused by the implantation and to activate dopants. Annealing treatments are intended to reduce or eliminate damage to the semiconductor material. For example, annealing occurs when the wafer is exposed to high temperatures ranging from 800° C. to 1100° C. Also, short pulses of laser or electron beam radiation can be used to heal damage to semiconductor crystals that occur during the implantation of impurities. However, prior art annealing techniques, especially high temperature annealing, may result in unwanted diffusion of impurities deteriorating quality of the resulting semiconductor device. 
   Thus, low temperature annealing techniques for promoting healing of damaged semiconductor regions are desired. For example, mechanical energy is used to anneal semiconductor wafers. As described in U.S. Pat. No. 6,001,715 to Manka et al., large amplitude sound waves created by laser ablation are used to anneal crystalline materials. 
   It is an object of the present invention to anneal semiconductors doped by ion-implantation or other doping methods which cause damage to the wafer. 
   It is an object of the present invention to provide a non-thermal process of annealing a doped semiconductor material. 
   It is an object of the present invention to provide a process of annealing a semiconductor material implanted with dopants. 
   It is an object of the present invention to provide a new and improved process of annealing semiconductors. 
   SUMMARY OF THE INVENTION 
   These and other objects have been met by a method of non-thermal annealing of a silicon material comprising irradiating a doped silicon material, such as a wafer, with electromagnetic radiation in a wavelength or frequency range coinciding with phonon frequencies in the doped semiconductor material. Although ideal pure crystals have a distinct phonon spectrum with very sharply defined wavelengths (see absorption peaks of  FIG. 1 ) that would need to be matched precisely by an external irradiation source to achieve phonon resonance, real-world silicon wafers used for manufacturing integrated circuits contain impurities, deliberately introduced dopants, and crystal lattice defects that tend to spread out the phonon absorption bands. Therefore, a wavelength in the spread out phonon absorption region of a doped semiconductor material will cause sufficient phonon resonance to produce annealing of the material. 
   Wavelengths in the phonon absorption region of semiconductor materials having impurities, dopants, and/or crystal lattice defects range from 10-25 μm and more particularly from 15-18 μm (corresponding to a frequency range of 12-30 THz and more particularly from 16.5-20 THz). This wavelength or frequency is in the extreme end of the microwave region and coincides with the phonon absorption region of the spectrum for annealing. Too short a wavelength (or too high a frequency) i.e. less than 10 μm, and the dominant absorption becomes free carrier absorption with significant heating. Likewise, too low a frequency (or too long a wavelength) less than 12 THz, and other heat-inducing absorption mechanisms will be dominant. Note that the invention operates at a frequency far above an ordinary microwave oven of 2.45 GHz frequency and far longer than the visible light or near infrared wavelengths (&lt;1 or 2 μm) of laser thermal annealing or Rapid Thermal Processing (RTP). 
   For non-thermal annealing of the present invention, the semiconductor material is placed in an apparatus including a cavity and a radiation source of a particular wavelength. In one embodiment, for maximum annealing effect, the semiconductor material should be positioned so that it coincides with the plane of a standing-wave peak. However, simply placing the wafer in the cavity in any desired position will suffice since the phonon absorption bands are relatively spread out. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an absorption spectrum of intrinsic silicon in the range of 5 to 35 μm. 
       FIG. 2  is a simplified diagrammatic view of the annealing process of the present invention. 
       FIG. 3  is a perspective view of an apparatus for carrying out an annealing of a semiconductor according to the present invention. 
       FIG. 4  is an absorption spectrum of a typical semiconductor in various absorption ranges including a phonon band absorption range. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   With reference to  FIG. 2 , the method according to the present invention comprises providing a typical semiconductor material  10 , for example silicon or another crystalline material which has a doped region. In addition to purposefully introduced dopants, the material may include other impurities. After having undergone doping by a process (such as ion implantation) which damages the semiconductor material, the material is annealed to repair the damage by exposing the material to electromagnetic radiation in a wavelength or frequency range coinciding with a phonon frequency of the doped semiconductor material. An electromagnetic wave having a frequency coinciding with the phonon frequencies of the doped semiconductor material is indicated by the schematic beam  14  and is incident on the material  10 . Reference numeral  12  indicates the position or region of the dopant after ion implantation or other doping process. Upon material exposure to the beam  14  within the frequency or wavelength coinciding with the phonon frequencies of the doped semiconductor material, the semiconductor material  10  is annealed. Enhanced phonon frequency is assumed to be responsible for the annealing mechanism. A specific wavelength range and a specific frequency range used for annealing in the present invention will be discussed below with regard to  FIG. 4 . 
   Referring to  FIG. 3 , in accord with the method of the present invention, the semiconductor material  10 , such as a silicon wafer  16 , is placed inside a resonator cavity designated  18  coupled by way of a wave guide or traveling wave tube  20  to an electromagnetic generator such as a microwave and/or far infrared generator or free electron laser (not shown). The cavity may comprise for example, a metal material, or may further comprise a liner (not shown) to prevent contamination. The cavity may include a cylindrical shape and is at least large enough to accommodate the semiconductor material  10  to be treated. The material is situated in an area within the cavity. In a preferred embodiment, for maximum annealing effect, the material should be positioned so that it coincides with a particular plane of exciting waves in the cavity. For this purpose, a wafer holder  17  capable of micro-positional adjustments or vibrations might be used. However, placing the material in the cavity  18  in any location where it may be irradiated usually suffices to achieve annealing. To achieve annealing, the generator is used to irradiate the semiconductor material within the cavity  18  with electromagnetic radiation in a wavelength or frequency range coinciding with phonon frequencies of the doped semiconductor material  10  or, more specifically, the doped region  12  of the semiconductor material for a pre-determined period. In one example, the semiconductor material is irradiated for a time period between 1-10 minutes, depending on the power incident on the wafer surface. 
   A wavelength in the phonon frequency will cause enhancement of phonon resonance to produce annealing of the semiconductor material  10 .  FIG. 4  indicates the location of the phonon bands in the absorption spectrum. Wavelengths in the phonon frequency region (Reststrahlen region) of the typical semiconductor material which is doped range from 10-25 μm and more particularly from 15-18 μm. Frequencies of phonons of the typical semiconductor material, in particular silicon, range from 12-30 THz and more particularly from 16.5-20 THz. The phonon absorption ranges are spread out as compared to the phonon absorption range of pure silicon as seen in  FIG. 1 . Therefore, an exact wavelength is not required to resonate phonons. Instead a wavelength within the specified range will suffice. In one example, a 15 THz microwave generator was used. 
   Still referring to  FIG. 4 , it is seen that too short a wavelength (or too high a frequency) i.e. less than 10 μm (greater than 30 THz), and the dominant absorption becomes free carrier absorption with significant heating. Other heat-inducing absorption mechanisms may also occur with too short a wavelength. Too long a wavelength (or too low a frequency) i.e. greater than 25 μm (less than 12 THz) and free carrier absorption or other heat-inducing absorption mechanisms will become dominant. The invention does not contemplate use of any ordinary microwave oven 2.45 GHz frequencies. 
   In one test example, a silicon wafer having a thickness of 150 mm is implanted with 1.0×10 16  atoms of Arsenic @ 40 keV. After doping, the wafer is placed in the cavity  18  and exposed to electromagnetic radiation in a wavelength or frequency range coinciding with phonon absorption resonances of the doped semiconductor material for a period of 6-10 minutes. The maximum wafer temperature reached because of secondary absorption mechanisms like free carrier absorption is around 400° C.-450° C. for irradiation “pulses” on the order of 6-10 minutes. In the example, annealing of a doped layer of the wafer occurs after and during irradiation. In this example, annealing of Arsenic is not a complete annealing, though annealing does occur without excessive exposure to high temperatures.