A method of manufacturing semiconductor on insulator

Abstract: After the oxygen ion is implanted to the surface of (100) Si single crystal substrate, the annealing of about 10 cycles is conducted under the nitrogen gas ambience, wherein the high temperature of 1150°C and low temperature of 450°C are alternately repeated during one heat cycle of about seven minutes and thereby a Si single crystal layer is formed near the surface and the SiO₂ insulator is then formed just under the Si single crystal layer.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a method of forming SOI (Semiconductor On Insulator) structure by SIMOX (Separation by Implanted Oxygen) method and particularly to a method of forming SOI structure containing less crystal defects. Since a substrate having the SOI structure (hereinafter called a SOI substrate) has excellent merits, when devices are formed thereon, that is, a parasitic capacitance is small and high speed operation of devices can be realized, device isolation can be done easily, immunity from radiation such as alpha ray is high and a high voltage resistant device may be formed, etc., practical application into MOS IC, etc. is now often attempted. Description of the Related Art A substance formed by the SIMOX method is an example of the SOI substrate. In the SIMOX method, a silicon dioxide (SiO₂) layer is formed in a silicon single crystal substrate by ion implantation method of oxygen. Usually, the oxygen ion of 80∼ 200 KeV is implanted in the dose of 10¹⁷∼10¹⁸cm⁻³ and thereafter the annealing is carried out at the temperature of about 1100°C. Thereby, a neighboring layer of surface which has changed to amorphous layer is then single-crystallized and a SiO₂ layer is formed in the region thereunder where the concentration of implanted oxygen becomes maximum in such a manner as taking also the oxygen in the vicinity of surface. The silicon single crystal layer in the vicinity of surface corresponds to SOI and this layer is used for formation of devices such as IC, etc. The SIMOX method and application into devices are described, for example, Sorin Cristoloveanu: "Electrical Evaluation of SIMOX Material and Integrated Devices" Material Research Society Symposium Proceedings Vol. 107, p.335-p.347, 1988. The SOI substrate formed by SIMOX method is hereinafter called a "SIMOX substrate" and silicon single crystal layer formed on the insulation film of "SIMOX substrate" is called the "SIMOX SOI". The manufacturing condition of the "SIMOX substrate" used actually is, for example, as follow. The original silicon single crystal substrate is an n-type wafer having specific resistance of 1 kΩ cm and surface orientation of (100). The oxygen ion is implanted to the surface of this wafer in the implantation energy of 150 KeV and dose of 1 X 10¹⁸cm ⁻². Under this condition, the project range of oxygen ion is 0.37 µ m. Thereafter, this substrate is subjected to the heat treatment for 60 min. at 1200°C in the nitrogen ambience. As a result, the SiO₂ insulation film formed thereby has the thickness of about 200 nm and the silicon single crystal layer, namely "SIMOX SOI" formed thereon has the thickness of about 200 nm. However, this "SIMOX SOI" usually has a crystal defect rate of 1 x 10⁷ cm⁻². If the "SIMOX SOI" having such many crystal defects is formed, a leak current increases and thereby it becomes difficult to obtain a device having good device characteristics. Various annealing processes are known for the preparation of semiconductor substrates. In one example disclosed in EP-A-0 060 676, the thermal annealing of silicon containing oxygen as a possible nucleus for the production of crystalline defects is obtained using an alternately repeated heat cycle. In another example disclosed in Journal of Applied Physics, vol. 59, No. 10, May, 1986, pp 3495-3502, the annealing of silicon containing oxygen by is obtained by use of a heating step for regrowth of thermal donors and a heating step for removal of thermal donors. In yet another example disclosed in EP-A-0 269 349, a SIMOX structure is obtained by the successive steps of implanting oxygen ions into a buried layer of the silicon substrate and a two-step annealing comprising a low temperature step (500<T<800°C) and a high temperature step (T>1200°C), followed by a second implantation and a second low-temperature step. SUMMARY OF THE INVENTION It is an object of the present invention to form "SIMOX SOI" which ensures less crystal defects and good device characteristics. This object may be attained by executing the method defined in claim 1. For this annealing through such heat cycle, the "SIMOX substrate" is radiated and heated by tungsten lamp in the heat treatment chamber of nitrogen gas ambience forming a heat treatment apparatus such as a rapid thermal annealer. The tungsten lamp is controlled to conduct the annealing by heat cycle for the "SIMOX substrate". With such heat cycle annealing, a silicon single crystal layer, namely "SIMOX SOI" is formed in the vicinity of the surface of "SIMOX substrate" and a SiO₂ insulation layer is formed just under the "SIMOX SOI". According to the result of experiment for evaluating crystal defect on the "SIMOX SOI" by the etch pit at the surface, the measured etch pit density of 1 x 10⁷cm⁻² is reduced to about 1 x 10⁶cm⁻² as a result of heat cycle annealing of 10 times of more. Moreover, it has been confirmed in the heat cycle annealing that the etch pit density of 1 x 10⁷cm⁻² before the heat cycle annealing has been reduced to about 1 x 10⁶cm⁻² after the heat cycle annealing of 10 times by selecting the high temperature to 1100°C or higher and the low temperature to 500°C or less. Therefore, the "SIMOX SOI" formed by the present invention assures excellent device characteristics. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram of a heat treatment apparatus for heat treatment of "SIMOX substrate"; Fig. 2 is a heat cycle diagram by the present invention for heat treatment of "SIMOX substrate"; Fig. 3 is an experimental curve indicating relationship between the etch pit density of "SIMOX SOI" and a number of heat cycles; Fig. 4 is an experimental curve indicating relationship between the etch pit density of "SIMOX SOI" and T H after the 10 cycles annealing with T L kept constant at 450°C ; Fig. 5 is an experimental curve indicating relationship between the etch pit density of "SIMOX SOI" and T L after the 10 cycles annealing with T H kept constant at 1150°C; Fig. 6 (a) is a step diagram for ion implantation of oxygen O⁺ to the surface of Si substrate; Fig. 6 (b) is a step diagram for forming a Si single crystal layer under the surface of sample and then forming a SiO₂ layer just under the Si single crystal layer by the heat cycle annealing; Fig. 6 (c) is a step diagram for channel doping of boron ion (B⁺) to the surface single crystal layer after the etching for device isolation; Fig. 6 (d) is a step diagram for conducting gate etching after the gate oxidation and then growth of n-type poly-Si and then forming the source drain by selectively implanting arsenic ion (As); Fig. 6 (e) is a step diagram for providing contact holes respectively to the source and drain after forming the SiO₂ layer at the sample surface; and Fig. 6 (f) is a step diagram for forming the wirings to the source and drain, respectively and completing an n-channel MOSFET. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be explained hereunder with reference to Fig. 1 to Fig. 6. "SIMOX substrate" 1 is carried into the heat treatment chamber 9 of the heat treatment apparatus such as a rapid thermal annealer shown in Fig. 1 and is then placed on a substrate holder 2. At the upper part of heat treatment apparatus, a plurality of tungsten lamps are provided as the light source 3 for heat treatment in order to irradiate the "SIMOX substrate" 1 through a quartz plate 4. The nitrogen gas is introduced into the heat treatment chamber 9 through a gas entrance 5 and is exhausted through a gas exit 6. The heat treatment chamber 9 is formed by external wall of quartz and the external surface thereof is evaporated with aluminum film 7 to obtain uniform chamber temperature. The temperature of "SIMOX substrate" 1 is controlled by a pyrometer 8 provided at the lower part of the heat treatment chamber. The total electrical power of tungsten lamps becomes about 20 KW in order to heat the "SIMOX substrate" up to 1150°C In this embodiment, the N₂ gas is introduced into the heat treatment chamber 9 but hydrogen (H₂) or an inert gas such as argon (Ar), helium (He), krypton (Kr) or xenon (Xe) may also be used. Moreover, in case the heat treatment is carried under the vacuum ambience by evacuating the heat treatment chamber 9, more uniform temperature distribution is assured for entire part of the "SIMOX substrate". Fig. 2 is a heat cycle diagram for heat treatment to "SIMOX substrate" 1. In this heat cycle, the high temperature T H is set to 1150°C and the low temperature T L to 450°C , the holding time of T H and T L is respectively set to 10 seconds to several tens of minutes and most desirable holding time is 3 to 4 minutes. Therefore, one heat cycle time is about 7 minutes. In the heat treatment for the "SIMOX substrate" 1 in accordance with the heat cycle of Fig. 1, the density of etch pits appearing in the etching of "SIMOX SOI" is measured in each heat cycle. A curve plotting the density value of etch pits for the number of heat cycles is shown in Fig. 3. In this step, the etching is carried out for 15 sec. using the Wright's etching solution. (Wright's etching solution is a solution obtained by the mixing of hydrofluoric acid of 60cc; nitric acid of 30cc; chromic acid (5 mol/ ℓ) of 30cc; copper nitrate of 2 g, water of 60cc and acetic acid of 60cc.) As is shown in Fig. 3, density of etch pit gradually reduces with increase of a number of heat cycles from the value 1 x 10⁷cm⁻² before the heat treatment and reduction is saturated from the fifth heat cycle. In the tenth heat cycle, density becomes about 1 x 10⁶cm⁻². Next, the experimental result for determining the optimum values for the high temperature T H and low temperature T L in the heat cycle of Fig. 1 is shown in Fig. 4 and Fig. 5. First of all, T L is kept to a constant value of 450°C in the heat cycle, while T H is set to several temperature values from 900°C to 1200°C . In any case, the "SIMOX substrate" is subjected to the annealing by 10 cycles, etch pit density of "SIMOX SOI" is measured after 10 cycles, and these values are plotted against T H in the heat cycle. A curve thus obtained is shown in Fig. 4. From Fig. 4, it is obvious when T H is about 1100°C or higher, density of etch pit is reduced to about 1 x 10⁶cm⁻² from the value 1 x 10⁷cm⁻² before the heat treatment. Moreover, T H in the heat cycle is fixed to 1150°C , while T L is set to several values in the range from 300°C to 700°C . In any case, the "SIMOX substrate" is subjected to the annealing by 10 cycles, etch pit density of "SIMOX SOI" is measured after 10 cycles, and these values are plotted against T L in the heat cycle. As shown in Fig. 5, when T L is about 500°C or lower, density of etch pit is reduced to about 1 x 10⁶cm⁻² from the value 1 x 10⁷cm⁻² before the heat treatment. In this embodiment, T H and T L of heat cycle for annealing the "SIMOX substrate" are respectively fixed to constant values. However, T H must be 1100°C in minimum and T L must be 500°C in maximum and these are not required to be constant. In case a MOSFET with channel length of 2 µ m is manufactured as an example utilizing SIMOX SOI, a leak current is 0.005 pA per gate width 1 µ m. It is equal to 1/20 of the leak current of a similar MOSFET fabricated by using the conventional method. Next, a method of manufacturing MOSFET utilizing the method of the present invention will be explained with reference to Figs. 6(a)∼6(f). As shown in Fig. 6(a), the oxygen ion O⁺ is implanted, with energy of 150 KeV and dose of 1 x 10¹⁸cm⁻², to the surface of Si substrate 11 having resistivity of 1 KΩ, and surface orientation of (100). This sample is subjected to the heat cycle annealing under the nitrogen ambience. The heat cycle annealing to be conducted is shown in Fig. 2 and a number of heat cycles is set to 10 cycles. As a result of this heat cycle annealing, the Si single crystal layer 12 is formed in the region up to the depth of 200 nm from the surface of sample and the SiO₂ layer 13 is formed, just under this layer, in the region up to the depth of 400 nm from the surface of sample. This condition is shown in Fig. 6(b). Next, as shown in Fig. 6(c), after a device region 14 and the adjacent device region 15 are formed for the device isolation to the surface single crystal layer, the boron ion (B⁺) in the dose of 1 x 10¹²cm⁻² is implanted to the device region 14 as the channel doping. As shown in Fig. 6(d), the SiO₂ layer 16 of 15 nm (150 Å) is formed by the gate oxidation at 1000°C , after which the n-type poly-Si is etched. Next, the source 18 and drain 19 are formed by implanting the arsenic ion (As⁺) with energy of 100 KeV and dose of 1 x 10¹⁶cm⁻². As shown in Fig. 6(e), after the SiO₂ layer is formed on the surface of sample and the contact holes 20, 21 are opened respectively to the source 18 and drain 19. Finally, as shown in Fig. 6(f), the aluminum (Aℓ) wirings 23, 24 are formed respectively to the source 18 and drain 19 through the contact holes 20, 21, thereby completing the n-channel MOSFET. In this embodiment, the MOSFET has been formed as a device but the device is not limited thereto and may be a bipolar transistor or photosensor.

Claims: A method of forming a semiconductor-on-insulator structure having an insulating layer of SiO₂ implanted within a silicon substrate (1; 11) and having a layer of single crystalline silicon (12) on that insulating layer, comprising the steps of: forming an implanted layer doped with oxygen ions by ion implantation into the silicon substrate (1; 11) and annealing the substrate by a heat treatment, characterized in that said annealing step is conducted in cycles in which a high temperature of 1100°C or higher and a low temperature of 500°C or lower are alternately repeated. The method according to claim 1, wherein said annealing cycles are repeated for five times or more. The method according to claim 1, wherein the high temperature value in each cycle is set in the range from 1200°C to 1400°C and the low temperature value in the range from 200°C to 400°C. The method according to claim 3, wherein the high temperature and low temperature holding time in each cycle is 10 seconds or longer. The method according to claim 1, wherein said annealing cycles are carried out under the ambience of gas selected from the group consisting of nitrogen, hydrogen, argon, helium, krypton and xenon. The method according to claim 1, wherein said annealing cycles are carried out under vacuum conditions. The method according to claim 1, wherein said annealing cycles are carired out by a radiation heating apparatus (2-6). The method according to claim 7, wherein tungsten lamps (3) are used as a radiator of radiation heating apparatus. The method according to claim 1, wherein the dose of oxygen ion to be implanted is selected from the range of 0.2 x 10¹⁸ cm⁻² ∼ 5 x 10¹⁸ cm⁻². The method according to claim 1, wherein semiconductor devices (16-22) are formed on the surface of said silicon layer (12).

Inventors: FUJIOKA HIROSHI, FUJIOKA, HIROSHI
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1990
Main classification: H01L21
IPC codes: H01L21, H01L27