Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/322—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to modify their internal properties, e.g. to produce internal imperfections
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
  • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
  • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
  • H01L21/04—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer
  • H01L21/18—Manufacture or treatment of semiconductor devices or of parts thereof the devices having potential barriers, e.g. a PN junction, depletion layer or carrier concentration layer the devices having semiconductor bodies comprising elements of Group IV of the Periodic Table or AIIIBV compounds with or without impurities, e.g. doping materials
  • H01L21/30—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26
  • H01L21/322—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to modify their internal properties, e.g. to produce internal imperfections
  • H01L21/3221—Treatment of semiconductor bodies using processes or apparatus not provided for in groups H01L21/20 - H01L21/26 to modify their internal properties, e.g. to produce internal imperfections of silicon bodies, e.g. for gettering
  • H01L21/3225—Thermally inducing defects using oxygen present in the silicon body for intrinsic gettering
• • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
  • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
  • Y10T—TECHNICAL SUBJECTS COVERED BY FORMER US CLASSIFICATION
  • Y10T428/00—Stock material or miscellaneous articles
  • Y10T428/21—Circular sheet or circular blank

Definitions

• the present invention generally relates to epitaxial semiconductor structures, especially epitaxial silicon wafers used in the manufacture of electronic components, and to methods for their preparation. More specifically, the epitaxial structures comprise a single crystal silicon substrate that is heavily doped with an N-type dopant (N+) or a P-type dopant (P+) and an epitaxial layer which is lightly doped with an N-type dopant (N-), wherein oxygen precipitation is suppressed in the substrate.
• N+ N-type dopant
• P+ P-type dopant
• N- N-type dopant
• Single crystal silicon the starting material for most processes for the fabrication of semiconductor electronic components, is commonly prepared by the Czochralski process, wherein a single seed crystal is immersed into molten silicon and then grown by extraction.
• molten silicon is contained in a quartz crucible, it is contaminated with various impurities, among which is mainly oxygen. As such, oxygen is present in supersaturated concentrations in the wafers sliced from single crystal silicon grown by this method.
• oxygen precipitate nucleation centers may form and ultimately grow into large oxygen clusters or precipitates. Depending upon their location, such precipitates can be beneficial or detrimental. When present in active device regions of the wafer, they can impair the operation of the device. When present outside these regions, oxygen precipitates may serve as a gettering site for metals.
• Falster et al. disclose a process for installing a non-uniform concentration of vacancies in a wafer in a rapid thermal annealer whereby in a subsequent oxygen precipitation heat-treatment, oxygen precipitates form in the vacancy-rich regions but not in the vacancy-lean regions.
• Falster discloses a process in which non-oxygen precipitating wafers are prepared by rapid thermally annealing the wafers in an oxygen-containing atmosphere or by slow-cooling the wafers through the temperature range at which vacancies are relatively mobile.
• epitaxial wafer structures comprising heavily doped substrates present somewhat different challenges.
• uncontrolled oxygen precipitation in heavily doped substrates can lead to the generation of relatively large concentrations of silicon self-interstitials at high temperatures because of their emission during oxygen precipitate growth.
• Relatively large concentrations of silicon self-interstitials tend to promote diffusion of dopant (or other impurities) from the heavily doped substrate into the more lightly doped, N- device layer, thereby potentially altering critical characteristics, such as avalanche breakdown voltage, in some power devices.
• one aspect of the present invention is an epitaxial silicon wafer comprising a heavily doped silicon substrate that has a resistivity of less than about 5 m ⁇ * cm and is substantially free from oxygen precipitate nuclei.
• the wafer also comprises an N- silicon epitaxial layer having a resistivity of greater than about 100 m ⁇ *cm.
• N-/N+ or N- /P+ epitaxial silicon wafers may be prepared with improved oxygen precipitation behavior and, as a result, a greater degree of control over diffusion of dopant (and other impurities) from the heavily doped substrate into the more lightly doped, N- epitaxial layer.
• the resulting epitaxial wafers will not form oxygen precipitates during a subsequent oxygen precipitation heat treatment (e.g., annealing the wafer at a temperature of 800 ° C for four hours and then at a temperature of 1000 ° C for sixteen hours).
• the starting material for the process of the present invention is a single crystal silicon wafer that has been sliced from a single crystal ingot grown in accordance with Czochralski crystal growing methods.
• the single crystal silicon wafer has a central axis; a front surface and a back surface that are generally perpendicular to the central axis; a circumferential edge; and a radius extending from the central axis to the circumferential edge.
• the wafer may be polished or, alternatively, it may be lapped and etched, but not polished.
• the wafer may have vacancy or self-interstitial point defects as the predominant intrinsic point defect.
• the wafer may be vacancy dominated from center to edge, self-interstitial dominated from center to edge, or it may contain a central core of vacancy dominated material surrounded by an axially symmetric ring of self-interstitial dominated material.
• Czochralski-grown silicon typically has an oxygen concentration within the range of about 5 x 10 17 to about 9 x 10 17 atoms/cm 3 (ASTM standard F-121 -83). Because the oxygen precipitation behavior of the wafer is essentially erased by the present process (i.e., the wafer is essentially rendered non-oxygen precipitating, even if subjected to an oxygen precipitation heat treatment), the starting heavily doped wafer may have an oxygen concentration falling anywhere within or even outside the range typically attainable by the Czochralski process.
• oxygen precipitate nucleation centers may form in the single crystal silicon ingot from which the heavily doped wafer is sliced.
• the presence or absence of these nucleation centers in the starting material is not critical to the present invention. Preferably, however, these centers are capable of being dissolved by the rapid thermal anneal heat-treatment of the present invention.
• the silicon wafer is heavily doped wafer with one or more N-type or P-type dopants.
• Typical N-type dopants include phosphorous and arsenic. In one embodiment, the dopant is phosphorous. In another embodiment, the dopant is arsenic. In yet another embodiment, phosphorous and arsenic are both used as dopants.
• Typical P-type dopants include boron, aluminum, and gallium. In one embodiment, the dopant is boron. In another embodiment, the dopant is aluminum, while in another embodiment, the dopant is gallium. In yet another embodiment, any combination of boron, aluminum, and gallium is used as the dopant.
• the total concentration of dopant(s) is such that the wafer has a resistivity of less than about 5 m ⁇ -cm, such wafers typically being referred to as N+ or P+ wafers.
• the dopant concentration is sufficient to provide the wafer with a resistivity of less than about 3 m ⁇ -cm. In certain embodiments, resistivities of less than about 2 m ⁇ -cm will be preferred. In one preferred embodiment, the dopant concentration is sufficient to provide the wafer with a resistivity of less than about 1 m ⁇ -cm.
• the resistivity values noted above correspond to an N-type dopant concentration that may be greater than about 1.24 x 10 19 at/cm 3 .
• the heavily doped wafer has N-type dopant(s) present in a concentration greater than about 2.25 x 10 19 at/cm 3 , such as greater than about 3.43 x 10 19 at/cm 3 .
• the heavily doped wafer has N-type dopant(s) present in a concentration greater than about 7.36 x 10 19 at/cm 3 .
• the resistivity values noted above correspond to a P-type dopant concentration that may be greater than about 2.1 x 10 19 at/cm 3 .
• the heavily doped wafer has P-type dopant(s) present in a concentration greater than about 3.7 x 10 19 at/cm 3 , such as greater than about 5.7 x 10 19 at/cm 3 .
• the heavily doped wafer has P-type dopant(s) present in a concentration greater than about 1.2 x 10 20 at/cm 3 .
• the heavily doped wafer is subjected to a heat treatment step to cause dissolution of any pre-existing oxygen clusters and any pre-existing oxidation induced stacking faults (OISF) nuclei.
• this heat treatment step is carried out in a rapid thermal annealer (RTA) in which the wafer is rapidly heated to a target temperature, then annealed at that temperature for a relatively short period of time.
• RTA rapid thermal annealer
• the wafer is rapidly heated to a temperature in excess of 1150 ° C, preferably at least 1175 ° C, typically at least about 1200 ° C, and, in some embodiments, to a temperature of about 1200 ° C to 1275 ° C.
• the wafer will generally be maintained at this temperature for at least one second, typically for at least several seconds (e.g., at least 3), and potentially for several tens of seconds (such as between about 10 and about 60 seconds, e.g., 20, 30, 40, or 50 seconds) depending upon the concentration, type and size of any preexisting defects.
• the rapid thermal anneal may be carried out in any of a number of commercially available RTA furnaces in which wafers are individually heated by banks of high power lamps. Rapid thermal annealer furnaces are capable of rapidly heating a silicon wafer, e.g., they are capable of heating a wafer from room temperature to 1200 ° C in a few seconds.
• One such commercially available RTA furnace is the 3000 RTP available from Mattson Technology (Freemont, CA).
• the annealing step will increase the number density of crystal lattice vacancies in the heavily doped wafer.
• oxygen-related defects such as ring OISF
• ring OISF are high temperature nucleated oxygen agglomerates catalyzed by the presence of a high concentration of vacancies.
• oxygen clustering is believed to occur rapidly at elevated temperatures, as opposed to regions of low vacancy concentration where behavior is more similar to regions in which oxygen precipitate nucleation centers are lacking.
• the density of vacancies in the heat-treated wafer is controlled in the process of the present invention to limit or even avoid oxygen precipitation in a subsequent oxygen precipitation heat treatment.
• the (number) density of vacancies in the annealed wafer can be controlled by limiting the cooling rate from the annealing temperature, by including a sufficient partial pressure of oxygen in the annealing atmosphere, or by doing both.
• the vacancy concentration in the annealed wafer may be controlled, at least in part, by controlling the atmosphere in which the heat- treatment is carried out.
• Experimental evidence obtained to date suggests that the presence of a significant amount of oxygen suppresses the vacancy concentration in the annealed wafer.
• the rapid thermal annealing treatment in the presence of oxygen results in the oxidation of the silicon surface and, as a result, acts to create an inward flux of silicon self-interstitials.
• This inward flux of self-interstitials has the effect of gradually altering the vacancy concentration profile by causing Frankel pair recombinations to occur, beginning at the surface and then moving inward.
• the annealing step is carried out in the presence of an oxygen-containing atmosphere in one embodiment. That is, the anneal is carried out in an atmosphere containing oxygen gas (O2), water vapor, or an oxygen-containing compound gas which is capable of oxidizing an exposed silicon surface.
• the atmosphere may thus consist entirely of oxygen or oxygen compound gas, or it may additionally comprise a non-oxidizing gas, such as argon.
• the atmosphere will preferably contain a partial pressure of oxygen of at least about 0.001 atmospheres (atm.), or 1 ,000 parts per million atomic (ppma). More preferably, the partial pressure of oxygen in the atmosphere will be at least about 0.002 atm. (2,000 ppma), still more preferably 0.005 atm. (5,000 ppma), and still more preferably 0.01 atm. (10,000 ppma).
• Intrinsic point defects are capable of diffusing through single crystal silicon with the rate of diffusion being temperature dependant.
• concentration profile of intrinsic point defects therefore, is a function of the diffusivity of the intrinsic point defects and the recombination rate as a function of temperature.
• the intrinsic point defects are relatively mobile at temperatures in the vicinity of the temperature at which the wafer is annealed in the rapid thermal annealing step, whereas they are essentially immobile for any commercially practical time period below or at temperatures of as much as 700 ° C.
• the concentration of vacancies in the annealed wafer is controlled, at least in part, by controlling the cooling rate of the wafer through the temperature range in which vacancies are relatively mobile. Such control is exercised for a time period sufficient to reduce the number density of crystal lattice vacancies in the cooled wafer prior to cooling the wafer below the temperature range in which vacancies are relatively mobile. As the temperature of the annealed wafer is decreased through this range, the vacancies diffuse to the wafer surface and become annihilated, leading to a change in the vacancy concentration profile.
• the extent of such change depends on the length of time the annealed wafer is maintained at a temperature within this range and the magnitude of the temperature, with greater temperatures and longer diffusion times generally leading to increased diffusion.
• the average cooling rate from the annealing temperature to the temperature at which vacancies are practically immobile is preferably no more than 20 ° C per second, more preferably no more than about 10 ° C per second, and still more preferably no more than about 5 ° C per second.
• the temperature of the annealed wafer following the high temperature anneal may be reduced quickly (e.g., at a rate greater than about 20 ° C/second) to a temperature of less than about 1150 ° C but greater than about 950 ° C, and then held for a time period that is dependent upon the holding temperature. For example, several seconds (e.g., at least about 2, 3, 4, 6 or more) may be sufficient for temperatures near 1150 ° C, whereas several minutes (e.g., at least about 2, 3, 4, 6 or more) may be required for temperatures near 950 ° C to sufficiently reduce the vacancy concentration.
• the cooling step may be carried out in the same atmosphere in which the heating step is carried out.
• Suitable atmospheres include, e.g., nitriding atmospheres (i.e., atmospheres containing nitrogen gas (N 2 ) or a nitrogen-containing compound gas that is capable of nitriding an exposed silicon surface, such as ammonia); oxidizing (oxygen-containing) atmospheres; non-oxidizing, non-nitriding atmospheres (such as argon, helium, neon, carbon dioxide); and combinations thereof.
• nitriding atmospheres i.e., atmospheres containing nitrogen gas (N 2 ) or a nitrogen-containing compound gas that is capable of nitriding an exposed silicon surface, such as ammonia
• oxidizing (oxygen-containing) atmospheres such as argon, helium, neon, carbon dioxide
• non-oxidizing, non-nitriding atmospheres such as argon, helium, neon, carbon dioxide
• the rapid thermal treatments employed herein may result in the out-diffusion of a small amount of oxygen from the surface of the front and back surfaces of the wafer, the resulting annealed wafer has a substantially uniform interstitial oxygen concentration as a function of distance from the silicon surface.
• the annealed wafer will have a substantially uniform concentration of interstitial oxygen from the center of the wafer to regions of the wafer that are within about 15 microns of the silicon surface, more preferably from the center of the silicon to regions of the wafer that are within about 10 microns of the silicon surface, even more preferably from the center of the silicon to regions of the wafer that are within about 5 microns of the silicon surface, and most preferably from the center of the silicon to regions of the wafer that are within about 3 microns of the silicon surface.
• a substantially uniform oxygen concentration shall mean a variance in the oxygen concentration of no more than about 50%, preferably no more than about 20%, and most preferably no more than about 10%.
• An epitaxial layer is deposited or grown on a surface of the annealed silicon wafer to an average thickness of at least about 5 cm by means generally known in the art to form the epitaxial silicon wafer.
• epitaxial growth is achieved by chemical vapor deposition, because this is one of the most flexible and cost effective methods for growing epitaxial layers on semiconductor material; see, e.g., U.S. Patent No. 5,789,309. Doping of the epitaxial layer may take place after or during the growth process.
• the resulting epitaxial layer has an N-type dopant concentration to provide the epitaxial layer with a resistivity of at least about 10 m ⁇ -cnn, such as at least about 100 m ⁇ -cnn.
• the epitaxial layer will typically have a resistivity of between about 100 m ⁇ -cnn and about 100 ⁇ -crn.
• the epitaxial layer will have a resistivity of between about 300 rn ⁇ -cnn and about 10 ⁇ -crn.
• the epitaxial layer will typically have a dopant concentration of less than about 4.8 x 10 18 at/cm, such as between about 4.3 x 10 13 at/cm and about 7.8 x 10 16 at/cm.
• the N-type epitaxial layer has a dopant concentration between about 4.4 x 10 14 at/cm and about 1.9 x 10 16 at/cm.
• the epitaxial layer is doped, as described, with one or more N-type dopants selected, for example, from the group consisting of phosphorous, arsenic, and antimony.
• the N-type dopant will be phosphorous, arsenic, or both phosporous and arsenic.
• the dopant is phosphorous.
• the dopant is arsenic.
• phosphorous and arsenic are both used as dopants.
• One advantage to using epitaxial deposition is that existing epitaxial growth reactors can be used in conjunction with a direct dopant feed during epitaxial growth. That is, the N-type dopant can be mixed with the carrier gas to dope the deposited epitaxial layer.
• the epitaxial layer is formed in conjunction with the annealing step detailed above.
• the epitaxial layer is formed such that the duration of the anneal step is satisfied.
• the cooling atmosphere, cooling rate, or both the cooling atmosphere and rate are controlled as detailed above. That is, in one variation of this embodiment, the atmosphere after the anneal and epitaxial layer formation is an oxygen-containing atmosphere that is capable of oxidizing an exposed silicon surface.
• the atmosphere will preferably contain a partial pressure of oxygen of at least about 0.001 atmospheres (atm), or 1 ,000 parts per million atomic (ppma). More preferably, the partial pressure of oxygen in the atmosphere will be at least about 0.002 atm (2,000 ppma), still more preferably 0.005 atm (5,000 ppma), and still more preferably 0.01 atm (10,000 ppma).
• the cooling rate of the wafer is controlled with or without controlling the cooling atmosphere.
• the cooling rate is controlled such that the average cooling rate from the annealing temperature to the temperature at which vacancies are practically immobile (e.g., about 950 ° C) is preferably no more than 20 ° C per second, more preferably no more than about 10 ° C per second, and still more preferably no more than about 5 ° C per second.
• the temperature may be reduced quickly (e.g., at a rate greater than about 20 ° C/second) to a temperature of less than about 1150 ° C but greater than about 950 ° C, and then held for a time period between several seconds to several minutes, depending upon the holding temperature. For example, at least about 2, 3, 4, 6 seconds or more may be sufficient for temperatures near 1150 ° C, whereas at least about 2, 3, 4, 6 minutes or more may be required for temperatures near 950 ° C.
• a polysilicon layer is deposited on the backside of the highly doped substrate prior to the annealing step described above.
• the grain boundaries of the polysilicon layer serve as a gettehng site for dopant.
• the polysilicon layer may be deposited by any means conventionally known in the art.
• the polysilicon layer may be deposited by chemical vapor deposition using silane (SiH 4 ) gas and arsenic doping, as more fully described in U.S. Pat. No. 5,792,700 or 5,310,698.
• Silicon structures manufactured according to this invention may be used in various technologies.
• the silicon structure of this invention is suitable for use in the manufacture of power devices, such as power diodes, thyhstors, and, in particular, power MOSFETs and JFETs. This list is in no way intended to be restrictive or comprehensive.

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• 2007
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