Method for forming carbon film

Abstract

To form a high-hardness carbon film by achieving both high energy of carbon ions and suppression of excessive temperature rise of a base material during film formation.SOLUTION: In a method for forming a carbonaceous film using a sputtering method, a bias voltage to be applied to a substrate 2 during a treatment for forming the carbonaceous film is set to 100 to 400V, the substrate 2 is cooled simultaneously with the treatment for forming the carbonaceous film, and the carbonaceous film is formed on the substrate 2 while the temperature of the substrate 2 is maintained at a temperature lower by 30 °C or more than the substrate temperature at the end of film formation in a case where the carbonaceous film is formed under a condition that the substrate is not cooled.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for forming a carbon film using a sputtering method. [Background technology]

[0002] There are known techniques for depositing various hard films, such as metal carbide films, metal nitride films, and diamond-like carbon films, on the surfaces of workpieces (workpieces) such as dies used in various processes such as press working and forging, as well as machine parts and electronic parts, in order to improve wear resistance and lubrication.

[0003] Diamond-like carbon (hereinafter sometimes referred to as "DLC"), a type of hard carbon film, is an amorphous carbon film with both the sp3 bonds of diamond and the sp2 bonds of graphite in its carbon atom skeleton. DLC is one of the hard films that has been put to practical industrial use, and there are active efforts being made to improve its quality, such as by increasing its hardness, improving its friction and wear resistance, and improving its adhesion to substrates.

[0004] Regarding a film formation method for obtaining high-hardness DLC, Non-Patent Document 1 discloses that the hardness of DLC depends on the bias voltage, and that increasing the bias voltage can improve hardness, but that increasing the bias voltage too much can actually result in a decrease in hardness. On the other hand, it is generally known that high-energy sputtered particles cause an increase in the temperature of the substrate and the formed DLC film, and Non-Patent Document 2 discloses that as the temperature of the DLC increases, the structure of the DLC converts from a diamond structure to a graphite structure, resulting in a decrease in hardness. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] Hiroshi Murakami and three others, "DLC Thin Film Synthesis by Filtered Arc Method", Nissin Electric Technical Review Vol.47, No.1 (2002.3), pp.15-19 [Non-patent document 2] C.Ziebert, 4 others, “Interfaces and temperature stability of stepwise graded DLC filmsstudied by nanoindentation and Raman spectroscopy”, Surface& Coatings Technology 200 1-4(2005), p.1127-1131 Summary of the Invention [Problem to be solved by the invention]

[0006] As mentioned above, increasing the energy of carbon ions by using a high bias voltage during film formation is effective in obtaining DLC ​​with high hardness, but this leads to an increase in the temperature of the substrate and DLC, so if excessive temperature increases in the substrate and DLC during film formation are not suppressed, the hardness will actually decrease. Previously, there was no established technology that could simultaneously increase the energy of carbon ions and suppress excessive temperature increases in the substrate and DLC during film formation, and there was a limit to how much energy could be increased to obtain DLC with high hardness.

[0007] The present invention has been made in view of the above circumstances, and aims to form a carbon film with high hardness by achieving both high energy carbon ions during film formation and suppression of excessive temperature rise in the substrate. [Means for solving the problem]

[0008] The present invention, which solves the above-mentioned problems, is a method for forming a carbon film by a sputtering method, characterized in that a bias voltage applied to a substrate during the carbon film formation process is set to 100 to 400 V, the substrate is cooled simultaneously with the carbon film formation process, and the carbon film is formed on the substrate while maintaining the temperature of the substrate at a temperature that is 30°C or more lower than the substrate temperature at the end of film formation when the carbon film is formed under conditions where the substrate is not cooled. [Effects of the Invention]

[0009] This method achieves both high energy carbon ions during film formation and suppression of excessive temperature rise in the substrate, making it possible to form a carbon film with high hardness. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a diagram showing an example of a layered structure of a DLC coating material in which a DLC film is formed on a substrate. [Figure 2] 1 is a diagram showing an example of a film formation apparatus for performing a film formation method according to an embodiment of the present invention. [Figure 3] FIG. 10 is a front view (viewed from the positive side in the X direction) of the plate that supports the substrate. [Figure 4] 10 is a vertical cross-sectional view of the housing of the substrate cooling mechanism as viewed from the positive side in the X direction. FIG. [Figure 5] FIG. 1 is a diagram showing a pulse pattern in a DLC film formation test. [Figure 6] FIG. 10 is a diagram showing the relationship between the cooling effect of the substrate and the hardness increasing effect in a DLC film formation test. DETAILED DESCRIPTION OF THE INVENTION

[0011] As a result of extensive investigations, the present inventors have found that in producing a carbon film by sputtering, when the temperature of the substrate on which the carbon film is to be produced is 300°C or less at the end of film production, cooling the substrate during film production can improve the hardness of the carbon film compared to when the substrate is not cooled, and have completed the present invention.

[0012] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In this specification and the drawings, elements having substantially the same functional configurations are designated by the same reference numerals, and redundant description will be omitted.

[0013] Fig. 1 is a diagram showing an example of the layer structure of a DLC coating material in which DLC, a type of carbon film, is formed on a substrate. The DLC coating material 1 shown in Fig. 1 is composed of a substrate 2, an intermediate layer 3 formed on the substrate 2, and a DLC film 4 formed on the intermediate layer 3. The substrate 2 is a steel material such as alloy tool steel for mold applications, such as SKD11, or high-speed tool steel, but the material of the substrate 2 is not particularly limited as long as it is a material known as a material on which DLC film 4 can be formed.

[0014] The intermediate layer 3 is a layer formed as a surface treatment prior to the deposition of the DLC 4, and contains, for example, chromium, tungsten, and carbon. The intermediate layer 3 serves to improve adhesion between the substrate 2 and the DLC 4, and is formed when necessary depending on the application of the DLC coating material 1. Note that, as in this embodiment, there may be another layer interposed between the substrate 2 and the DLC 4. However, in this specification, "a state in which a DLC film is formed on a substrate" includes a state in which the DLC film is formed directly on the surface of the substrate, and a state in which another layer is interposed between the substrate and the DLC and the DLC film is formed on the surface of that layer.

[0015] The carbon film deposition method described in this embodiment is a method for depositing DLC4 using a sputtering method, but first, an example of a deposition apparatus for carrying out the deposition method will be described with reference to Figures 2 to 4. Note that the X direction, Y direction, and Z direction in the figures are perpendicular to one another.

[0016] 2, a carbon film forming apparatus 10 according to this embodiment includes a chamber 11 in which a film forming process is performed, a base 12 fixed to the bottom surface of the chamber 11, and a support pillar 13 extending vertically upward (positive side in the Z direction) from the center of the base pillar 12. An attachment mechanism 14 is provided near the upper end of the support pillar 13 to fix a housing 31 of a substrate cooling mechanism 30 (described later) to the support pillar 13.

[0017] A plate 15 for supporting the substrate 2 is provided on the outside of the wall surface of the housing 31 opposite to the wall surface on the mounting mechanism 14 side (positive side in the X direction). The plate 15 is fixed to the housing 31 by mechanical fastening means such as screws while in contact with the housing 31. A jig 16 for fixing the substrate 2 to the plate 15 is attached to the wall surface on the opposite side to the housing 31 side of the plate 15 (positive side in the X direction). The plate 15 is made of a material with high thermal conductivity, such as aluminum nitride.

[0018] FIG. 3 is a front view of the plate 15 as viewed from the positive side in the X direction. The substrate 2 illustrated in FIG. 3 is disk-shaped, and the jig 16 corresponding to this substrate shape has an annular top surface 16a and a cylindrical side wall surface 16b. The top surface 16a of the jig 16 is fixed to the plate 15 by a mechanical fastening means such as screws, and a space for accommodating the substrate 2 is provided inside the jig 16. The dashed line around the substrate 2 illustrated in FIG. 3 indicates the peripheral shape of the substrate 2, and the inner diameter of the annular top surface 16a is smaller than the diameter of the substrate 2. Therefore, the substrate 2 accommodated inside the jig 16 is fixed in position relative to the plate 15 with its film-forming surface exposed, without falling out of the jig 16. The shape of the jig 16 can be appropriately changed depending on the shape of the substrate 2, and the arrangement position and number of the substrates 2 relative to the plate 15 can also be appropriately changed.

[0019] A thermocouple 17 is installed on the surface of the plate 15, and the surface temperature of the plate 15 can be measured by this thermocouple 17. As described above, the plate 15 is made of a material with high thermal conductivity such as aluminum nitride, and the temperature of the substrate 2 in contact with the plate 15 can be considered to be approximately the same temperature as the surface temperature of the plate 15. In other words, the surface temperature of the plate 15 measured by the thermocouple 17 during film formation can be considered to be the temperature of the substrate 2 during film formation.

[0020] Referring again to FIG. 2, a graphite target 18 is installed as a carbon target on a wall portion facing the plate 15 inside the chamber 11. A bias power supply 19 that applies a bias voltage to the substrate 2 is connected to the plate 15. The bias power supply 19 may be a DC power supply or a pulse power supply. A sputtering pulse power supply 20 that applies a pulsed voltage to generate sputtering is connected to the graphite target 18. Note that since the bias voltage is a negative voltage, the sign of the voltage value is negative, but the negative sign will be omitted in this specification.

[0021] A gas inlet 21 for supplying a sputtering gas such as argon gas or neon gas is provided on the wall of chamber 11 opposite to the wall on the graphite target 18 side (negative side in the X direction). The wall on which gas inlet 21 is provided is also provided with an exhaust pipe 22 connected to a vacuum pump (not shown). During the DLC film formation process, the pressure inside chamber 11 is controlled by adjusting the amount of sputtering gas supplied from gas inlet 21.

[0022] Next, the substrate cooling mechanism 30 for cooling the substrate 2 during the film formation process will be described. As shown in Fig. 2, the substrate cooling mechanism 30 has a hollow housing 31 and a refrigerant supply pipe 32 connected to the bottom of the housing 31. This refrigerant supply pipe 32 is connected to a refrigerant supply source 33 installed outside the chamber 11. The refrigerant supply source 33 is capable of adjusting the refrigerant temperature, and refrigerant adjusted to a set temperature can be supplied from the refrigerant supply pipe 32 into the housing 31. The refrigerant may be liquid or gas, and for example, a fluorine-based insulating coolant is used.

[0023] 4 is a vertical cross-sectional view of the housing 31 as seen from the positive side in the X direction. A refrigerant circulation space 34 is formed inside the housing 31. A partition plate 35 that divides the circulation space 34 is provided in the center of the bottom surface of the housing 31. The partition plate 35 extends vertically upward (toward the positive side in the Z direction) from the center of the bottom surface of the housing 31, and a gap exists between the upper end of the partition plate 35 and the top surface of the housing 31. A refrigerant discharge pipe 36 is connected to the bottom surface of the housing 31, and is provided at a distance from the refrigerant supply pipe 32. The partition plate 35 is located between the refrigerant supply pipe 32 and the refrigerant discharge pipe 36.

[0024] As indicated by the arrows in the housing 31 in FIG. 4 , the refrigerant supplied from the refrigerant supply pipe 32 into the housing 31 rises between the wall surface of the housing 31 and the partition plate 35, passes between the top surface of the housing 31 and the upper end of the partition plate 35, flows toward the refrigerant discharge pipe 36, and is discharged from the refrigerant discharge pipe 36. This distributes the refrigerant throughout the flow space 34 within the housing 31, lowering the temperature of the entire wall surface of the housing 31. As a result, the plate 15 ( FIG. 2 ) in contact with the housing 31 is cooled, and the substrate 2 in contact with the plate 15 is also cooled. That is, according to the substrate cooling mechanism 30 described above, the substrate 2 is cooled by heat conduction via the plate 15 by the refrigerant circulating within the housing 31. The refrigerant flow path formed by the partition plate 35 described above is an example and may be changed as appropriate depending on the size and number of substrates 2 to be installed.

[0025] Furthermore, the coolant discharge pipe 36 is connected to the above-mentioned coolant supply source 33, and the coolant discharged from the coolant discharge pipe 36 is adjusted to a set temperature by the coolant supply source 33, and then supplied again to the coolant supply pipe 32. In other words, the substrate cooling mechanism 30 has a structure in which the coolant circulates. The cooling capacity of the substrate 2 by the substrate cooling mechanism 30 can be controlled by changing the set temperature, flow rate, total amount, etc. of the coolant, taking into consideration the sputtering conditions, the conductivity and heat capacity of the coolant, etc.

[0026] The above describes one example of the film formation apparatus 10. The materials of the components constituting the film formation apparatus 10 are appropriately selected so as not to impede the formation of the DLC 4 on the substrate 2 by sputtering the graphite target 18.

[0027] (DLC film formation method) Next, a method for depositing DLC ​​as a carbon film using the above-described film deposition apparatus 10 will be described.

[0028] First, as a pretreatment before DLC deposition, argon bombardment is performed as needed to clean the surface of the substrate 2. Next, to improve adhesion between the substrate 2 and the DLC 4, an intermediate layer 3 is formed on the substrate 2 as needed. Specifically, for example, a film containing chromium, tungsten, and carbon is formed as the intermediate layer 3.

[0029] Next, argon gas and neon gas as sputtering gases are supplied into chamber 11 from gas inlet 21 so that the flow rate ratio of argon gas to neon gas is 1:1. The pressure inside chamber 11 is maintained at, for example, 100 Pa or less by adjusting the flow rate of the gas introduced into chamber 11.

[0030] Thereafter, the bias power supply 19 and the sputtering pulse power supply 20 are turned ON. As a result, argon ions that have been converted into plasma between the substrate 2 and the graphite target 18 collide with the graphite target 18, and the generated carbon ions are deposited as DLC 4 on the surface of the substrate 2, thereby progressing film formation. Meanwhile, a bias voltage is applied to the substrate 2 during film formation, and the higher this bias voltage, the more easily the temperature of the substrate 2 rises.

[0031] In the film-forming process for forming the DLC4 film, the bias voltage applied to the substrate 2 is set to 100 to 400 V. If the bias voltage is less than 100 V, the energy of the carbon ions is low, making it impossible to form a DLC4 film of good quality. On the other hand, if the bias voltage exceeds 400 V, the plasma discharge mode tends to shift to arc discharge, making it impossible to perform a normal DLC film-forming process.

[0032] In the film formation method according to this embodiment, when a film formation process is performed, the substrate cooling mechanism 30 described above is operated to supply a coolant into the housing 31. This reduces the temperature of the housing 31 and the plate 15 in contact with the housing 31, and cools the substrate 2 supported by the plate 15. That is, according to the film formation method according to this embodiment, by cooling the substrate 2 simultaneously with the formation of the DLC film, it is possible to form the DLC film while suppressing a temperature rise in the substrate 2.

[0033] The temperature of the substrate 2 during the film formation process is maintained at a temperature at least 30°C lower than the substrate temperature at the end of film formation when DLC4 is formed without cooling the substrate 2 (hereinafter referred to as the "uncooled temperature"). As will be shown in the examples described later, when DLC4 is formed while maintaining the temperature of the substrate 2 at a temperature at least 30°C lower than the uncooled temperature, it is possible to form a DLC4 film that is harder than DLC4 formed without cooling the substrate 2. The uncooled temperature may be measured in advance by a DLC4 film formation test, or may be estimated based on the film formation conditions.

[0034] If the difference between the temperature of the substrate 2 during the film formation process and the temperature when not cooled is less than 30°C, the structural change of DLC4 from a diamond structure to a graphite structure is not sufficiently suppressed, and the effect of increasing hardness is small. Therefore, from the viewpoint of increasing the hardness of DLC4, the temperature of the substrate 2 during the film formation process is preferably maintained at a temperature at least 35°C lower than the temperature when not cooled, and more preferably at a temperature at least 40°C or 50°C lower. Furthermore, the difference between the temperature of the substrate 2 during the film formation process and the temperature when not cooled may be 100°C or higher as long as it does not interfere with the DLC4 film formation process, but is preferably 90°C or lower, and more preferably 80°C or lower or 70°C or lower.

[0035] Furthermore, the temperature of the substrate 2 during the film formation process is preferably 80° C. or less. On the other hand, when forming a DLC4 film without cooling the substrate 2 in order to measure the temperature when not cooled, the temperature of the substrate 2 during the film formation process is preferably 300° C. or less.

[0036] According to the film formation method described above, the energy of carbon ions can be increased during film formation by setting the bias voltage to 100 to 400 V, while softening due to structural changes in DLC can be suppressed by maintaining the temperature of the substrate 2 during film formation at a temperature at least 30°C lower than the temperature when not cooled. In other words, DLC can be formed while achieving both high energy carbon ions and suppression of excessive temperature rise in the substrate during film formation, and a carbon film with high hardness can be obtained.

[0037] While the present invention has been described above by way of example, it is understood that the present invention is not limited to such examples. It is clear that a person skilled in the art can conceive of various modifications and alterations within the scope of the technical ideas set forth in the claims, and that such modifications and alterations are also within the technical scope of the present invention.

[0038] For example, the components of the above-described embodiments can be combined in any manner, and such combinations will naturally provide the functions and advantages of the individual components involved in the combination, as well as other functions and advantages that will be apparent to those skilled in the art from the description herein. [Example]

[0039] A DLC film formation test was conducted to evaluate a film formation method in which the substrate was cooled simultaneously with DLC film formation.

[0040] (Basic conditions for DLC film formation) In this deposition test, a Domino mini (manufactured by Oerikon Balzers) was used as the deposition apparatus, and DLC deposition was performed on multiple test substrates made of SKD11, each 22 mm in diameter and 7 mm thick. A film containing chromium, tungsten, and carbon was deposited on the surface of the substrate as a surface treatment to ensure adhesion. The carbon target measured 70 mm x 450 mm, and the deposition apparatus was equipped with a cooling mechanism for cooling the substrate, similar to the deposition apparatus shown in Figure 2. In this example, Fluorinert FC-40 (manufactured by 3M) was used as the coolant passed through the cooling mechanism.

[0041] Argon gas (Ar) and neon gas (Ne) were supplied as sputtering gases into the chamber at a flow rate ratio of 1:1, and the flow rates of the argon gas and neon gas were each set to 100 sccm so that the degree of vacuum inside the chamber was maintained at 0.6 Pa. In addition, the on / off time setting of the sputtering voltage was controlled in a pulsed manner using the pattern shown in Figure 5, and the sputtering voltage value was set so that the peak current value was 500 A.

[0042] The bias voltage conditions and cooling conditions for each test are explained below. The film formation time was 60 minutes.

[0043] Example 1 The bias voltage applied to the substrate was set to 100 V, and the temperature of the coolant supplied to the substrate cooling mechanism was set to 20°C, and the DLC film was formed while cooling the substrate.

[0044] <Example 2> The bias voltage applied to the substrate was set to 200 V, and the temperature of the coolant supplied to the substrate cooling mechanism was set to 20°C, and the DLC film was formed while cooling the substrate.

[0045] Example 3 The bias voltage applied to the substrate was set to 300 V, and the temperature of the coolant supplied to the substrate cooling mechanism was set to 20°C, and the DLC film was formed while cooling the substrate.

[0046] Example 4 The bias voltage applied to the substrate was set to 400 V, and the temperature of the coolant supplied to the substrate cooling mechanism was set to 20°C, and the DLC film was formed while cooling the substrate.

[0047] <Example 5> The bias voltage applied to the substrate was set to 200 V, and the temperature of the coolant supplied to the substrate cooling mechanism was set to 40°C, and the DLC film was formed while cooling the substrate.

[0048] <Comparative Example 1> Without applying a bias voltage to the substrate, the temperature of the coolant supplied to the substrate cooling mechanism was set to 20°C, and the DLC film was formed while cooling the substrate.

[0049] <Comparative Example 2> The bias voltage applied to the substrate was set to 200 V, and the temperature of the coolant supplied to the substrate cooling mechanism was set to 60°C, and the DLC film was formed while cooling the substrate.

[0050] <Conventional Examples 1 to 5> In Conventional Examples 1 to 5, DLC films were formed without supplying a cooling liquid to the substrate cooling mechanism. That is, in Conventional Examples 1 to 5, the substrate was not cooled during DLC ​​film formation. In Conventional Example 1, the bias voltage applied to the substrate was 0 V, in Conventional Example 2, the bias voltage applied to the substrate was 100 V, in Conventional Example 3, the bias voltage applied to the substrate was 200 V, in Conventional Example 4, the bias voltage applied to the substrate was 300 V, and in Conventional Example 5, the bias voltage applied to the substrate was 400 V.

[0051] After forming the DLC film under the above conditions, the substrate temperature at the end of film formation for each example (the surface temperature of plate 15 shown in Figure 2) was recorded, and the cooling effect (the temperature at the end of film formation when the substrate was cooled - the temperature at the end of film formation without cooling) was calculated compared to Conventional Examples 1-5, in which the substrate was not cooled during film formation. Furthermore, the surface hardness of the DLC for each example was measured.

[0052] Further, Raman spectroscopy was carried out under the following analytical conditions to measure the peak intensity ratio of the D band to the G band (ID / IG ratio) and the half-width of the G band. Exposure time: 10 seconds Excitation wavelength: 532nm Grating: 1800 l / mm Slit width: 65μm Aperture (spot diameter): 10 μm Laser intensity: 0.5mW

[0053] The differences in deposition conditions for each of the above-described examples, as well as the measurement results of the DLC surface hardness, ID / IG ratio, and G-band half-width, are shown in Tables 1 and 2 below. Figure 6 also shows the relationship between the cooling effect of the substrate (deposition end temperature without cooling - deposition end temperature with cooling) and the increase in hardness (DLC hardness with cooling of the substrate - DLC hardness without cooling). The substrates compared when calculating the "cooling effect" and "hardness increment" are two substrates with the same bias voltage application conditions. For example, the cooling effect and hardness increment of the substrate of Example 1 shown in Table 1 below are calculated for comparison with the substrate of Conventional Example 2, which has the same bias voltage of 100 V.

[0054] [Table 1]

[0055] [Table 2]

[0056] As shown in Tables 1 and 2 above, in Examples 1 to 5 where the cooling effect of the substrate was 30°C or more, the DLC hardness was found to be increased compared to other examples where the cooling effect was less than 30°C and the bias voltage conditions were the same.

[0057] As the proportion of sp3 bonds in DLC increases, the G peak position shifts to the lower wavenumber side and the ID / IG ratio decreases. As shown in Tables 1 and 2 above, in Examples 1 to 5, the ID / IG ratio is smaller than in other examples where the cooling effect is less than 30°C and the bias voltage conditions are the same. Therefore, it is presumed that the proportion of sp3 bonds in the DLC of Examples 1 to 5 is increased.

[0058] The half-width of the G band indicates the degree of crystallinity due to sp2 bonds, and a larger value indicates greater bond disorder, leading to increased film density and Young's modulus. Since the half-width of the G band is larger in Examples 1 to 5 than in other examples where the cooling effect is less than 30°C and the bias voltage conditions are the same, the DLC of Examples 1 to 5 has increased film density and Young's modulus, which is presumably responsible for the increased hardness.

[0059] The above describes embodiments of the present invention. The effects described herein are merely illustrative or exemplary and are not limiting. In other words, the technology disclosed herein may achieve other effects that will be apparent to those skilled in the art from the description of this specification, in addition to or in place of the above effects. [Industrial Applicability]

[0060] The present invention can be applied when forming a carbon film on a substrate. [Explanation of symbols]

[0061] 1 DLC coating material 2 Base material 3. Middle class 4 DLC 10 Film deposition equipment 11 Chambers 12 Foundation 13 Posts 14 Mounting mechanism 15 plates 16 Jig 16a Top 16b Side wall 17 Thermocouple 18 Graphite Target 19 Bias power supply 20. Pulse power supply for sputtering 30 Base material cooling mechanism 31 Case 32 Refrigerant supply pipe 33 Refrigerant supply source 34 Refrigerant circulation space 35 Divider 36 Refrigerant discharge pipe

Claims

1. A method for forming a carbon film using a sputtering method, comprising: a bias voltage applied to the substrate during the carbon film formation process is set to 100 to 400 V; a film formation method, characterized in that the substrate is cooled simultaneously with the carbon film formation process, and the carbon film is formed on the substrate while maintaining a temperature of the substrate at a temperature that is 30° C. or more lower than a substrate temperature at the end of film formation when the carbon film is formed under conditions where the substrate is not cooled.

2. 2. The film forming method according to claim 1, wherein the temperature of the substrate is maintained at 80[deg.] C. or less during the film forming process of the carbon film.

3. 3. The film forming method according to claim 1, wherein the substrate outside the housing is cooled by circulating a coolant through a hollow housing supporting the substrate.

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