Method for reducing InAs / GaSb surface defects by controlling As background pressure

Abstract

The invention relates to the field of semiconductor material molecular beam epitaxial growth manufacturing, and discloses a method for controlling As background pressure to reduce InAs / GaSb surface defects, which comprises the following steps: S1, substrate preparation and pretreatment: selecting a proper GaSb single crystal substrate, and carrying out selection, cleaning, degassing and deoxidation treatment on the GaSb substrate; s2, carrying out epitaxial growth of a GaSb buffer layer, and carrying out epitaxial growth of the GaSb buffer layer on the processed GaSb substrate; s3, control of As background pressure intensity: in the growth process of the GaSb buffer layer, the As background pressure intensity in the cavity is controlled and stabilized by adjusting the opening degree of an As needle valve; s4, growth of an InAs / GaSb superlattice: growing an InAs and GaSb superlattice structure layer by layer under the condition of well controlled As background pressure intensity. By accurately controlling the As background pressure intensity, the'bright spot 'defect and other surface defects formed on the InAs / GaSb surface can be effectively reduced, and the surface quality of the superlattice material is improved.

Description

Technical Field

[0001] The invention relates to the field of semiconductor material molecular beam epitaxial growth manufacturing, in particular to a method for controlling As background pressure to reduce InAs / GaSb surface defects. Background Art

[0002] Represented by InAs, GaSb, and AlSb InAs / GaSb compound semiconductor materials play a very important role in the field of photoelectric detection due to their advantages such as small lattice mismatch and flexible and adjustable energy band. With the gradual maturity of GaSb substrate preparation technology and the development of molecular beam epitaxy (MBE) technology, the broken bandgap superlattice structure composed of InAs / GaSb materials has been widely used in high-speed, low-power long-wave detectors and lasers. At present, the material structure can be optimized through energy band design to improve the absorption coefficient of the material. The separation of heavy and light holes in the energy band structure can inhibit Auger recombination, reduce the related dark current and increase the working temperature of the detector. In the long-wave band, the electron effective mass of InAs / GaSb superlattice materials is large, which is three times that of HgCdTe, which reduces the tunneling current and can obtain a high detection rate. The above advantages make it possible for type II superlattice materials to surpass traditional mercury cadmium telluride detectors in the field of infrared detection, but the As element has a huge impact on the quality of superlattice epitaxy in the actual epitaxy process, and its needle valve size directly affects the background pressure of epitaxial growth. Large pressure changes will introduce a large number of deep-level defects in periodic superlattice materials, exacerbating the Shockley-Read-Hall (SRH) recombination process and defect-assisted tunneling process inside the device, thus restricting device performance. Therefore, it is urgent to carry out research on a new epitaxial growth method for superlattice materials for application in military and civilian infrared detection fields.

[0003] Long-wave infrared detectors based on InAs / GaSb type II superlattice have great application prospects and value in deep space exploration, space-based missile warning, space remote sensing, free-space laser communication and other fields. However, in the actual epitaxial process, the V group gas molecules in the background pressure will competitively adsorb to the surface of the GaSb substrate and compete with the epitaxial molecules for adsorption. This may form undesirable compounds, affecting its crystal quality and defect density. At the same time, high background pressure may lead to higher defect density, thereby reducing the quality and performance of the film. In the epitaxial process of InAs / GaSb superlattice materials, the proportion of As will directly affect the crystallization quality of the epitaxial wafer. Usually, the As needle valve is opened in advance to stabilize the beam for a period of time. If the As valve is not opened in advance, the actual deposited As element is small when growing the InAs interface, and it is difficult to obtain an ideal InAs interface. However, it was found in the experiment that the opening of the As valve will directly affect the background pressure in the MBE cavity. When the GaSb layer grows, As atoms will compete with Sb atoms for adsorption, and it is very easy to form a GaAsSb or GaAs interface, resulting in poor surface morphology and increased roughness of the epitaxial wafer. When the heating temperature of the As source furnace is constant, the change of the background pressure in the MBE chamber can be effectively controlled within the order of 10-8 by changing the size of the As needle valve, avoiding the influence of the As background pressure on the GaSb interface caused by opening the As valve in advance, and obtaining an epitaxial wafer without surface defects. InAs / GaSbII type superlattice is grown layer by layer on the GaSb buffer layer, and there are obvious "bright spot" defects on its surface. Experiments have found that this defect has a strong dependence on the size of the As background pressure during epitaxial growth. At present, there are relatively few reports internationally on the causes of nano-scale defects caused by the As background affecting the epitaxial quality when growing InAs / GaSb materials on the GaSb buffer layer, and how to solve such defects. In response to the above problems, a method for controlling the As background pressure to reduce InAs / GaSb epitaxial defects is proposed. Summary of the invention

[0004] In view of the shortcomings of the prior art, the present invention provides a method for controlling the As background pressure to reduce InAs / GaSb surface defects, thereby solving the problem that during the epitaxial growth of InAs / GaSb type II superlattice on a GaSb substrate, the GaAsSb ternary alloy is easily affected by the As element background to form surface defects.

[0005] To achieve the above objectives, the present invention is implemented by the following technical solution: A method for controlling As background pressure to reduce InAs / GaSb surface defects, comprising the following steps:

[0006] S1: Substrate preparation and pretreatment

[0007] Selecting a suitable GaSb single crystal substrate, and performing selection, cleaning, degassing and deoxidation treatment on the GaSb substrate;

[0008] S2: Epitaxial growth of GaSb buffer layer

[0009] epitaxially growing a GaSb buffer layer on the processed GaSb substrate;

[0010] S3: Control of As background pressure

[0011] During the growth of the GaSb buffer layer, the As background pressure in the chamber is controlled and stabilized by adjusting the opening of the As needle valve;

[0012] S4: Growth of InAs / GaSb superlattice

[0013] Under controlled As background pressure conditions, InAs and GaSb superlattice structures are grown layer by layer.

[0014] Preferably, the substrate preparation and pretreatment includes the following specific steps:

[0015] S101: Select a suitable GaSb single crystal substrate and clean it with an organic solvent;

[0016] S102: rinsing the cleaned substrate with deionized water;

[0017] S103: placing the substrate in a molecular beam epitaxy system for heating and degassing treatment;

[0018] S104: The substrate is heated to perform a deoxidation treatment, and the surface state of the substrate is monitored by reflected high-energy electron diffraction.

[0019] Preferably, the thickness of the GaSb single crystal substrate selected in step S101 is 300-400 μm, the temperature of the degassing treatment in step S103 is 280-320°C, the time is 2-3 hours, the vacuum degree is <10^-10 mbar, and the temperature of the deoxidation treatment in step S104 is 500-540°C, and the duration is 2-3 minutes.

[0020] Preferably, the epitaxial growth of the GaSb buffer layer comprises the following specific steps:

[0021] S201: adjusting the substrate temperature to a suitable GaSb growth temperature;

[0022] S202: epitaxially grow a GaSb buffer layer on the substrate, and monitor the changes in surface morphology during the epitaxial growth process by RHEED.

[0023] Preferably, the GaSb growth temperature selected in step S201 is in the range of 430-450° C., the growth thickness of the GaSb buffer layer in step S202 is 500-600 nm, and the growth rate is 0.5-1.0 μm / h.

[0024] Preferably, the control of the As background pressure comprises the following specific steps:

[0025] S301: opening the As needle valve in advance during the growth of the GaSb buffer layer;

[0026] S302: gradually adjust the opening of the As needle valve to control and stabilize the background pressure in the chamber.

[0027] Preferably, in step S301, the initial opening of the As needle valve is 70%-80%, and the holding time is 60-90 minutes. In step S302, the opening of the As needle valve is controlled within the range of 6%-10%, so that the background pressure is stabilized between 7×10^-8 and 1×10^-7 Torr.

[0028] Preferably, the growth of the InAs / GaSb superlattice comprises the following specific steps:

[0029] S401: After the growth of the GaSb buffer layer is completed, the As needle valve opening is increased to a level required for the growth of the InAs layer;

[0030] S402: growing InAs and GaSb superlattice structures layer by layer at a suitable growth temperature;

[0031] S403: After the growth is completed, the chamber temperature is slowly lowered and the sample is taken out of the growth chamber.

[0032] Preferably, in the step S401, the opening of the As needle valve is adjusted to 16%-20% when the InAs layer is grown, in the step S402, the growth temperature of the InAs layer is 480-500°C, the growth temperature of the GaSb layer is 430-450°C, and in the step S403, the cooling rate is controlled at 5-10°C / min, and the final temperature drops to below 100°C.

[0033] The invention provides a method for controlling As background pressure to reduce InAs / GaSb surface defects.

[0034] It has the following beneficial effects:

[0035] 1. The present invention can effectively reduce the "bright spot" defects and other surface defects formed on the surface of InAs / GaSb by accurately controlling the As background pressure, thereby improving the surface quality of the superlattice material.

[0036] 2. The present invention controls the As background pressure within a suitable range, which helps to reduce deep energy level defects and crystal dislocations in the lattice, thereby improving the crystallization quality of the material.

[0037] 3. The present invention reduces surface defects and improves crystal quality, can reduce dark current and noise inside the material, and improve the performance of devices such as infrared detectors, especially in detection applications in the long-wave infrared band.

[0038] 4. The present invention makes the epitaxial growth process more stable and more repeatable by precisely controlling the opening of the As needle valve and the background pressure, thereby improving production efficiency and product consistency. BRIEF DESCRIPTION OF THE DRAWINGS

[0039] Figure 1 This is a schematic diagram showing the effect of the size of the As needle valve on the background pressure of the present invention;

[0040] Figure 2 This is a schematic diagram of the influence of the As background pressure corresponding to the As valve size of the present invention being 16.18%;

[0041] Figure 3 This is a schematic diagram of the influence of the As background pressure corresponding to the As valve size of the present invention being 8.6%. DETAILED DESCRIPTION

[0042] The technical solution of the present invention will be described clearly and completely below in conjunction with the accompanying drawings of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0043] Please refer to the attached Figure 1 - Attachment Figure 3 The embodiment of the present invention provides a method for controlling the As background pressure to reduce InAs / GaSb surface defects, comprising the following steps:

[0044] S1: Substrate preparation and pretreatment

[0045] Background and Purpose:

[0046] Before epitaxial growth, cleaning and pretreatment of GaSb substrates are crucial to ensure the purity and activity of the substrate surface to avoid interference of foreign impurities in the subsequent growth process. Effective degassing and deoxidation treatment of the substrate can remove water molecules, oxygen molecules and other volatile impurities adsorbed on the surface, thereby improving the crystal quality of epitaxial materials.

[0047] Detailed steps:

[0048] S101: Select and clean substrate

[0049] Select a suitable GaSb (001) single crystal substrate with a thickness range of 300-400 μm. This thickness range is selected based on thermal treatment stability and subsequent processing requirements. Clean the substrate with organic solvents such as acetone and isopropanol, each solvent for 5-10 minutes to remove organic contaminants on the surface.

[0050] S102: Deionized water rinse

[0051] The cleaned substrate is rinsed with deionized water for 3-5 minutes to ensure that residual solvents and contaminants are completely removed and the surface of the substrate is clean.

[0052] S103: Degassing treatment

[0053] The rinsed substrate is placed in a molecular beam epitaxy (MBE) system, and the system vacuum is maintained at <10^-10mbar. The substrate is heated to 280-320℃ for 2-3 hours for degassing. This step mainly removes residual moisture and other volatile substances on the surface and inside of the substrate to ensure the cleanliness of the substrate at high temperature.

[0054] S104: Deoxidation treatment

[0055] The substrate temperature is raised to 500-540°C and maintained for 2-3 minutes. The substrate surface state is monitored in real time by reflection high energy electron diffraction (RHEED) to ensure complete removal of surface oxides and obtain a smooth and clean substrate surface.

[0056] S2: Epitaxial growth of GaSb buffer layer

[0057] Background and Purpose:

[0058] The growth of GaSb buffer layer is to provide a good substrate, so that the subsequent InAs / GaSb superlattice material can be uniformly epitaxially grown on it. The quality of the buffer layer directly affects the crystallization quality and surface morphology of the superlattice material.

[0059] Detailed steps:

[0060] S201: Temperature adjustment

[0061] The substrate temperature is adjusted to 430-450°C, which is the appropriate temperature range for epitaxial growth of GaSb buffer layer. Too high or too low temperature will lead to a decrease in the crystal quality of the buffer layer.

[0062] S202: Epitaxial Growth

[0063] A GaSb buffer layer with a thickness of 500-600nm is grown on the substrate at a rate of 0.5-1.0μm / h. The surface morphology changes during the epitaxial growth process are monitored in real time by RHEED to ensure that the buffer layer has good crystal quality and a smooth surface.

[0064] S3: Control of As background pressure

[0065] Background and Purpose:

[0066] During the epitaxial growth of the GaSb buffer layer, the control of the As background pressure is crucial. By precisely adjusting the opening of the As needle valve, the As background pressure in the MBE chamber can be controlled, thereby reducing the surface defects formed during the epitaxial growth process.

[0067] Detailed steps:

[0068] S301: As needle valve initial setting

[0069] Before starting to grow the GaSb buffer layer, the As needle valve is opened in advance, with an initial opening of 70%-80% and maintained for 60-90 minutes. This process ensures that the As beam reaches a stable state at the beginning of epitaxial growth.

[0070] S302: Adjust the As needle valve

[0071] During the epitaxial growth process, the opening of the As needle valve is gradually adjusted and finally controlled within the range of 6%-10%, so that the background pressure is stabilized between 7×10^-8 and 1×10^-7 Torr. The stability of this background pressure is crucial to reducing surface defects.

[0072] S4: Growth of InAs / GaSb superlattice

[0073] Background and Purpose:

[0074] The growth of InAs / GaSb superlattice requires strict control of the growth conditions of each layer to ensure the layered structure and good surface morphology of the material. The adjustment of As needle valve and temperature control are important factors that determine the quality of superlattice growth.

[0075] Detailed steps:

[0076] S401: Increase the opening of the As needle valve

[0077] After the growth of the GaSb buffer layer is completed, the As needle valve opening is increased to 16%-20% to meet the As supply demand for the growth of the InAs layer.

[0078] S402: Layer-by-layer growth of superlattice

[0079] At a suitable temperature, the InAs and GaSb superlattice structures are grown layer by layer. The growth temperature of the InAs layer is 480-500°C, and the growth temperature of the GaSb layer is 430-450°C, ensuring the high-quality growth of each layer.

[0080] S403: Slowly cool down and take out the sample

[0081] After the superlattice structure is grown, the cavity temperature is slowly lowered, and the cooling rate is controlled at 5-10°C / minute. The final temperature drops to below 100°C, and then the sample is taken out of the growth chamber.

[0082] The following is an introduction in conjunction with specific embodiments:

[0083] The German ultra-high vacuum (<10-11mbar) molecular beam epitaxy equipment KomponentOCTOPLUS400 system was used, the As2 beam was provided by a cracking source furnace, and the AFM atomic force microscope was used to analyze the crystal surface morphology and defect density.

[0084] Embodiment 1:

[0085] It was found that the As background pressure is closely related to the needle valve size. A higher background pressure is not conducive to the epitaxial growth of superlattice materials. By changing the size of the As needle valve, the valve value corresponding to a lower background pressure is obtained.

[0086] The method comprises the following steps: first, a two-inch GaSb (001) substrate with a thickness of 350 mm is placed in a growth chamber, and the substrate is heated to 300°C for high-temperature degassing, and the degassing time is not less than two hours. Next, the temperature is raised to 520°C for deoxidation, and the surface morphology of the material is detected by RHEED until the dots and lines are clearly visible, and the growth begins after three minutes. The temperature is then lowered to 440°C, and a GaSb buffer layer with a thickness of 500 nm is grown on the GaSb substrate. The temperature is then lowered to 420°C, and a beam flow monitor (BFM) is pushed under the substrate to monitor the beam size and background pressure. The As needle valve is first set to 80%, and the valve size is gradually reduced after stabilization for 90 minutes. Wait for 5 minutes before each recording to ensure the stability of the pressure. Finally, the sample is slowly cooled to 100°C and taken out of the growth chamber.

[0087] Embodiment 2:

[0088] To further verify the effect of background pressure on reducing nanoscale "bright spot" defects, a comparative experiment was conducted by growing the same superlattice material with two different needle valve sizes during the epitaxial InAs / GaSb superlattice.

[0089] The method comprises the following steps: first, a two-inch GaSb (001) substrate with a thickness of 350 mm is placed in a growth chamber, and the substrate is heated to perform high-temperature degassing at 300° C., and the degassing time is not less than two hours. Next, the temperature is raised to 520° C. for deoxidation, and the surface morphology of the material is detected by RHEED until the points and lines are clearly visible, and the growth begins after three minutes. The temperature is then lowered to 440° C., and a GaSb buffer layer with a thickness of 500 nm is grown on the GaSb substrate. The temperature is then lowered to 420° C., and different growth methods are adopted for the GaSb buffer layer: the As needle valve is opened in advance when the GaSb layer is epitaxially grown, and the valve sizes are 16.18% and 8.6% respectively, wherein the valve size of Sb is kept unchanged, and the temperatures of the In source furnace and the Ga source furnace are kept constant. Finally, the sample is slowly cooled to 100° C. and taken out of the growth chamber.

[0090] Expected Results

[0091] The relationship between the As needle valve size and background pressure obtained in Example 1 is as follows Figure 1 As shown. During the molecular beam epitaxy process, when growing InAs / GaSb superlattice, if the As needle valve is not opened in advance, the beam current will hardly reach the preset value, and the lack of As will cause needle defects on the surface of the epitaxial material. Therefore, when growing GaSb, the As needle valve will be opened in advance, and the freely diffused As will form a background pressure in the chamber. Excessive background pressure will have a negative impact on the growth of GaSb, and the background pressure is usually required to be maintained at a low level (10-7Torr~10-8Torr). A higher As background will cause the following problems:

[0092] (1) Impurity introduction: As in high background pressure may cause additional impurities to be adsorbed onto the GaSb epitaxial layer, affecting the purity and electrical properties of the material;

[0093] (2) Oxidation and other reactions: Under high background pressure, As and Sb atoms compete for adsorption, which easily forms GaAsSb or GaAs interface, affecting the chemical composition and crystal structure of GaSb;

[0094] (3) Changes in growth rate: High background pressure may affect the energy distribution of the As beam, thereby affecting the growth rate and the surface morphology of the epitaxial layer.

[0095] like Figure 1 The effect of the As needle valve size on the background pressure shows that when the needle valve is less than 10%, the corresponding background pressure is 8.77×10-8Torr, indicating that the background pressure effect caused by the early opening of As is very small, which meets the background conditions for high-quality epitaxy of GaSb. Therefore, the size of the As valve needs to be precisely controlled during the growth process to avoid the "bright spot" defect problem on the surface of the superlattice material caused by excessive As background.

[0096] After taking out the sample from Example 2, the effects of different valve sizes on epitaxial quality when the As needle valve is opened in advance during the growth of the GaSb layer are compared. AFM observations show that the superlattice surface morphology is strongly dependent on the size of the As valve opening in advance, such as Figure 2 and Figure 3 As shown. When the As valve size is 16.18%, the corresponding As background pressure is about 1.29×10-6Torr. At this time, the InAs / GaSb interface is greatly affected by the background, point defects appear on the surface, and the surface roughness is 0.805nm. When the As valve is reduced to 8.6%, the As background is reduced to the order of 10-8. At this time, there are no obvious defects on the superlattice surface, and the surface roughness is 0.319nm.

[0097] When the As needle valve is reduced to below 10%, the background pressure in the epitaxial process drops to the order of 10-8, effectively avoiding the problem of competitive adsorption of As during the growth of the GaSb layer. The resulting InAs / GaSb superlattice surface has no obvious defects, effectively eliminating the point defects that are easily formed during the epitaxy of superlattice materials, thereby better paving the way for the preparation of high-performance antimonide superlattice infrared detector devices.

[0098] Summary Conclusion

[0099] The present invention proposes a theoretical model for the cause of point-like "bright spot" defects on the surface of InAs / GaSb type II superlattice, and a growth method for controlling the As background pressure used to effectively eliminate the defects. The As element valve is opened in advance during the growth of the GaSb epitaxial layer, and the needle valve size is controlled within 10% according to the BMF test result. While ensuring that the As beam current reaches stability during the growth of the InAs layer, the As background pressure in the GaSb layer epitaxy process is effectively controlled to be on the order of 10-8. The present invention effectively solves the problems of competitive adsorption of As atoms under high background pressure, poor surface morphology of epitaxial wafers, and increased roughness, and can improve the quality of type II superlattice epitaxial growth. The invention has broad application prospects and technical advantages for superlattice infrared detectors.

[0100] Although embodiments of the present invention have been shown and described, it will be appreciated by those skilled in the art that various changes, modifications, substitutions and variations may be made to the embodiments without departing from the principles and spirit of the present invention, and that the scope of the present invention is defined by the appended claims and their equivalents.

Claims

1. A method for controlling As background pressure to reduce InAs / GaSb surface defects, characterized in that: The following steps are involved: S1: Substrate preparation and pretreatment Selecting a suitable GaSb single crystal substrate, and performing selection, cleaning, degassing and deoxidation treatment on the GaSb substrate; S2: Epitaxial growth of GaSb buffer layer epitaxially growing a GaSb buffer layer on the processed GaSb substrate; S3: Control of As background pressure During the growth of the GaSb buffer layer, the As background pressure in the chamber is controlled and stabilized by adjusting the opening of the As needle valve; S4: Growth of InAs / GaSb superlattice Under controlled As background pressure conditions, InAs and GaSb superlattice structures are grown layer by layer.

2. A method for controlling As background pressure to reduce InAs / GaSb surface defects according to claim 1, characterized in that: The substrate preparation and pretreatment includes the following specific steps: S101: Select a suitable GaSb single crystal substrate and clean it with an organic solvent; S102: rinsing the cleaned substrate with deionized water; S103: placing the substrate in a molecular beam epitaxy system for heating and degassing treatment; S104: The substrate is heated to perform a deoxidation treatment, and the surface state of the substrate is monitored by reflected high-energy electron diffraction.

3. A method for controlling As background pressure to reduce InAs / GaSb surface defects according to claim 2, characterized in that: The thickness of the GaSb single crystal substrate selected in step S101 is 300-400 μm, the temperature of the degassing treatment in step S103 is 280-320° C., the time is 2-3 hours, the vacuum degree is <10^-10 mbar, and the temperature of the deoxidation treatment in step S104 is 500-540° C., and the duration is 2-3 minutes.

4. The method of controlling As background pressure to reduce InAs / GaSb surface defects according to claim 1, characterized in that: The epitaxial growth of the GaSb buffer layer comprises the following specific steps: S201: adjusting the substrate temperature to a suitable GaSb growth temperature; S202: epitaxially grow a GaSb buffer layer on the substrate, and monitor the changes in surface morphology during the epitaxial growth process by RHEED.

5. A method for controlling As background pressure to reduce InAs / GaSb surface defects according to claim 4, characterized in that: The GaSb growth temperature range selected in step S201 is 430-450° C., the growth thickness of the GaSb buffer layer in step S202 is 500-600 nm, and the growth rate is 0.5-1.0 μm / h.

6. The method of controlling As background pressure to reduce InAs / GaSb surface defects according to claim 1, characterized in that: The control of the As background pressure includes the following specific steps: S301: opening the As needle valve in advance during the growth of the GaSb buffer layer; S302: gradually adjust the opening of the As needle valve to control and stabilize the background pressure in the chamber.

7. A method for controlling As background pressure to reduce InAs / GaSb surface defects according to claim 6, characterized in that: In step S301, the initial opening of the As needle valve is 70%-80%, and the holding time is 60-90 minutes. In step S302, the opening of the As needle valve is controlled within the range of 6%-10%, so that the background pressure is stabilized between 7×10^-8 and 1×10^-7 Torr.

8. The method of controlling As background pressure to reduce InAs / GaSb surface defects according to claim 1, characterized in that: The growth of the InAs / GaSb superlattice comprises the following specific steps: S401: After the growth of the GaSb buffer layer is completed, the As needle valve opening is increased to a level required for the growth of the InAs layer; S402: growing InAs and GaSb superlattice structures layer by layer at a suitable growth temperature; S403: After the growth is completed, the chamber temperature is slowly lowered and the sample is taken out of the growth chamber.

9. A method for controlling As background pressure to reduce InAs / GaSb surface defects according to claim 8, characterized in that: The opening of the As needle valve in step S401 is adjusted to 16%-20% when the InAs layer is grown. The growth temperature of the InAs layer in step S402 is 480-500°C. The growth temperature of the layer is 430-450°C, and the cooling rate in the step S403 is controlled at 5-10°C / min. Eventually the temperature dropped below 100°C.

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