Surface treatment method and apparatus therefor

Abstract: A method of preventing damages of a semiconductor having an insulator film at its surface caused by holes induced in the insulator film and move to and are trapped at or near the interface between the insulator film and a substrate upon applying surface treatment for the surface of the semiconductor, as well as an apparatus suitable to practicing the method are disclosed. The surface treatment method comprises trapping to eliminate the holes in the insulator film by an electric field before the movement of the holes at or near the interface, neutralizing the induced holes by re-combination, or previously forming a hole trapping level in the insulator film thereby trapping the induced holes to the trapping level.

Description: BACKGROUND OF THE INVENTION The present invention relates to a surface treatment method according to the preamble of claim 1 suitable to elimination of damages induced in an insulator film present at the surface of a semiconductor device, as well as an apparatus for practicing such a method. In the surface treatment of a semiconductor by using high energy particles generated, for example, in plasmas, it has been known that various damages are caused in the insulator film of the semiconductor device due to such high energy particles irradiated to the semiconductor. For eliminating such damages, a method of annealing the semiconductor after surface treatment has been adopted. Damages in the insulator film are mainly caused by holes induced in the insulator film due to various particles irradiated to a specimen for conducting the surface treatment. The various particles means those elementary particles such as ions, neutrals, electrons, photons or mesons. The holes are induced near the surface of the insulator film. When the holes, after thus induced move through the insulator film and are trapped at or near the interface between the insulator film and the substrate, this causes changes (damages) in the device characteristics of semiconductor such as the threshold voltage shift, dielectric breakdown, decline in life time for minority carriers, etc. That is, when the holes are trapped at or near the interface, the trapped holes generate near interface states or interface states, or holes trapped at or near the interface are recombined with thermally excited electrons from the substrate to generate photons, which break the bonds of the crystal lattice at or near the interface to newly generate near interface states or interface states. It is considered that the characteristic degradation as described above is caused by such mechanisms. Accordingly, these damages caused by the holes can not be eliminated completely by merely annealing the semiconductor device after the surface treatment. That is, if the holes induced in the insulator film in the process of the surface treatment are trapped at or near the interface between the insulator film and the substrate, this may cause damages. However, since the conventional surface treatment method and surface treatment apparatus are not adapted to inhibit the holes induced in the insulator film from being trapped at or near the interface between the insulator film and the substrate, it can not be effectively suppress the occurrence of the damages as described above. EP 2421 teaches a method for neutralising positive charges already trapped in the insulated gate of a MOS device by injecting electrons into the insulated gate from the substrate side. SUMMARY OF THE INVENTION The object of the present invention is to provide a surface treatment method and a surface treatment apparatus capable of avoiding that holes induced in the insulator film in the course of the surface treatment are trapped at or near the interface between the insulator film and the substrate. This object is met by the invention as set out in claims 1 and 7. According to the present invention, preventing the induced holes from being trapped at or near the interface can be improved by previously cooling a semiconductor specimen to be applied with surface treatment in a series of processes for the surface treatment of the semiconductor including the process of practicing such a method. Electrons are injected at a low energy from the outside of the surface of the insulator film into the neighbourhood of the surface of the insulator film, after the holes are induced near the surface of the insulator film by the application of the surface treatment and before the holes move near the interface between the insulator film and the substrate, that is, while the induced holes are still present near the surface of the insulator film. That is, the injected electrons recombine the induced holes to neutralize the holes. For injecting electrons at low energy into the insulator film there can be used, for example, a method of disposing material that generates photoelectrons upon irradiation of photons above the surface of the insulator film, irradiating photons thereto thereby generating photoelectrons and injecting the photoelectrons into the insulator film. For the insulator film as the object to be treated in the present invention, there can be mentioned Si oxide film, Si nitride film or Ta oxide film usually used for the formation of semiconductor devices but there is no particular restriction only thereto and any other optional insulator films may be used. As the material for generating photoelectrons, there can be mentioned those compound semiconductors such as Ag-O-Cs, Ga-As, Ga-Cs, In-Ga-As, Sb-Cs, Sb-Rb-Cs, Cs-Te, Cs-I, etc. in view of the reason, for example, that they have good efficiency for generating photoelectrons, can produce photoelectrons with photons in a wavelength region from visible rays to ultraviolet rays. When the semiconductor specimen to be applied with the surface treatment is previously cooled in the series of surface treatment process including the process of practicing the method, since the hole mobility is abruptly reduced along with the lowering of the temperature, holes induced in the insulator film does not move instantly due to diffusion from the place where they are induced (that is in the vicinity of the surface) to the place at or near the interface relative to the substrate. Accordingly, if electrons are injected in this state, trapping of the holes near or at the interface between the insulator film and the substrate can be prevented completely. Further, since the thermal kinetic energy of the hole is extremely small in a state cooled to the low temperature as described above, the holes are easily trapped. The standard for the cooling temperature of the specimen can be determined as below. That is, the moving velocity of the holes is reduced by cooling and, briefly speaking, it is necessary that the holes induced in the insulator film do not move as far as near or at the interface between the insulator film and the substrate before practicing the method of eliminating or neutralizing the holes. Accordingly, the specimen may be cooled so as to obtain such a hole mobility as capable of satisfying the relationship: T 1 > T 2 in which T 1 represents a period of time required for holes from their induction in the insulator film till reaching near or at the interface between the insulator film and the substrate by the movement through the insulator film, and T 2 represents a period of time required from the induction of the holes in the insulator film till the conduction of any one of the methods as described above for eliminating the holes out of the insulator film or neutralizing. T 2 is about several tens of seconds in the usual surface treatment process. Assuming for example, that an SiO 2 film of 100 nm film thickness is used as an insulator film and that the holes move only by the diffusion, T 1 is about 10 2 seconds at a cooling temperature of lower than 200 K, by which the conditions for the equation (1) above can be satisfied sufficiently. Further, there is a possibility that the surface of the insulator film is charged up to about +10 V under the effect of irradiated particles upon surface treatment. In such a case, since the induced holes drift in the direction of the interface between the insulator film and the substrate due to the charge-up potential, T 1 is decreased. According to the present invention, the cooling temperature is lowered to less than 150 K, so that T 1 can be more than 10 2 sec. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic structural view for a surface treatment apparatus; Fig. 2 is a graph illustrating the temperature dependency of the hole mobility in SiO 2 ; Pig. 3 is a schematic structural view of a portion of an after-treatment device in the surface treatment apparatus; Fig. 4 is a schematic structural view of a surface treatment apparatus as an embodiment of the present invention; and Fig. 5 is a schematic structural view for a portion of the after-treatment device in the surface treatment apparatus shown in Fig. 4. DESCRIPTION OF PREFERRED EMBODIMENT The present invention will now be described by way of an embodiment with reference to the accompanying drawings. Fig. 1 illustrates an entire structure of a surface treatment apparatus. The apparatus comprises plasma device for surface treatment, a hole elimination device for eliminating holes induced in an insulator film present on a surface to be treated upon surface treatment by the plasma device and a guide tube for connecting both of the devices. In the apparatus of the constitution shown in Fig. 1, a microwave from a magnetron 1 is at first guided through a wave guide tube 2 into a discharge tube 3. A gas used for the surface treatment is discharged from a gas reservoir 23 by way of a gas inlet 6 by a vacuum pump 10 and then introduced into a vacuum vessel (surface treatment chamber) 5, and plasmas are generated by the microwave power introduced in the discharge tube 3. The generated plasmas are collimated into a high density state by the axial magnetic field from a solenoid coil 4. Surface treatment is applied for a sample 7 placed on a sample holder 8 by various particles generated from the plasmas. In this case, the sample 7 and the sample holder 8 can be cooled by a cooler 9. Upon applying the surface treatment by using the plasmas, holes are induced in the insulator film of the sample 7 under the irradiation of the various particles from the plasmas, and Fig. 2 shows the mobility of the thus induced holes in the insulator film (SiO 2 film in this case) as a function of the temperature of the sample. The graph was obtained by re-plotting data shown in a literature (R.C. Huges: Appl. Phys. Lett., vol. 26, No. 8 (1975), p 436). According to the figure, the mobility of the hole in the SiO 2 film, for example, at an absolute temperature of 122 K is 3.3 x 10 24 m 2 /Vs and movement of the hole in the SiO 2 film of 100 nm thickness takes about 10 -8 sec of time assuming that the surface of the film was charged up to about +10 V. It has been known that the holes are induced by the irradiated particles only at the extreme surface layer of the insulator film. For instance, it has been known that even vacuum ultraviolet rays that penetrate most deeply among various particles generated in the plasmas, form holes at a depth within 10 nm from the surface of the insulator film. That is, if surface treatment is applied while cooling a sample to a temperature of lower than 150 K and cooling is continued at that temperature after the surface treatment, the induced holes can be kept as they are to the induced position, that is, at the surface layer of the insulator film. After the predetermined surface treatment to the sample 7 has been completed in the surface treatment vacuum vessel 5, a valve 12 is opened and the sample 7 completed with the surface treatment is displaced together with the sample holder 8 through a guide tube 11 into a vacuum chamber 14 for after-treatment kept at a predetermined vacuum degree by an evacuation device 21, by using a sample holder displacing rod 13 having a displacing rod handle 22 and then placed on a cooling device 20 (refer to Fig. 3). In this way, by placing the sample holder 8 and the sample 7 on the cooling device 20, after-treatment for eliminating or neutralizing the holes is conducted while keeping the sample 7 in a state cooled to a low temperature. Description will now be made of an embodiment of the present invention with reference to Figs. 4 and 5. Fig. 4 shows a surface treatment apparatus according to the present invention comprising a plasma device for surface treatment, a device for eliminating holes induced in the insulator film upon surface treatment and a guide pipe for connecting both of the devices. After applying a required surface treatment while cooling a sample 1 in the same procedures as described above, a valve 12 is opened and a sample 7 and a sample holder 8 are displaced through a guide pipe 11 into an after-treatment vacuum device 14 kept at a predetermined vacuum degree by an exhaust device 21 by using a sample holder displacing rod 13 having a displacing rod handle 22, and then they are placed on a cooling device 20. An after-treatment step is applied for neutralizing holes in the insulator film of the sample while cooling the sample 7 on the sample holder 8 placed on the cooling device 20 at a low temperature by the cooling device 20. Fig. 5 shows a state of the device when the after-treatment step is conducted. At first, photons P are generated by a photon generation device and irradiated to a compound semiconductor 28 that emits photoelectrons upon irradiation of photons. Photoelectrons e emitted from the compound semiconductor under the photoelectric effect are injected into the insulator film of the sample 7. The injected photoelectrons neutralize to eliminate the holes in the insulator film. According to the present invention, since the holes induced in the insulator film by the surface treatment can be neutralized, it is possible to prevent the damages of semiconductor caused by the trapping of the holes near or at the interface between the insulator film and the substrate.

Claims: A surface treatment method comprising a first step of applying a surface treatment to the surface of a sample containing an insulator film on a semiconductor substrate, the surface treatment including irradiating the exposed surface of said insulator film with various kinds of particles thereby inducing holes near the surface of the insulator film, and a second step of recombining the induced holes before they can move near the interface between the insulator film and the substrate, characterised in that the first step is applied while cooling the sample to a temperature below 150 K, the cooling being continued at that temperature after the first step and during said second step, and that the second step comprises injecting electrons into the insulator film through its exposed, irradiated surface. The method of claim 1, wherein the insulator film is an Si oxide, Si nitride or Ta oxide film. The method of claim 1 or 2, wherein said first step comprises a surface treatment using a plasma. The method of anyone of claims 1 to 3, wherein the electrons are injected by disposing above the exposed, irradiated surface of the insulator film, but without contact to the surface, a material that generates photoelectrons upon irradiation with photons, and irradiating said material with photons thereby injecting photoelectrons into the insulator film. The method of claim 4, wherein said photoelectron generating material is a compound semiconductor. The method of claim 5, wherein the compound semiconductor is Ag-O-Cs, Ga-As, Ga-Cs, In-Ga-As, Sb-Cs, Sb-Rb-Cs, Cs-Te or Cs-I. A surface treatment apparatus for carrying out the method of any one of claims 1 to 6, comprising a vacuum vessel (5) forming a surface treatment chamber, means (23) for introducing into said vacuum vessel (5) a gas used for the surface treatment of a sample comprising a semiconductor substrate and an insulator film thereon, means (10) for evacuating the vessel (5), means (1, 2, 4) for generating particles used for said surface treatment, means (26∼30) for recombining holes induced by said surface treatment near the surface of said insulator film, and cooling means (9, 20) capable of lowering the sample temperature to a value below 150 K when the sample is subjected to said surface treatment and said treatment of recombining holes, wherein said means for recombining holes comprises means (27∼30) for injecting electrons into the insulator film through the surface which is irradiated by the particles used for the surface treatment. The apparatus of claim 7, wherein said particle generating means (1, 2, 4) is a plasma generating device. The apparatus of claim 7 or 8, wherein the electron injecting means (27∼30) comprises photoelectron generating means (28, 29) disposed above, and out of contact with, the surface of said insulator film, comprising a material which generates photoelectrons upon irradiation with photons, and means for displacing said photoelectron generating means to a position which does not hinder the performance of said surface treatment. The apparatus of claim 9, wherein said photoelectron generating material is a compound semiconductor material. The apparatus of claim 10, wherein the compound semiconductor material is Ag-O-Cs, Ga-As, Ga-Cs, In-Ga-As, Sb-Cs, Sb-Rb-Cs, Cs-Te or Cs-I.

Inventors: MIZUTANI TATSUMI, SUZUKI KEIZO, YUNOGAMI TAKASHI, MIZUTANI, TATSUMI, SUZUKI, KEIZO, YUNOGAMI, TAKASHI
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, G01Q30