CONTAMINANT REMOVAL METHOD

Abstract: A contaminant adhering to the surface of a substrate such as a compound semiconductor substrate is removed by reacting it with a reactive atomic species such as atomic hydrogen in order to weaken the interaction between the contaminant and the substrate. Subsequently or simultaneously with this, the substrate is irradiated with electron beams or light or other electromagnetic radiation to cause the contaminant to be detached from the substrate.

Description: CONTAMINANT REMOVAL METHOD This invention relates to a method for removal of contaminants from a substrate and is more particularly concerned with the removal of contaminants from a semiconductor substrate. High performance semiconductor devices place very stringent requirements on ultra clean surfaces with atomic flatness. Hence, the cleaning of semiconductor surfaces is a vital step in the MBE growth programme. The main purpose is to reduce the existence of defects that originate at the substrate/epitaxial interface. The presence of native oxide and carbon contaminants on the interface between the substrate surface and the regrown MBE layer, as a result of exposure to air for processing, induces a high concentration of deep levels that cause depletion of free carriers at the interface, introduces scattering effects that reduce mobility, and disrupts smooth surface morphology. If the surface contaminants cannot be removed efficiently without disrupting the surface crystallinity and stoichiometry, then it becomes difficult to grow a high quality epitaxial layer on top. A thick buffer layer would need to be grown on top of the surface before growth of high quality crystal layers can be carried out. This is very time consuming and therefore costly, since an extra step is introduced in processing. Conventional surface cleaning methods rely on thermal or ion-beam sputter cleaning in UHV to remove the oxide and carbon contaminants. Ion sputtering generally not only removes the unwanted species, but also dislodges the substrate atoms and damages the surface crystallinity. Thereafter, the damaged surface needs to be annealed at high temperatures to regain surface stoichiometry and evenness. Substrate heating to even higher temperatures, usually in excess of 600°C for GaAs MBE, is necessary in thermal cleaning to desorb surface adsorbates. The heating of semiconductor substrates to high temperatures poses several problems. At these high temperatures, the surface can reconstruct or roughen after desorption of contaminants. Other likely consequences are diffusion of contaminants into the bulk and diffusion of dopants from patterned regions. In recent years, cleaning techniques based upon hydrogen or halogen containing plasmas and hydrogen atom irradiation have been developed and are currently used. These benefit from lower energies and therefore introduce lower degrees of damage to the substrate. However these techniques are used with elevated substrate temperatures and therefore still suffer from the disadvantages mentioned above. Generic methods like carbon elimination by low energy argon ion bombardment, where the beam is mixed in with hydrogen atoms, have Others have proposed that post-MBE arsenic beam irradiation, i.e. arsenic passivation, on the GaAs surface at a low temperature is effective for protecting and achieving clean interfaces for GaAs MBE regrowth. It is an object of the present invention to provide a method for the removal of contaminants from a substrate which obviates these disadvantages. According to the present invention there is provided a method for the removal of a contaminant from a substrate, comprising the steps of forming a reactive species, the reacting the reactive species with the contaminant on the substrate so as to weaken the interaction between the substrate and the contaminant, and irradiating the substrate to cause the contaminant to be detached from the substrate. The present invention is particularly suitable for the removal of contaminants which are present on the surface of the substrate. Preferably the substrate is a semiconductor, and more preferably the substrate is a compound semiconductor for example GaAs, InSb, CdTe or HgxCdi-xTe. Preferably the reactive species forming step is performed in the region of the substrate (e.g. within a chamber in which the substrate to be cleaned is disposed). The forming step may be effected by splitting the reactive species from a fluid, preferably a molecular gas. The reactive species may be formed by splitting a molecular gas such as hydrogen, a halogen (e.g. chlorine), oxygen or ozone to form a reactive atomic species. Advantageously the irradiating step may be performed by electron irradiation, more advantageously the electron irradiation is low energy (e.g. less than 10OeV) electron irradiation. Alternatively the irradiating step may be performed by electromagnetic radiation such as by light. By appropriate choice of the reactive species and/or the irradiating step it is possible to remove contaminants in a chemically selective manner. Also according to the present invention there is provided a method of removing a contaminant from a substrate, comprising the steps of bringing a reactive species (e.g. a reactive atomic species such as hydrogen, oxygen or a halogen in atomic form) into contact with the substrate; and simultaneously irradiating the substrate, whereby to cause the contaminant to be removed. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of the apparatus used in the method. Figure 2 shows vibrational spectra from a GaAs substrate after a range of contaminant removal procedures. Figure 3 shows the relative composition of a GaAs substrate during a range of contaminant removal procedures. An ultra-high vacuum (UHV) chamber 10 is fitted with an adjustable gas inlet valve 12, an electron gun 14 and a resistively heated metal filamentl 8. Mounted within the chamber 10 upon a sample manipulator (not shown) is a GaAs substrate 20 with a thermocouple 22 mounted upon the front surface thereof and a coiled wire filament 24. The substrate 20 is in the line of sight of the filament 18. Also fitted to the UHV chamberl O but not shown is a high-resolution electron energy loss spectrometer (H REELS) for collecting vibrational spectra from the substrate 20. A molecular gas, in this example hydrogen, is admitted to the UHV chamber 10 through the gas inlet valve 12 to a pressure of approximately 8x10 "7 mbar. The hydrogen gas is atomised by the heated metal filament 18, this filament is typically of tantalum foil or a coiled tungsten filament. The sample 20 is irradiated with low energy electrons, typically 50 eV, from the electron gun 14, which has a finite spatial resolution. The substrate 20 is heated by the filament 24 on the reverse side to that irradiated by the electron gun 14. The sample temperature is monitored by use of the thermocouple 22 mounted directly upon the front face of the substrate 20. The atomic hydrogen reacts with carbon containing species bound to the substrate surface to fulfil the valency of the atom bonding to the surface, thereby weakening the substrate - atom bond, the transfer of energy from the electron beam to the weakly bound carbon containing species enhances the movement of the carbon containing species away from the substrate 20. Referring now to Figure 2, the spectrum I shows a vibrational spectrum from a clean GaAs (100) surface. The spectrum II is a vibrational spectrum from a GaAs (100) sample which has been deliberately contaminated to a controlled extent by exposure to air, a feature labelled A corresponds to the presence of a surface oxide species with features labelled B, C and D corresponding to the presence of hydrocarbon fraction species (e.g. CnHx) bound to the surface. Spectrum III is the vibrational spectrum of the air contaminated substrate of I after exposure to atomic hydrogen (H*) and irradiation with low energy (50eV) electrons. This shows a decrease in intensity of the features B to D with the feature A remaining approximately constant in intensity. The Spectrum IV shows the effect of exposure to atomic hydrogen at an elevated temperature (140°C) in the absence of a source of electrons following the treatment detailed for spectrum III. There is continued removal of carbonaceous species with only minimal removal of oxide from the substrate. The rate of removal of carbonaceous species is comparable to that of the room temperature, electron irradiated substrate, see Figure 3. Spectrum V shows the cumulative effect of the treatments detailed for spectrum III and IV and exposure to atomic hydrogen, electron irradiation and heating to 140°C. The rate of removal of carbonaceous species is not increased over that at room temperature with exposure to atomic hydrogen and electron irradiation. Thus whilst it can be seen that all three treatments serve to remove carbonaceous contaminants, exposure to atomic hydrogen and electron irradiation give an effect which is chemically selective. Referring now to Figure 3, showing the relative elemental composition of an air contaminated GaAs substrate, as determined by Auger electron spectroscopy, whilst undergoing cumulative contaminant removal as described for the vibrational spectra of Figure 2 above. During the initial contaminant removal period (0 - 2 hours), utilising only atomic hydrogen (H * ) and low energy electrons, there is a marked decrease in the amount of carbonaceous contaminants present bound to the substrate as exemplified by the decrease in value of the curve A. There is also a corresponding increase in the fractional amount of both Ga and As present in the surface layers. Further cleaning procedures (2 - 6) hours, as detailed for the vibrational spectra, whilst evidencing a continued removal of carbon containing species the rate of this removal is less than the initial cleaning procedure which utilised only atomic hydrogen and low energy electrons. It will be appreciated that this method is not limited to the use of hydrogen as a reactant gas and that a wide range of gases for example Cb, CH4, O3 and O2 may all be used. Further the method has the possibility of application to large number of substrates such as compound semiconductors, (InSb, CdTe, InP, GaP, GaSb, In As, AlAs, AlSb, CdS, CdSe, ZnS, ZnSe, ZnTe, HgTe, Cdi-xHgxTe, PbS, PbSe, PbTe, PbTe, SnTe, Gai-xA As, CulnSe2), elemental semiconductors such as Si and Ge and conductors and insulators. The use of a photon to promote the transfer of the weakly bound carbon containing species away from the surface is also considered.

Claims: CLAIMS 1. A method for the removal of a contaminant from a substrate, comprising the steps of (a) forming a reactive species; (b) reacting the reactive species with the contaminant on the substrate so as to weaken the interaction between the substrate and the contaminant; and (c) irradiating the substrate to cause the contaminant to be detached from the substrate. 2. A method as claimed in claim 1 , wherein the substrate is a semiconductor. 3. A method as claimed in claim 1 wherein the substrate is a compound semiconductor. 4. A method as claimed in any preceding claim, wherein step (a) is performed in the region of the substrate. 5. A method as claimed in any preceding claim, wherein step (a) is performed within a chamber in which the substrate to be cleaned is disposed. 6. A method as claimed in any preceding claim, wherein step (a) is effected by splitting the reactive species from a fluid. 7. A method as claimed in claim 6, wherein the fluid is a molecular gas. 8. A method as claimed in claim 7, wherein the molecular gas is selected from hydrogen, a halogen, oxygen and ozone. 9. A method as claimed in any preceding claim, wherein step (c) is performed using electron irradiation. 10. A method as claimed in claim 9, wherein the electron irradiation is low energy electron irradiation. 1 1. A method as claimed in any one of claims 1 to 8, wherein step (c) is performed using light or other electromagnetic radiation. 12. A method of removing a contaminant from a substrate, comprising bringing a reactive species into contact with the substrate; and simultaneously irradiating the substrate, whereby to cause the contaminant to be removed.

Inventors: BARNARD JOHN CAMERON, CHEN YU, PALMER RICHARD EDWARD, SCHMIDT JENS, SILLER LIDIJA, BARNARD, JOHN, CAMERON, CHEN, YU, PALMER, RICHARD, EDWARD, SCHMIDT, JENS, SILLER, LIDIJA
Applicants: BARNARD JOHN CAMERON, CHEN YU, PALMER RICHARD EDWARD, SCHMIDT JENS, SILLER LIDIJA, UNIV BIRMINGHAM, BARNARD, JOHN, CAMERON, CHEN, YU, PALMER, RICHARD, EDWARD, SCHMIDT, JENS, SILLER, LIDIJA, THE UNIVERSITY OF BIRMINGHAM
Application year: 1999
Main classification: H01L21
IPC codes: B01J19, B08B7, H01L21