Patent Application
20200381271
This disclosure relates to substrate processing. More particularly, the present disclosure relates to improved heating and cooling of a substrate during substrate processing.
Substrate processing for making electrical devices, electronic devices, optical devices, mechanical devices, and so forth, may entail operations such as layer deposition, substrate etching, ion implantation, annealing, or other processes, where heating and cooling of a substrate takes place. Many types of processing systems employ resistive heating, where a substrate is placed in contact with a substrate holder of a platen directly or indirectly resistively heated. Heat is then conducted from the platen or holder to the substrate. Other types of processing systems may employ lamp heating, such as halogen lamps, to heat a substrate primarily by radiation.
When substrates are processed under vacuum or reduced pressure below atmospheric pressure, thermal conduction decreases between a heated platen or holder and a substrate, as compared to thermal conduction at atmospheric pressure. Thus, heating of a substrate using a heated platen or holder may be sluggish for processed at reduced pressure of under vacuum.
Likewise, radiative heating of substrates may be inefficient for many types of substrates due to the low absorbance of the substrate. For example, silicon wafers are semitransparent to infrared radiation, such as radiation emitted by halogen lamps at substrate temperatures below approximately 500° C. Moreover, other substrates remain highly transparent to radiation including infrared radiation up to much higher temperatures. Accordingly, lamp heating of substrates may be inefficient and result in sluggish heating rates.
Regarding substrate cooling, once substrate processing at elevated temperatures is completed, relatively rapid cooling of the substrate to lower temperature such as room temperature may be useful to increase throughput and reduce overall process time. Effectively cooling substrates such as semiconductor wafers (wafers) in a vacuum environment is challenging due to the poor thermal contact between the wafer and surroundings. Due to metal contamination and particle generation concerns, the use of thermal gaskets or other mechanical means to increase thermal contact conductance (TCC) or simply, “thermal conductance,” from a substrate is not widely deployed. Even using electrostatic clamping and backside gas cooling yields an effective TCC is estimated to be less than 1000 W/m2-K, a thermal conductance not sufficient to generate very rapid substrate cooling. One approach for wafer cooling is to transfer a wafer from vacuum in a processing chamber to a loadlock, where the loadlock is maintained under atmospheric pressure, and to accordingly cool the wafer using natural convection. Under this approach, as an example, approximately 12 minutes may be required to cool a Si wafer from 150° C. to 60° C. In addition, the wafer may endure >10° C. temperature differential during cooling.
With respect to these and other considerations the present disclosure is provided.
In one embodiment A heating system for heating a substrate is provided. The heating system may include a susceptor, where the susceptor has a substrate support surface. The heating system may further include a heat transfer layer, disposed on the substrate support surface, where the heat transfer layer comprising an array of aligned carbon nanotubes.
In a further embodiment, a cooling apparatus for cooling a substrate may include a cooling block. The cooling block may include a housing, where the housing has a substrate support surface. The cooling block may also include a phase change component, disposed within the housing, and a heat transfer layer, disposed on the substrate support surface, the heat transfer layer comprising an array of aligned carbon nanotubes.
In another embodiment, a cooling system for cooling a substrate may include a cooling block. The cooling block may include a housing, having a substrate support surface and a lower surface, opposite the substrate support surface. The cooling block may also include a phase change component, disposed within the housing. The cooling system may also include a first heat transfer layer, disposed on the substrate support surface, where the first heat transfer layer includes a first array of aligned carbon nanotubes. The cooling system may include a cooling structure, disposed opposite the lower surface, and a second heat transfer layer, disposed in contact with the lower surface and the cooling structure, where the second heat transfer layer includes a second array of aligned carbon nanotubes.
The present embodiments will now be described more fully hereinafter with reference to the accompanying drawings, where various embodiments are shown. The subject of this disclosure, however, may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so this disclosure will be thorough and complete, and will fully convey the scope of the subject of this disclosure to those skilled in the art. In the drawings, like numbers refer to like elements throughout.
In the following description and/or claims, the terms “on,” “overlying,” “disposed on” and “over” may be used in the following description and claims. “On,” “overlying,” “disposed on” and “over” may be used to indicate where two or more elements are in direct physical contact with each other. However, “on,”, “overlying,” “disposed on,” and over, may also mean where two or more elements are not in direct contact with each other. For example, “over” may mean where one element is above another element but not contact each other and may have another element or elements in between the two elements. Furthermore, the term “and/or” may mean “and”, it may mean “or”, it may mean “exclusive-or”, it may mean “one”, it may mean “some, but not all”, it may mean “neither”, and/or it may mean “both”, although the scope of claimed subject matter is not limited in this respect.
Various embodiments involve apparatus and systems to rapidly heat as well as rapidly cool a substrate, including under vacuum conditions. The term “elevated temperature” as used herein, refers to substrate temperatures generally greater than approximately 50° C. Various embodiments are particularly useful for heating and cooling substrates when disposed in an atmosphere below ambient pressure, such as below 100 Torr, below 1 Torr, or below 1 mTorr pressure. Accordingly, the present embodiments provide enhanced thermal coupling between a temperature sensor and substrate via improved thermal and mechanical properties of a thermal transfer layer.
The susceptor 104 may be constructed of a known material, such as a ceramic, graphite or a metallic material. In various non-limiting embodiments, over a given operating range, such as room temperature to 500° C. or room temperature to 1000 C, the susceptor material used to form susceptor 104 may be an opaque material to radiation such as infrared radiation, and may have a high emissivity.
The heat transfer assembly 102 may be designed to abut against a substrate, such as substrate 110. The substrate 110 may be a flat body, such as a wafer or plate, where the substrate defines a substrate plane coincident with or parallel to the X-Y plane of the Cartesian coordinate system shown. The susceptor 104 may also have a flat surface, and in some embodiments may extend over an area matching or exceeding the area of the substrate 110 within the X-Y plane. According to various embodiments of the disclosure, the array of aligned carbon nanotubes within the heat transfer layer 106 extend along an alignment direction, perpendicular to the substrate plane, meaning along the Z-axis, forming a layer of vertical array of carbon nanotubes (VACNT). The heat transfer layer 106 may be formed wherein the carbon nanotubes are formed as a growing layer of vertically aligned nanotubes on the susceptor 104 or may be formed by applying a pre-formed layer of vertically aligned nanotubes onto the susceptor 104.
As shown in
There are multiple advantages provided by the arrangement of
Notably, known halogen lamp heating of a Si wafer is not efficient due to the nature of the silicon material. Silicon is largely semitransparent in the infrared range (above ˜1 μm wavelength), as shown is shown in
This function determines the fraction of the total emissive power in a wavelength band. F0→1.1μm·3000K=0.34, indicating (1-0.34)=66% of the emissive power resides in wavelength >1.1 μm for a heat source of 3000K. This property, taken together with the transmission curve shows a silicon wafer will not absorb a significant portion of the total emissive energy. An opaque susceptor with high emissivity (ε>0.8), on the other hand, can more readily absorb the incoming radiative energy. According to some embodiments, the susceptor 104 may be formed from graphite (ε>0.8), heavily-doped Si, or other materials having a high ε coating. The VACNTs are known to have high emissivity, an thus may be used to coat a susceptor body in some embodiments.
By using VACNTs as a heat transfer layer 106, between the susceptor 104 and the substrate 110, heat is more efficiently transferred from the susceptor 104 to the substrate 110, especially at relatively lower temperatures, such as below 750 C. This heat transfer can overcome a low thermal contact conductance between susceptor 104 and substrate 110, imposed by vacuum. For example, in known systems, when processing a substrate under vacuum conditions (meaning below a few Torr pressure around the substrate), even with the assistance of backside gas supplied to a substrate holder or susceptor, the thermal conductance between susceptor and substrate does not exceed 1,000 W/m2-K. As a result, a large ΔT (>10° C.) is generally observed for known systems operating under vacuum conditions using a susceptor and substrate, without a VACNT layer.
In particular, arrays (forests) of vertical aligned carbon nanotubes (VACNTs) have exceptional thermal and mechanical properties. These properties render the VACNTs as excellent materials to overcome thermal contact resistance under vacuum. The high thermal conductivities of CNTs, (predictions suggest a value 6,600 W/m-K for single-wall, and 3,000 W/m-K for multiwalled) significantly enhance the heat flow between the substrate 110 and the susceptor 104. Research has shown the thermal conductance of carbon nanotubes may be in the range of 50,000 up to 143,000-250,000 W/m2-K. Such high thermal conductance will render the substrate temperature nearly identical to the temperature of the susceptor, such as within 0.5° C. of the susceptor temperature. Moreover, carbon nanotubes exhibit a degree of mechanical compliance, where the mechanical compliance of carbon nanotubes may allow the heat transfer layer 106 to fill any gaps between substrate 110 and susceptor 104, due to surface roughness, thus providing better thermal path between susceptor 104 and substrate 110.
In operation, the use of the heating apparatus 100 provides for more rapid substrate heating when the heating assembly 108 is turned on or powered. Since the susceptor 104 has a high absorbance of radiation including in the IR range, the use of lamps such as halogen ramps will rapidly and efficiently transfer radiant energy into the susceptor 104. As the susceptor 104 heats up, thermal transfer to the substrate 110 will be very efficient due to the high thermal conductance of the carbon nanotubes.
In other embodiments of the disclosure, enhanced substrate cooling approaches are provided. These approaches may employ apparatus including a cooling block and a heat transfer layer. The cooling block may include a housing having a substrate support surface, and a phase change component, disposed within the housing. The heat transfer layer may include an array of aligned carbon nanotubes, such as those described above.
In these embodiments, the cooling block and heat transfer layer may provide an enhance substrate cooling approach, where, due to carbon nanotube's superb mechanical and thermal properties, a TCC>>10,000 W/m2-K is achieved. This large thermal conductance, combined with the use of a phase change material embedded in a cooling block, may bring a substrate, such as a wafer to room temperature from 150° C. or 500° C. in less than 1 minute while maintaining temperature uniformity <1° C. during cooling.
In various embodiments, the housing of a cooling block may be in the shape of an enclosure containing phase change material (PCM), where a vertical aligned carbon nanotube (VACNTs) coating is disposed on top of the enclosure. The heat transfer layer may be formed wherein the carbon nanotubes are formed as a growing layer of vertically aligned nanotubes on the enclosure or may be formed by applying a pre-formed layer of vertically aligned nanotubes onto the enclosure.
As detailed below, in different embodiments, an enclosure or cooling block containing phase change material may be a stand-alone apparatus, may be mounted onto a cooled structure, such as a water-cooled chamber wall, or may be affixed to a cooled structure such as by contacting a cooled chamber wall through another layer of vertically aligned nanotubes.
In the embodiment of
As shown in
As in previously described embodiments, the nanotubes may be single-walled or multi-walled. Because of high thermal conductance of the arrays of vertically aligned carbon nanotubes, the heat transfer layer 206 rapidly conducts heat from the substrate 110 to the cooling block 204 During a cooling operation, when a substrate 110 is placed on the cooling block 204 sensible heat from the substrate 110 is absorbed by the phase change material. The latent heat of fusion the PCM aids in maintaining a uniform temperature of the cooling block 204, i.e., the melting point of the PCM. In accordance with different embodiments of the disclosure, the PCM material may be selected based on the desired substrate temperature after processing, as well as the latent heat of fusion. For example, to achieve a substrate temperature of 40° C. after cooling, use of a PCM having a melting temperature of approximately 40° C. may be appropriate, since the temperature of the PCM material of the phase change component 208 will not exceed 40° C. until sufficient heat has been absorbed to completely melt the PCM material. Said differently, to ensure 40° C. temperature is rapidly reached, the PCM material may be provided with sufficient mass and sufficiently high latent heat of fusion to ensure the phase change component 208 is not completely melted when sufficient heat is transferred from the substrate 110 to cooling block 204 to cool the substrate from a process temperature to 40° C.
Notably, in some embodiments, the phase change material in cooling block 204 may be replaced by a heat pipe with appropriate heat removal capabilities.
In operation, when a substrate 110 is processed at high temperature at a first location is a system or apparatus, the substrate may be transferred after processing to a position where the substrate 110 is brought into contact with the cooling block 204 and heat transfer layer 206. Once brought into contact with the cooling block 204, including heat transfer layer 206, the substrate 110 will rapidly cool at least to the temperature of the cooling block 204.
Based on the high thermal conductance of aligned carbon nanotubes, an analytical model was developed to predict the time needed to cool the substrate 110 to 30° C. under different conditions, assuming an enclosure temperature of 25° C. Notably, while research has shown the thermal conductance of carbon nanotubes may be in the range of 50,000 up to 143,000-250,000 W/m2-K, more conservative values for thermal conductance are used to illustrate the benefit of a heat transfer layer of vertically aligned carbon nanotubes. Table 1 shows a comparison of cooling cycle duration for cooling a silicon wafer from two different elevated temperatures according to different scenarios. In a radiation-only scenario, the substrate is removed from a heat source and allowed to cool in vacuum by self-radiation. In a load lock cooling scenario, the substrate is placed in a load lock with an atmosphere approximately 1 atm. In the columns for TCC=1,000 W/m2-K, TCC=25,000 W/m2-K, the substrate is instantly placed on the cooling block, such as in the arrangement of
In accordance with various embodiments of the disclosure, the PCM material selected for use in a cooling block may we be either an inorganic or organic material. For scenarios to cool a substrate rapidly to room temperature, one can choose the family of paraffin waxes, which family in general contains different members having melting points in the range of 5° C.−60° C., depending on the exact chemical composition. A short list of related hydrocarbons is shown in II. As shown, the temperature to be reached by a substrate may be controlled by choice of the melting temperature of the PCM material, where a family of materials providing melting point increments as small as 3 degrees are readily available.
As discussed above, cooling of a substrate by the cooling block 204 involves transfer of heat from the substrate at a given initial temperature to the cooling block 204. To cool a 150° C. 300 mm-Si wafer, having a mass of 128 gram, to 25° C., the sensible heat needed to be removed is 11,320 J. To process 25 wafers, 1.6 kg n-Heptadecane (e.g. a Ø300 mm cylinder with a height of 29 mm) is needed. To process more wafers without a hard stop, the amount or size of the PCM material would increase linearly with increased number of wafers. Thus, the embodiment of
For cooling larger numbers of wafers, other embodiments provide configurations where a cooling block is thermally grounded.
In the embodiment of
In sum, there are notable advantages of the embodiments of
The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize the usefulness is not limited thereto and the present disclosure may be beneficially implemented in any number of environments for any number of purposes.
This application claims priority to U.S. Provisional Patent Application No. 62/853,324, filed May 28, 2019, entitled SYSTEM AND APPARATUS FOR ENHANCED SUBSTRATE HEATING AND RAPID SUBSTRATE COOLING, and incorporated by reference in its entirety.
| Number | Date | Country | |
|---|---|---|---|
| 62853324 | May 2019 | US |
