Filament for heater pedestals

Abstract

Various embodiments of this specification relate to pedestals for semiconductor processing, and methods for their use and manufacture. In a particular embodiment, a pedestal for supporting a semiconductor substrate during processing includes a body having an inner portion and an outer portion surrounding the inner portion, an inner filament disposed in the inner portion of the body having a first temperature coefficient of resistance (TCR), and an outer filament disposed in the outer portion of the body having a second TCR, the second TCR being different from the first TCR.

Description

Technical Field

[0001] Incorporation by Reference The PCT application is filed herewith simultaneously with this specification as part of this application. Each application claiming any benefit or priority identified in the simultaneously filed PCT application is incorporated herein by reference in its entirety for all purposes.

[0002] Various embodiments of this specification relate to filaments for use in a heater pedestal for processing a semiconductor substrate, and methods and apparatus for using such filaments.

Background Art

[0003] Semiconductor processing often involves controlling the temperature of the semiconductor substrate being processed. One way to control the substrate temperature involves placing the substrate on a temperature-controlled pedestal within the processing chamber. The pedestal may also be referred to as a substrate support, a chuck, etc.

[0004] The description of the background art provided herein is for the purpose of generally presenting the context of the present disclosure. To the extent that it is described in this background art section, the research of the inventors, as well as aspects of the description that may not be regarded as prior art at the time of filing, are not admitted as prior art to the present disclosure, either expressly or implicitly.

Summary of the Invention

Means for Solving the Problems

[0005] Various embodiments of this specification relate to methods and apparatus for processing semiconductor substrates, and methods for manufacturing such apparatus. In various embodiments, the method or apparatus includes a pedestal having certain characteristics. For example, in one aspect of the disclosed embodiments, a pedestal is provided for supporting a semiconductor substrate during processing, the pedestal comprising a body having an inner portion and an outer portion surrounding the inner portion, an inner filament disposed in the inner portion of the body having a first temperature coefficient of resistance (TCR), and an outer filament disposed in the outer portion of the body having a second TCR, the second TCR being different from the first TCR.

[0006] In various embodiments, the inner and outer filaments may be configured to be powered together by a single power source. For example, the inner and outer filaments may be physically and electrically connected to each other.

[0007] The pedestal may include any number of parts, each having a filament inside. For example, in various embodiments, the body of the pedestal may further include an additional annular portion, which is located between the inner and outer portions of the body of the pedestal, and the pedestal may further include an additional filament located in the additional annular portion, which has a third TCR, the third TCR being different from the first and second TCRs.

[0008] In some embodiments, the second TCR may be greater than the first TCR. In some such embodiments, the second TCR may be at least about 0.1% greater than the first TCR. In other embodiments, the second TCR may be lower than the first TCR.

[0009] Several materials may be used for the inner and outer filaments. In various embodiments, the inner and outer filaments may comprise different materials. In some such embodiments, the inner and outer filaments may comprise different metals. In these and other embodiments, the inner and outer filaments may comprise the same material but may be doped with different dopants or to different degrees. In various embodiments, the inner and / or outer filaments may undergo heat treatment and / or plasma treatment to modify the first TCR and / or second TCR, respectively.

[0010] In some embodiments, the body of the pedestal may further include a second outer portion surrounding the outer portion, and the pedestal may further include a second outer filament positioned in the second outer portion, wherein the inner and outer filaments are configured to be powered by a first power source, and the second outer filament is configured to be powered by a second power source such that the inner and outer filaments can be powered independently of the second outer filament.

[0011] Another aspect of the disclosed embodiment provides a method for processing a substrate, the method comprising the steps of receiving the substrate on a pedestal in a processing chamber, the pedestal being a body having an inner portion and an outer portion surrounding the inner portion, the body having an upper surface in contact with the substrate, the body comprising an inner filament disposed in the inner portion of the body having a first temperature coefficient of resistance (TCR), and an outer filament disposed in the outer portion of the body having a second TCR, the second TCR being different from the first TCR, the step of supplying power to the pedestal to raise the pedestal to a high temperature, and processing the substrate at a high temperature.

[0012] In various embodiments, the inner and outer filaments may be powered together by a single power source. In these or other embodiments, the temperature profile on the upper surface of the pedestal body may be high and substantially uniform.

[0013] In some embodiments, the steps of supplying power to the pedestal to raise the pedestal to a high temperature and processing the substrate at a high temperature may include (1) a step of supplying power to the pedestal to raise the pedestal to a first temperature and processing the substrate at the first temperature in a first time, and (2) a step of supplying power to the pedestal to raise the pedestal to a second temperature and processing the substrate at the second temperature in a second time, wherein the first and second temperatures differ by at least about 50°C, and the temperature profile on the upper surface of the pedestal body is substantially uniform at both the first and second temperatures.

[0014] Another aspect of the disclosed embodiments provides a method for manufacturing a pedestal for supporting a semiconductor substrate during processing, the method comprising the steps of: providing an inner filament having a first temperature coefficient of resistance (TCR); providing an outer filament having a second TCR, wherein the second TCR differs from the first TCR, and both the inner and outer filaments are configured to be powered together by a single power source; providing a pedestal body; and sealing the inner and outer filaments within the pedestal body.

[0015] In some embodiments, the second TCR may be greater than the first TCR. In some such embodiments, the second TCR may be at least about 0.1% greater than the first TCR. In other embodiments, the second TCR may be lower than the first TCR.

[0016] These and other embodiments are described further below with reference to the drawings. [Brief explanation of the drawing]

[0017] [Figure 1A] A diagram showing a temperature-controlled pedestal for processing a semiconductor substrate. [Figure 1B] A diagram showing a temperature-controlled pedestal for processing a semiconductor substrate. [Figure 1C] A diagram showing a temperature profile that can occur across the entire surface of the pedestal during processing. [Figure 1D] A diagram showing a temperature profile that can occur across the entire surface of the pedestal during processing. [Figure 1E] A diagram showing an example of the resistance of a filament versus temperature. [Figure 1F] A diagram showing the resistance of a pedestal that can provide a uniform temperature over only a small temperature range versus temperature. [Figure 2A] A diagram showing temperature-controlled pedestals for processing a semiconductor substrate according to various embodiments. [Figure 2B] A diagram showing temperature-controlled pedestals for processing a semiconductor substrate according to various embodiments. [Figure 2C] A graph showing the resistance of an exemplary pedestal versus temperature according to the embodiments shown in FIGS. 2A and 2B. [Figure 2D] A graph showing the temperature profile across an exemplary pedestal according to the example of FIGS. 2A - 2C. [Figure 3] A diagram showing an exemplary deposition apparatus according to various embodiments.

DETAILED DESCRIPTION OF THE INVENTION

[0018] In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present disclosure. The disclosed embodiments may be practiced without some or all of these specific details. In other instances, well-known process operations have not been described in detail in order not to unnecessarily obscure the disclosed embodiments. The disclosed embodiments are described in conjunction with specific embodiments, but it is understood that the disclosed embodiments are not intended to be limiting.

[0019] Many semiconductor processing operations can be optimized by controlling the temperature of the substrate being processed. Such operations can include, but are not limited to, deposition, etching, plasma processing, heat treatment, etc. Embodiments herein are not limited to any particular type of semiconductor substrate processing. Rather, the embodiments disclosed herein can be applied to many different types of processing.

[0020] One technique available for controlling the temperature of a semiconductor substrate during processing is to provide a temperature-controlled pedestal on which the substrate is supported. One purpose of the pedestal is to heat the substrate to a desired processing temperature. Such a pedestal can include heating elements and / or cooling elements for controlling the temperature, if desired. In many cases, the heating element includes a filament that is heated by resistive heating. The filament heats the pedestal to a high temperature, for example, in many embodiments, from about 250°C to about 700°C. Temperatures outside this range may be used.

[0021] It can be difficult to ensure that all portions of the pedestal in contact with the substrate are at a uniform temperature. Without the desired temperature uniformity, the substrate processing results can be insufficient and non-uniform. For example, in many cases, the variation in substrate temperature across the surface of the substrate is desirably less than about 5°C. This narrow substrate temperature range facilitates uniform processing results on the substrate, such as the deposition of a film having uniform properties across the surface of the substrate. Without proper temperature control, the die-to-die variation across the substrate resulting from processing at different temperatures can rise to unacceptable levels.

[0022] Furthermore, designing a pedestal that can achieve the desired temperature uniformity at different target processing temperatures is particularly challenging. As will be discussed further below, many temperature-controlled pedestals are designed to operate at a single temperature or a very narrow temperature range. When operating outside these temperatures, the pedestal often fails to provide an acceptablely uniform temperature across its entire surface, resulting in temperature gradients in both the pedestal and the substrate being processed. This leads to suboptimal processing results.

[0023] A pedestal comprises one or more filaments surrounded by a body. In some cases, the filaments include metals, ceramic metal-based materials (e.g., cermet materials), metal alloys, carbon-based materials, graphite-based materials, and the like. In various embodiments, the filaments may include one or more metals selected from the group consisting of molybdenum, tungsten, iron, chromium, aluminum, nickel, platinum, tantalum, rhodium, and combinations thereof. Specific examples of filament materials include, but are not limited to, tungsten, iron-chromium-aluminum, nickel-chromium, molybdenum, tungsten, platinum, tantalum-rhodium, platinum-rhodium, and the like.

[0024] In many semiconductor processing applications, particularly those involving high temperatures, the preferred material for the pedestal body is ceramic due to its ability to be heated to high temperatures without material changes that would impair the pedestal's function. However, ceramic materials have significantly lower thermal conductivity compared to other materials such as metals. As a result, the temperature does not fully equilibrium within the ceramic pedestal. Furthermore, due to the geometric shape of the pedestal, different parts of the pedestal lose heat at different rates, resulting in different nominal pedestal temperatures at different locations on the pedestal. Typically, heat loss is greatest at the outer edges of the pedestal. These heat losses can occur through radiation, convection, and conduction, all of which are temperature-dependent.

[0025] Figures 1A and 1B show a pedestal 102 used in semiconductor processing. Figure 1A is a top view of the pedestal 102, and Figure 1B is a side view of the pedestal 102. The pedestal includes an inner portion 106 and an outer portion 108. The inner portion 106 and the outer portion 108 typically form a single body, with the outer portion 108 surrounding the inner portion 106. In other words, although the inner portion 106 and the outer portion 108 are labeled as two separate elements, it is understood that these elements refer to different parts of a single pedestal body, as shown in Figures 1A and 1B. A filament 110 is wound around the pedestal 102, which includes both the inner portion 106 and the outer portion 108. The filament is in electrical contact with a power supply 112 that provides power (e.g., current, I) to heat the pedestal 102 through the filament 110.

[0026] Without careful design of the filament 110 and its placement within the pedestal 102, heat loss from the outer portion 108 may exceed heat loss from the inner portion 106. As a result, a temperature gradient is formed throughout the pedestal 102, with the highest temperature near the center of the pedestal (e.g., in the inner portion 106) and the lowest temperature near the edge of the pedestal (e.g., in the outer portion 108). This temperature gradient is shown in Figure 1C.

[0027] Figure 1C focuses on a case where a pedestal becomes relatively hot near its center and relatively cold near its edge, but it should be understood that other types of thermal gradients can also be formed during processing. For example, some pedestals may become relatively cold near their center and relatively hot near their edge during processing. Such thermal gradients can occur in particular cases where the body of the pedestal is supported on a stem, and the stem experiences significant heat loss. The stem of the pedestal supports the body of the pedestal at its center. If there is significant heat loss through the stem, it can cause the body of the pedestal to become relatively cold near its center (closer to where heat is lost through the stem) and relatively hot near its edge. Although most of the examples herein are presented in a situation where the center is relatively hot and the edges are colder, as shown in Figure 1C, the techniques herein can be applied to any other type of pedestal exhibiting a temperature gradient. Furthermore, although most of the examples herein are presented in the context of a pedestal having two parts (e.g., an inner part and an outer part), any number of parts can be defined on the pedestal to fine-tune the heating to facilitate uniform temperature treatment.

[0028] Various techniques have been developed to counteract these temperature gradients. For example, instead of using a single filament, multiple filaments may be used. The filaments may be placed in different parts of the pedestal to create different heating zones on the pedestal. For example, the first filament may be placed in the inner part of the pedestal and the second filament in the outer part of the pedestal, thereby creating an inner heating zone and an outer heating zone. Additional filament / heating zones may be provided as needed. Each filament can be powered independently, thereby allowing different parts of the pedestal to be heated at different rates, achieving a more uniform temperature across the entire pedestal. Furthermore, independent heating control for different heating zones allows a single pedestal to provide the desired temperature uniformity over a relatively wide range of target processing temperatures. Such multi-zone pedestals are very valuable because a single pedestal can be used for many different processes at different target temperatures. On the other hand, multi-zone pedestals are considerably more complex and expensive to manufacture. Each additional heating zone adds a significant cost to the pedestal. Such costs may relate to processes such as brazing of associated electrical connections, and / or hardware such as additional power supplies and wiring. It is often desirable to keep these costs low in order to minimize equipment costs associated with substrate processing.

[0029] Another technique that can be used to counteract temperature gradients across a pedestal is to use a pedestal with a single filament carefully positioned to provide a uniform temperature across the pedestal. For example, the filament may be arranged (e.g., wound) within the pedestal to provide non-uniform coil density and / or non-uniform filament density. This non-uniform filament arrangement results in different heating rates within different parts of the pedestal, thereby balancing the different heat loss rates from different parts of the pedestal. Of course, it should be understood that multiple filaments can be used instead of a single filament. Such filaments would result in a pedestal that is electrically connected to each other, all powered by a single power source, and is considered to have only a single heating zone. This is in contrast to the technique described above, which involves providing multiple power sources to independently power the filaments and form different independently controllable heating zones on the substrate.

[0030] The single-heating-zone technology is advantageous because it requires only a single filament powered by a single power source. Therefore, this technology is relatively affordable to implement compared to the multi-heating-zone methods described above. However, this technology produces a pedestal that provides the desired level of temperature uniformity only over a very limited temperature range (e.g., in some cases, a range of about 25°C or less). For example, a first pedestal may provide an acceptablely uniform pedestal temperature over a range of about 250°C to about 275°C, or a second pedestal with a different filament arrangement may provide an acceptablely uniform pedestal temperature over a range of about 275°C to about 300°C, and so on.

[0031] Figure 1D shows a graph illustrating temperature versus position on a pedestal for a pedestal with a single heating zone. In this example, the pedestal is designed to operate at a temperature of approximately 400°C. Two different scenarios are shown. In the first scenario, indicated by line 150, the pedestal operates at approximately 400°C. The temperature across the entire surface of the pedestal is extremely uniform because the pedestal is designed to operate at that temperature. In the second scenario, indicated by line 152, the pedestal operates at a temperature of approximately 600°C, about 200°C higher than its intended operating temperature. In this case, a temperature gradient is formed across the entire surface of the pedestal, with the highest temperature near the center of the pedestal and the lowest temperature near the edge of the pedestal. The substrate processed in the first scenario, indicated by line 150, shows high-quality, uniform results. In contrast, the substrate processed in the second scenario, indicated by line 152, shows relatively lower quality, more uniform results due to the temperature gradient formed on the substrate during processing.

[0032] The pedestal described in relation to Figure 1D can provide a uniform temperature in the first scenario, but not in the second scenario. This is a common problem for pedestals that have only a single heating zone. The pedestal cannot simply cope with the greater heat loss near the edge of the pedestal that occurs at the higher temperature in the second scenario.

[0033] Various embodiments of this specification address the aforementioned problems by providing a pedestal having a single power supply and multiple filaments, where the filaments have different characteristics but are all driven by the same power supply. More specifically, the filaments are selected to have different temperature coefficients of resistance (TCR).

[0034] Each material possesses an inherent property known as electrical resistivity, which refers to the resistance the material exerts against the flow of electric current. Electrical resistance is a corresponding external property and, furthermore, depends, for example, on the geometric shape of the relevant structure and the path of the current. This resistance to current manifests as heat (e.g., Joule heating) and forms the basis of resistive heating.

[0035] Electrical resistivity (and therefore electrical resistance) is often highly dependent on temperature. Typically, resistivity and resistance increase with temperature. This sensitivity of resistivity to temperature is a material property called TCR. For example, a typical TCR value for materials such as molybdenum is approximately 0.005 / °C (with respect to a reference temperature such as 0°C). This TCR represents the rate at which the resistance of 1 ohm at a reference temperature (e.g., 0°C) increases with temperature.

[0036] Previously, when multiple filaments were driven by a single power supply, the filaments were uniform to each other (e.g., made of the same material) and therefore had the same TCR. In some other embodiments, a pedestal may have multiple heating zones, each having multiple power supplies, where at least one of the power supplies powers the heating zones described herein, each having filaments with different TCRs in different parts of the heating zone driven by the power supply. In other words, the techniques described herein are particularly useful for manufacturing pedestals having only a single heating zone with a single power supply, but these techniques are not limited in that way and can also be used to manufacture pedestals having multiple heating zones and multiple power supplies.

[0037] Figure 1E shows an example of resistance versus temperature for a molybdenum (or other) filament. In this example, the molybdenum filament has a TCR of approximately 0.005 / °C and a resistance of approximately 1 ohm at a reference temperature of approximately 0°C. It can be seen that for every 200°C increase in temperature, the resistance increases by approximately 1 ohm (or more generally, the resistance value at 0°C). The slope of the line shown in Figure 1E is the TCR. At 200°C, the resistance is 2 ohms; at 400°C, the resistance is 3 ohms; at 600°C, the resistance is 4 ohms, and so on. When a current (e.g., approximately 10 amperes) is driven through this resistance, a Joule heating power is produced, given by the current squared × resistance. Thus, at 0°C, the resistance provides 100W; at 200°C, the resistance provides 200W; at 400°C, the resistance provides 300W; at 600°C, the resistance provides 400W, and so on.

[0038] As mentioned above, resistivity is an inherent material property, while resistance is an external property that depends on the geometric shape. Therefore, a filament (or assembly of filaments) with a uniform TCR can generate different resistances in different parts of the pedestal, depending on the geometric shape of the filaments within the pedestal.

[0039] In contrast, different techniques are used in various embodiments of this specification. In short, this technique involves selecting filaments having different TCRs for different parts of the pedestal. In many embodiments, the pedestal includes both an inner and an outer portion, as described, for example, in relation to Figure 1A. The TCR and resistance of the filament passing through the inner portion of the pedestal may differ from those of the filament passing through the outer portion of the pedestal. Typically, the filaments are arranged such that the resistance of the filament passing through the outer portion is greater than that of the filament passing through the inner portion (the inner and outer portions have approximately equal surface areas) to cope with the greater heat loss occurring near the edges of the pedestal compared to the center. Of course, the opposite may be true if it is desired to counteract relatively large heat loss near the inner portion of the pedestal and relatively low heat loss near the outer portion of the pedestal.

[0040] In the example shown in Figure 1F, a power of 630W is desirable to maintain the pedestal at a uniform temperature of approximately 400°C, with 300W accounting for heat loss through the inner portion of the pedestal and 330W accounting for heat loss through the outer portion of the pedestal (the inner and outer portions have approximately equal surface area and are powered together by a single power source). The resistive molybdenum coil filament is embedded in the pedestal such that the filament portion in the inner portion of the pedestal (represented by line 160) has a cumulative resistance of approximately 3 ohms at approximately 400°C, and the filament portion in the outer portion of the pedestal (represented by line 162) has a cumulative resistance of approximately 3.3 ohms at approximately 400°C. When a current of approximately 10A is passed through the filament to heat the filament, pedestal, and substrate, the filament portion on the inside of the pedestal supplies approximately 300W of power, the filament portion on the outside of the pedestal supplies approximately 330W of power, and therefore supplies a total power of approximately 630W as needed to maintain the pedestal at the desired temperature of approximately 400°C.

[0041] In relation to this same exemplary pedestal operating at different temperatures of approximately 600°C, modeling determines that 3600W of power is required to maintain the pedestal at a uniform temperature, with approximately 1600W accounting for heat loss from the inner portion of the pedestal and approximately 2000W accounting for heat loss from the outer portion of the pedestal. As shown in Figure 1F, at 600°C, the filament portion of the inner portion of the pedestal (represented by line 160) has a resistance of approximately 4 ohms, and the filament portion of the outer portion of the pedestal (represented by line 162) has a resistance of approximately 4.4 ohms. Increasing the heating current to 20A supplies the desired 1600W to the inner portion of the pedestal (20A 2 *4 ohms = 1600W). However, the outer part of the pedestal is 1760W (20A). 2It is heated only at a power of 4.4 ohms (1760W), which is 240W short of the power required to maintain the outer portion of the pedestal at the desired temperature of 600°C. As a result, the edges of the pedestal are below the desired 600°C, and the edges of the substrate being processed are below their intended processing temperature.

[0042] The temperature profile generated across the entire surface of the pedestal can be illustrated with reference to Figure 1D. For example, line 150 represents the temperature profile across the entire surface of the pedestal when it is heated to a temperature of approximately 400°C, which is the temperature at which it is designed to operate. In this case, the temperature profile is substantially uniform across the pedestal. In contrast, line 152 represents the temperature profile across the entire surface of the pedestal when it is heated to a temperature of approximately 600°C, which is substantially higher than the temperature at which it is designed to operate.

[0043] Simply increasing the current supplied to the filament does not solve the problem because it does not change the ratio of power supplied to the inner and outer parts of the pedestal. In other words, if the current supplied to the filament is increased to provide the desired heating to the outer part of the pedestal, the inner part of the pedestal will become too hot, and the central region of the substrate being processed on the pedestal will also exceed the desired processing temperature.

[0044] As described above, this problem can be solved by providing filaments with non-uniform TCRs in different regions of the pedestal. Figures 2A and 2B show an exemplary pedestal 202, with Figure 2A showing the pedestal 202 from above and Figure 2B showing the pedestal 202 from the side. The pedestal 202 includes an inner portion 206 and an outer portion 208 (additional annular portions may be defined as desired). The inner filament 210a has a first TCR and is located within the inner portion 206. The outer filament 210b has a second TCR (different from the first TCR) and is located within the outer portion 208. If additional annular portions are defined, each may have a filament having a specific TCR. The TCRs of the different filaments may differ to a certain degree in some embodiments. For example, a pedestal may have a second TCR that is at least about 0.1%, at least about 0.5%, at least about 1%, at least about 3%, at least about 5%, at least about 10%, at least about 20%, or at least about 30% greater (or less) than a first TCR. Generally, the desired difference in TCR can be selected based on the desired range of operating temperature and the desired sensitivity.

[0045] In various embodiments, the inner filament 210a and the outer filament 210b are electrically connected to each other and driven by the same power supply 212. In other words, the inner filament 210a and the outer filament 210b are not independently controllable with respect to the current / power supplied to them by the power supply. In various embodiments, the inner filament 210a is located entirely within the inner portion 206 of the pedestal 202, and the outer filament 210b is located entirely within the outer portion 208 of the pedestal, and the inner filament 210a and the outer filament 210b are connected to each other at the interface between the inner portion 206 and the outer portion 208 of the pedestal 202. In some embodiments, the inner filament 210a and the outer filament 210b may be part of a single filament. In such cases, the inner filament 210a and / or the outer filament 210b may undergo a process to modify their TCR so that the TCRs of the inner and outer filaments are different from each other. Of course, either or both of the inner filament 210a and the outer filament 210b can be made from multiple filaments connected to each other.

[0046] Figure 2C shows resistance versus temperature for both the inner filament 210a and the outer filament 210b of the pedestal shown in Figures 2A and 2B. In this example, line 250 relates to the inner filament 210a and line 260 relates to the outer filament 210b.

[0047] In the example described in relation to Figures 2A to 2D, it is determined by the model that 630W of power is required to maintain the pedestal 202 at a uniform temperature of approximately 400°C, with 300W of power accounting for heat loss through the inner portion 206 of the pedestal and 330W accounting for heat loss through the outer portion 208 of the pedestal (the inner and outer portions have approximately equal surface area and are powered together by a single power source). The inner filament 210a (represented by wire 250) is embedded in the inner portion 206 of the pedestal and has a cumulative resistance of approximately 3 ohms at approximately 400°C. The outer filament 210b (represented by wire 260) is embedded in the outer portion 208 of the pedestal and has a cumulative resistance of approximately 3.3 ohms at approximately 400°C. When a current of approximately 10A is passed through the filaments to heat the filaments, pedestal, and substrate, the inner filament 210a supplies approximately 300W of power, the outer filament 210b supplies approximately 330W of power, and therefore supplies a total power of approximately 630W as needed to maintain the pedestal 202 at a desired temperature of approximately 400°C.

[0048] In relation to this same exemplary pedestal 202 operating at different temperatures of approximately 600°C, modeling determines that 3600W of power is required to maintain the pedestal 202 at a uniform temperature, with approximately 1600W accounting for heat loss from the inner portion 206 of the pedestal and approximately 2000W accounting for heat loss from the outer portion 208 of the pedestal. As shown in Figure 2C, at 600°C, the inner filament 210a (represented by line 250) has a resistance of approximately 4 ohms, and the outer filament 210b (represented by line 260) has a resistance of approximately 5 ohms (this differs from the example in Figure 1F where the outer filament has a resistance of only 4.4 ohms at 600°C, as the outer filament had the same TCR as the inner filament). Increasing the heating current up to 20A will result in the desired 1600W (20A) 2 *4 ohms = 1600W) is supplied to the inner filament 210a (and the inner portion 206 of the pedestal for that purpose). Similarly, a heating current of 20A is supplied to the desired 2000W (20A 2A force of 5 ohms (2000W) is supplied to the outer filament 210b (and the outer portion of the pedestal 208 for that purpose). This provides a total of approximately 3600W, which was the power required to maintain the pedestal 202 at a uniform temperature of approximately 600°C.

[0049] Figure 2D shows the temperature profile generated across the entire surface of the pedestal in the examples shown in Figures 2A-2C. For example, line 270 represents the temperature profile across the entire surface of the pedestal when it is operating at approximately 400°C, and line 280 represents the temperature profile across the entire surface of the pedestal when it is operating at approximately 600°C. At both temperatures, a substantially uniform temperature profile is achieved.

[0050] The embodiments in Figures 2A–2C offer substantial improvements over the examples in Figures 1A–1F, with the greatest advantage being a substantially larger temperature operating window for a single pedestal with only a single power supply. This larger operating window means the pedestal can be used for a wide variety of applications occurring at substantially different temperatures, while still providing high-quality, uniform results across the entire surface of the substrate. Furthermore, because only a single power supply is used, the cost of such a pedestal is relatively lower compared to other pedestals that can operate over a similarly wide temperature range (e.g., pedestals with multiple heating zones independently controlled by multiple power supplies).

[0051] Several techniques can be used to ensure that the inner and outer filaments have different TCRs. For example, the inner and outer filaments may be made of different materials. In some cases, the inner and outer filaments may be made of different metals. In one example, the inner filament may be molybdenum and the outer filament may be tungsten (or vice versa). Appropriate metallurgical processes may be used to bond the filaments together. In some embodiments, the inner and outer filaments may be made of the same metal, but at least one of them may be doped with a material that is substantially absent in the other filament. For example, the inner filament may be molybdenum, and the outer filament may be molybdenum doped with one or more dopants such as carbon, nitrogen, and phosphorus. Similarly, the outer filament may be molybdenum, and the inner filament may be molybdenum doped with one or more dopants such as carbon, nitrogen, and phosphorus.

[0052] In another example, one or both of the inner and outer filaments may be treated (e.g., heat treatment, plasma treatment, etc.) to modify the TCR of the treated filament. This technique can be used to provide inner and outer filaments made of similar materials with different TCRs. In the exemplary heat treatment, the filament may be exposed to a high temperature for a specific period of time.

[0053] As described above, in some cases, plasma treatment may be used to modify the TCR of one or more filaments. Various types of plasma may be used, including but not limited to capacitively coupled plasma, inductively coupled plasma, transformer-coupled plasma, and microwave-coupled plasma. Various power levels and frequencies may be used as desired for the specific application.

[0054] In some embodiments, powder metallurgy techniques may be used to manufacture the filament or a portion thereof.

[0055] Regardless of the technology used to manufacture the filaments, both the inner and outer filaments (and any additional filaments, if used) are enclosed and sintered within the pedestal body. Therefore, any connections between individual filaments are expected to be durable.

[0056] As used herein, the term “substantially uniform” applied to the temperature profile on the pedestal is intended to mean that the difference between the highest and lowest temperatures on the pedestal is approximately 5°C or less. The temperature considered is the temperature of the upper surface of the pedestal, for example, the surface that comes into contact with the substrate during processing.

[0057] In some specific embodiments, the highest and lowest temperatures on the pedestal may differ from each other by about 1°C or less, or about 2°C or less, or about 3°C ​​or less, or about 4°C or less. In other embodiments, the temperature variation may be higher, for example, the highest and lowest temperatures on the pedestal may differ from each other by only about 10°C or less, or about 20°C or less. In various embodiments, the highest temperature on the pedestal may be up to a certain extent greater than the lowest temperature on the pedestal. For example, the highest temperature on the pedestal may be up to about 1%, or about 2%, or about 3%, or about 5%, higher than the lowest temperature on the pedestal, calculated in Celsius. As an example, for a pedestal operating at high temperatures that provides a highest temperature of about 600°C and a lowest temperature of about 580°C on the surface of the pedestal, the highest temperature is considered to be about 3.4% higher than the lowest temperature (e.g., (600-580) / 580=0.034).

[0058] In various embodiments, the pedestal can achieve a desired level of temperature uniformity over a wide range of operating temperatures. For example, the pedestal may achieve a desired level of temperature uniformity (e.g., as described above) over a range of at least about 50°C, or at least about 100°C, or at least about 200°C, or at least about 300°C, or at least about 500°C. In one example, a pedestal that provides substantial temperature uniformity at both 400°C and 600°C is understood to provide substantial temperature uniformity over a range of at least about 200°C.

[0059] Device The pedestals and filaments described herein may be incorporated into any type of semiconductor substrate processing apparatus. As stated above, the techniques described herein are useful for many different types of substrate processing, including but not limited to deposition, etching, and substrate processing. In particular, the techniques described herein are useful for apparatus operating at different temperatures at different times. By using filaments with different TCRs in different parts of the pedestal, the pedestal can achieve substantially uniform temperatures over a wide range of temperatures. This uniformity is achieved without the complexity and cost associated with multi-zone heating using multiple individually controllable power supplies.

[0060] For the sake of brevity, this section describes only a single exemplary apparatus. It should be understood that the concepts disclosed herein are equally applicable to different types of apparatus.

[0061] Figure 3 schematically illustrates an embodiment of a process station 300 that may be used to deposit materials using atomic layer deposition (ALD) and / or chemical vapor deposition (CVD), both of which may be plasma-enhanced. For simplicity, the process station 300 is shown as a standalone process station having a process chamber body 302 for maintaining a low-pressure environment. However, it will be understood that multiple process stations 300 may be included in a common process tool environment. Furthermore, it will be understood that in some embodiments, one or more hardware parameters of the process station 300, including those described in detail below, may be programmed by one or more computer controllers.

[0062] The process station 300 is in fluid communication with a reactant delivery system 301 to deliver process gas to a distribution showerhead 306. The reactant delivery system 301 includes a mixing vessel 304 for blending and / or adjusting the process gas for delivery to the showerhead 306. One or more mixing vessel inlet valves 320 may control the introduction of process gas into the mixing vessel 304. Similarly, a showerhead inlet valve 305 may control the introduction of process gas into the showerhead 306.

[0063] Some reactants may be stored in liquid form before vaporization at the process station and subsequent delivery to the process station. For example, the embodiment in Figure 3 includes a vaporization point 303 for vaporizing liquid reactants supplied to a mixing vessel 304. In some embodiments, the vaporization point 303 may be a heated vaporizer. Reactant vapor produced from such a vaporizer may condense in the downstream delivery piping. Exposure of the condensed reactants to unsuitable gases may generate small particles. These small particles can clog piping, interfere with valve operation, and contaminate substrates. Some techniques to address these problems involve sweeping and / or exhausting the delivery piping to remove residual reactants. However, sweeping the delivery piping can increase process station cycle time and reduce process station throughput. Therefore, in some embodiments, the delivery piping downstream of the vaporization point 303 may be thermally tracked. In some examples, the mixing vessel 304 may also be thermally tracked. In a non-limiting example, the piping downstream of the vaporization point 303 has an increasing temperature profile in the mixing vessel 304, extending from approximately 100°C to approximately 150°C.

[0064] In some embodiments, the reactant liquid may be vaporized by a liquid injector. For example, the liquid injector may inject pulses of liquid reactant into the carrier gas flow upstream of the mixing vessel. In one scenario, the liquid injector may vaporize the reactants by flushing the liquid from high pressure to low pressure. In another scenario, the liquid injector may atomize the liquid into dispersed microdroplets, which are then vaporized in a heated delivery pipe. It will be understood that smaller droplets can vaporize faster than larger droplets, reducing the delay between liquid injection and complete vaporization. Faster vaporization may result in a shorter length of piping downstream from the vaporization point 303. In one scenario, the liquid injector may be directly attached to the mixing vessel 304. In another scenario, the liquid injector may be directly attached to the showerhead 306.

[0065] In some embodiments, a liquid flow controller may be provided upstream of the vaporization point 303 to control the mass flow rate of the liquid for vaporization and delivery to the process station 300. For example, the liquid flow controller (LFC) may include a thermal mass flow meter (MFM) located downstream of the LFC. The plunger valve of the LFC may then be adjusted in response to a feedback control signal provided by a proportional-integral-derivative (PID) controller electrically connected to the MFM. However, stabilizing the liquid flow using feedback control may take more than one second. This allows for an extended time to introduce the liquid reactant. Therefore, in some embodiments, the LFC may be dynamically switched between a feedback control mode and a direct control mode. In some embodiments, the LFC may be dynamically switched from feedback control mode to direct control mode by disabling the LFC's sensing tube and PID controller.

[0066] The showerhead 306 distributes process gas toward the substrate 312. In the embodiment shown in Figure 3, the substrate 312 is located below the showerhead 306 and is shown resting on the pedestal 308. It will be understood that the showerhead 306 may have any preferred shape and may have any preferred number and arrangement of ports for distributing process gas toward the substrate 312. Although not shown in detail in Figure 3, it will be understood that the pedestal 308 may include the features described herein. These features include, but are not limited to, an inner portion for housing inner filaments and an outer portion for housing outer filaments, and have different TCRs for the inner and outer filaments. These features enable the pedestal 308 to operate at a substantially uniform temperature over a wide range of target operating temperatures. Additional portions and filaments may be provided as needed, as described above.

[0067] In some embodiments, a minute volume 307 is located below the showerhead 306. By performing the ALD and / or CVD process in a minute volume rather than the entire volume of the process station, it is possible to reduce reactant exposure and sweep times, reduce the time required to change process conditions (e.g., pressure, temperature, etc.), and limit the exposure of the process station robot to the process gas. Exemplary minute volume sizes include, but are not limited to, volumes of 0.1 liters to 2 liters. This minute volume also affects productivity throughput. The deposition rate per cycle decreases, but the cycle time is also reduced. In some cases, the latter effect is dramatic enough to improve the overall throughput of the module for a given target film thickness.

[0068] In some embodiments, the pedestal 308 may be raised or lowered to expose the substrate 312 to the microvolume 307 and / or vary the volume of the microvolume 307. For example, during the substrate transfer stage, the pedestal 308 may be lowered to load the substrate 312 onto the pedestal 308. During the deposition process stage, the pedestal 308 may be raised to position the substrate 312 within the microvolume 307. In some embodiments, the microvolume 307 may completely enclose a portion of the substrate 312 and the pedestal 308 to create a region of high flow impedance during the deposition process.

[0069] Optionally, the pedestal 308 may be lowered and / or raised during a portion of the deposition process to modulate the process pressure, reactant concentration, etc., within the microvolume 307. In one scenario where the process chamber body 302 remains at base pressure during the deposition process, lowering the pedestal 308 may allow for the exhaust of the microvolume 307. Exemplary ratios of microvolume to process chamber volume include, but are not limited to, a volume ratio of 1:300 to 1:10. In some embodiments, it will be understood that the height of the pedestal may be programmed by a suitable computer controller.

[0070] In an alternative scenario, the plasma density may be varied during plasma activation and / or processing cycles included in the deposition process by adjusting the height of the pedestal 308. At the end of the deposition process stage, the pedestal 308 may be lowered during another substrate transfer stage to allow for the removal of the substrate 312 from the pedestal 308.

[0071] While the exemplary microvolume variations described herein refer to a height-adjustable pedestal, it will be understood that in some embodiments, the volume of the microvolume 307 may be varied by adjusting the position of the showerhead 306 relative to the pedestal 308. Furthermore, it will be understood that the vertical position of the pedestal 308 and / or the showerhead 306 may be varied by any suitable mechanism within the scope of this disclosure. In some embodiments, the pedestal 308 may include a pivot axis for rotating the orientation of the substrate 312. In some embodiments, it will be understood that one or more of these exemplary adjustments may be performed programmatically by one or more suitable computer controllers.

[0072] Returning to the embodiment shown in Figure 3, the showerhead 306 and pedestal 308 electrically communicate with the RF power supply 314 and the matching network 316 to power the plasma. In some embodiments, the plasma energy may be controlled by controlling one or more of the process station pressure, gas concentration, RF source power, RF source frequency, and plasma power pulse timing. For example, the RF power supply 314 and the matching network 316 may operate at any suitable power to form a plasma having a desired composition of radical species. Examples of suitable powers are included above. Similarly, the RF power supply 314 may supply RF power at any suitable frequency. In some embodiments, the RF power supply 314 may be configured to control a high-frequency RF power supply and a low-frequency RF power supply independently of each other. Exemplary low-frequency RF frequencies may be, but are not limited to, 50 kHz to 300 kHz. Exemplary high-frequency RF frequencies may be, but are not limited to, 1.8 MHz to 2.45 GHz. It will be understood that any suitable parameters may be discretely or continuously modulated to provide plasma energy for surface reactions. In a non-limiting example, the plasma power may be pulsed intermittently to reduce ion collisions with the substrate surface compared to the case of a continuously powered plasma.

[0073] In some embodiments, the plasma may be monitored in situ by one or more plasma monitors. In one scenario, plasma power may be monitored by one or more voltage-current sensors (e.g., VI probes). In another scenario, plasma density and / or process gas concentration may be measured by one or more emission spectrometers (OES). In some embodiments, one or more plasma parameters may be programmed to adjust based on measurements from such in situ plasma monitors. For example, an OES sensor may be used in a feedback loop to provide programmed control of plasma power. In some embodiments, it will be understood that other monitors may be used to monitor the plasma and other process characteristics. Such monitors may include, but are not limited to, infrared (IR) monitors, acoustic monitors, and pressure transducers.

[0074] In some embodiments, the plasma may be controlled via input / output control (IOC) sequencing instructions. For example, instructions for setting the plasma conditions of a plasma process stage may be included in the corresponding plasma activation recipe stage of the deposition process recipe. In some cases, process recipe stages may be arranged sequentially so that all instructions for the deposition process stage are executed concurrently with that process stage. In some embodiments, instructions for setting one or more plasma parameters may be included in recipe stages preceding the plasma process stage. For example, a first recipe stage may include instructions for setting the flow rates of the inert gas and / or reactant gas, instructions for setting the plasma generator to a power setpoint, and a time delay instruction for the first recipe stage. A second subsequent recipe stage may include instructions for enabling the plasma generator and a time delay instruction for the second recipe stage. A third recipe stage may include instructions for deactivating the plasma generator and a time delay instruction for the third recipe stage. It will be understood that these recipe stages may be further subdivided and / or repeated in any preferred manner within the scope of this disclosure.

[0075] In some deposition processes, plasma strikes last for several seconds or more. In certain embodiments, much shorter plasma strikes may be used. These may be on the order of 10 milliseconds to 1 second, typically around 20 milliseconds to 80 milliseconds, with 50 milliseconds being a specific example. Such very short RF plasma strikes require very rapid stabilization of the plasma. To achieve this, the plasma generator may be configured such that impedance matching is preset to a specific voltage and the frequency can float. Conventionally, high-frequency plasmas are generated at an RF frequency of about 13.56 MHz. In various embodiments disclosed herein, the frequency is made capable of floating to a value different from this standard value. By floating the frequency while fixing impedance matching to a given voltage, the plasma can be stabilized much more quickly, and the result may be important when using very short plasma strikes associated with some types of deposition cycles.

[0076] In some embodiments, the pedestal 308 may be temperature-controlled via a heater 310 (similar to power supply 212 in Figure 2B). Furthermore, in some embodiments, pressure control of the deposition process station 300 may be provided by a butterfly valve 318. As shown in the embodiment of Figure 3, the butterfly valve 318 throttles the vacuum provided by a downstream vacuum pump (not shown). However, in some embodiments, pressure control of the process station 300 may also be adjusted by changing the flow rate of one or more gases introduced into the process station 300.

[0077] In some implementations, the controller is part of a system that may be part of the examples described above. Such a system may include a semiconductor processing apparatus that includes one or more process tools, one or more chambers, one or more platforms for processing, and / or specific processing components (such as wafer pedestals, gas flow systems, etc.). These systems may be integrated with electronics to control pre-processing, in-processing, and post-processing operations of semiconductor wafers or substrates. The electronics may be referred to as “controllers” that can control various components or sub-components of one or more systems. Depending on the processing requirements and / or the type of system, the controller may be programmed to control any of the processes disclosed herein, including the delivery of process gases, temperature settings (e.g., heating and / or cooling), pressure settings, vacuum settings, power settings, radio frequency (RF) generator settings in some systems, RF matching circuit settings, frequency settings, flow rate settings, fluid delivery settings, position and operation settings, the transfer of wafers to and from tools and other transfer tools, and / or load locks connected to or interfaced with specific systems.

[0078] Broadly speaking, a controller may be defined as an electronic device having various integrated circuits, logic, memory, and / or software that receive and issue instructions, control operations, enable cleaning operations, enable endpoint measurements, etc. Integrated circuits may include chips in the form of firmware that store program instructions, chips defined as digital signal processors (DSPs), application-specific integrated circuits (ASICs), and / or one or more microprocessors, or microcontrollers (e.g., software) that execute program instructions. Program instructions may be instructions communicated to the controller in the form of various individual settings (or program files) that define operating parameters for performing a particular process on a semiconductor wafer or system. In some embodiments, operating parameters may be part of a recipe defined by a process engineer to achieve one or more processing steps during the fabrication of one or more layers, materials, metals, oxides, silicon, silicon dioxide, surfaces, circuits, and / or dies of a wafer.

[0079] In some implementations, the controller may be part of, or coupled to, a computer integrated with, coupled to, networked to, or in some way networked to the system, or a combination thereof. For example, the controller may be all or part of a “cloud” or fab-host computer system and may enable remote access to wafer processing. The computer may enable remote access to the system to monitor the current progress of a fabrication operation, examine the history of past fabrication operations, examine trends or performance metrics from multiple fabrication operations, modify parameters of the current process, set processing steps to follow the current process, or start a new process. In some examples, a remote computer (e.g., a server) may provide process recipes to the system over a network that may include a local network or the internet. The remote computer may include a user interface that allows input or programming of parameters and / or settings, which are then communicated from the remote computer to the system. In some examples, the controller receives instructions in the form of data specifying the parameters of each processing step performed during one or more operations. It should be understood that the parameters may be specific to the type of process being performed and the type of tool the controller is configured to interface with or control. Therefore, as described above, the controllers may be distributed, for example, by comprising one or more individual controllers that are networked together and operate toward a common purpose, such as the processes and controls described herein. An example of a distributed controller for such purposes is one or more integrated circuits on a chamber that communicate with one or more integrated circuits located remotely (such as at the platform level or as part of a remote computer) which are combined to control the processes on the chamber.

[0080] Exemplary systems may include, but are not limited to, plasma etching chambers or modules, deposition chambers or modules, spin rinse chambers or modules, metal plating chambers or modules, clean chambers or modules, bevel edge etching chambers or modules, physical vapor deposition (PVD) chambers or modules, chemical vapor deposition (CVD) chambers or modules, atomic layer deposition (ALD) chambers or modules, atomic layer etching (ALE) chambers or modules, ion implantation chambers or modules, track chambers or modules, and any other semiconductor processing systems related to or usable in the fabrication and / or manufacture of semiconductor wafers.

[0081] As described above, depending on one or more process steps performed by the tool, the controller may communicate with one or more of the following: other tool circuits or modules, other tool components, cluster tools, other tool interfaces, adjacent tools, nearby tools, tools located throughout the factory, a main computer, another controller, or tools used for material transport to carry wafer containers to and from tool locations and / or load ports in the semiconductor manufacturing plant.

[0082] Where used herein, the term “approximately” applied to a number is understood to mean ±10% of the relevant value.

[0083] conclusion While the embodiments described above have been explained in some detail to clarify understanding, it will be clear that certain changes and modifications can be made within the scope of the appended claims. It should be noted that there are many alternative ways of implementing the processes, systems, and apparatus of this embodiment. Therefore, this embodiment should be considered illustrative and not limiting, and embodiments should not be limited to the details given herein. [Explanation of symbols]

[0084] 102 Pedestal 106 Inner part 108 Outer part 110 filaments 112 Power supply 202 Pedestal 206 Inner part 208 Outer part 210a inner filament 210b outer filament 212 Power supply 300 process stations 301 Reactant Delivery System 302 Process Chamber Body 303 Vaporization point 304 Mixing container 305 Shower head inlet valve 306-way shower head 307 Microvolume 308 Pedestal 310 Heater 312 circuit boards 314 RF power supply 316 Consistent Network 318 Butterfly Valve 320 Mixing container inlet valve

Claims

1. A pedestal for supporting a semiconductor substrate during processing, wherein the pedestal is A body having an inner portion and an outer portion surrounding the inner portion, An inner filament disposed in the inner portion of the main body, having a first temperature coefficient of resistance (TCR), An outer filament disposed on the outer part of the main body, wherein the outer filament has a second TCR, and the second TCR is different from the first TCR, and the outer filament A pedestal equipped with a pedestal.

2. The pedestal according to claim 1, wherein the inner filament and the outer filament are configured to be powered together by a single power source.

3. The pedestal according to claim 1, wherein the body of the pedestal further comprises an additional annular portion, the additional annular portion being positioned between the inner portion and the outer portion of the body of the pedestal, and the pedestal further comprises an additional filament positioned in the additional annular portion, the additional filament having a third TCR, the third TCR being different from the first TCR and the second TCR.

4. The pedestal according to claim 1, wherein the second TCR is larger than the first TCR.

5. The pedestal according to claim 4, wherein the second TCR is at least about 0.1% larger than the first TCR.

6. The pedestal according to claim 1, wherein the second TCR is lower than the first TCR.

7. The pedestal according to claim 1, wherein the inner filament and the outer filament are made of different materials.

8. The pedestal according to claim 7, wherein the inner filament and the outer filament comprise different metals.

9. The pedestal according to claim 1, wherein the inner filament and the outer filament comprise the same material but are doped with different dopants or to different degrees.

10. The pedestal according to claim 7, wherein the inner filament and / or the outer filament are heat-treated and / or plasma-treated to modify the first TCR and / or the second TCR, respectively.

11. The pedestal according to claim 1, wherein the body of the pedestal further comprises a second outer portion surrounding the outer portion, the pedestal further comprises a second outer filament disposed in the second outer portion, the inner filament and the outer filament are configured to be powered by a first power source, and the second outer filament is configured to be powered by a second power source such that the inner filament and the outer filament can be powered independently of the second outer filament.

12. A method for processing a substrate, wherein the method is The step of receiving the substrate on a pedestal in a processing chamber, wherein the pedestal is A body having an inner portion and an outer portion surrounding the inner portion, wherein the body has an upper surface that contacts the substrate, An inner filament disposed in the inner portion of the main body, having a first temperature coefficient of resistance (TCR), An outer filament disposed on the outer portion of the main body, wherein the outer filament has a second TCR, and the second TCR is different from the first TCR, and the outer filament It has steps, The steps include supplying power to the pedestal in order to raise the temperature of the pedestal, The steps of processing the substrate at the high temperature and Methods that include...

13. The method according to claim 12, wherein the inner filament and the outer filament are powered together by a single power source.

14. The method according to claim 12, wherein the temperature profile on the upper surface of the body of the pedestal is substantially uniform at the high temperature.

15. The method according to claim 12, wherein the steps of supplying power to the pedestal to raise the pedestal to the high temperature and processing the substrate at the high temperature include (1) a step of supplying power to the pedestal to raise the pedestal to a first temperature and processing the substrate at the first temperature for a first period of time, and (2) a step of supplying power to the pedestal to raise the pedestal to a second temperature and processing the substrate at the second temperature for a second period of time, wherein the first temperature and the second temperature differ by at least about 50°C, and the temperature profile on the upper surface of the body of the pedestal is substantially uniform at both the first temperature and the second temperature.

16. A method for manufacturing a pedestal for supporting a semiconductor substrate during processing, wherein the method is: The steps include providing an inner filament having a first temperature coefficient of resistance (TCR), A step of providing an outer filament having a second TCR, wherein the second TCR differs from the first TCR in that both the inner filament and the outer filament are powered together by a single power source, The steps include providing the pedestal body and The steps include sealing the inner filament and the outer filament within the pedestal body. Methods that include...

17. The method according to claim 16, wherein the second TCR is greater than the first TCR.

18. The method according to claim 17, wherein the second TCR is at least about 0.1% greater than the first TCR.

19. The method according to claim 16, wherein the second TCR is lower than the first TCR.

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