# Patent

## Title
Composite substrate for layered heaters

## Abstract

A method of forming a heater assembly for use in semiconductor processing includes thermally securing a heater substrate to an application substrate; and applying a layered heater to the heater substrate after the heater substrate is secured to the application substrate. The application of the layered heater includes applying a first dielectric layer onto the heater substrate, applying a resistive heating layer onto the first dielectric layer, and applying a second dielectric layer onto the resistive heating layer. The heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of at least one of the first dielectric layer and a coefficient of thermal expansion of the resistive heating layer.

## Background
FIELD
The present disclosure relates to layered heaters, and more specifically to layered heaters bonded to a semiconductor processing apparatus with improved reliability at elevated temperatures.
BACKGROUND
The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
A layered heater typically includes a plurality of functional layers applied on a substrate by layered processes. The plurality of functional layers may include a dielectric layer on the substrate, a resistive heating layer on the dielectric layer, and a protective layer on the resistive heating layer. The materials for the different functional layers and the substrate are carefully chosen to have compatible coefficient of thermal expansion (CTE) to reduce shear stress generated at the joining interfaces at elevated temperatures. The shear stress may cause generation of cracks or delamination at the joining interfaces, resulting in heater failure.
Only a limited number of materials can be used to form the different functional layers by a specific layered process, thereby limiting the selection of materials for the substrate, which should have a CTE matching the CTE of the dielectric layer applied on the substrate or matching the CTE of the heating layer. For example, when alumina ceramic is used to form the dielectric layer, alumina nitride or molybdenum is generally used to form the substrate due to its chemical and CTE compatibility with the alumina ceramic.
The layered heater may need to be joined to a heating target in some applications. For example, the layered heater may be joined to an electrostatic chuck to form a heated electrostatic chuck. However, the limited selection of materials for the substrate makes joining the layered heater to the electrostatic chuck difficult. When the substrate of the layered heater has a CTE that does not match the CTE of the chuck body, the heated electrostatic chuck is likely to fail due to generation of cracks or delamination at the joining interface at elevated temperatures.
SUMMARY
In one form, a method of forming a heater assembly for use in semiconductor processing is provided that comprises thermally securing a heater substrate to an application substrate, and applying a layered heater to the heater substrate after the heater substrate is secured to the application substrate. The application of the layered heater comprises applying a first dielectric layer onto the heater substrate, applying a resistive heating layer onto the first dielectric layer, and applying a second dielectric layer onto the resistive heating layer, wherein the heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of at least one of the first dielectric layer and a coefficient of thermal expansion of the resistive heating layer.
In still another form, a method of forming a heater assembly for use in semiconductor processing is provided that comprises brazing a heater substrate to an application substrate, and applying a layered heater to the heater substrate after the heater substrate is secured to the application substrate. The application of the layered heater comprises applying a bond coat layer by a thermal spray process onto the heater substrate, applying a first dielectric layer by a thermal spray process onto the bond coat layer, applying a resistive heating layer by a thermal spray process onto the first dielectric layer, and applying a second dielectric layer by a thermal spray process onto the resistive heating layer, wherein the heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of at least one of the first dielectric layer and a coefficient of thermal expansion of the resistive heating layer.
In yet another form, a method of forming a heater assembly for use in semiconductor processing is provided that comprises thermally securing a heater substrate to an application substrate at an elevated temperature, and applying at least one functional layer to the heater substrate after the heater substrate is secured to the application substrate. The heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of a material of the functional layer, the material of the functional layer not being capable of withstanding the elevated temperature of the thermal securing step.
Further yet, a heater assembly for use in semiconductor processing is provided that comprises an application substrate, a heater substrate secured to the application substrate by a thermal bonding process, and a functional layer disposed onto the heater substrate by a layered process. The heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of the functional layer.
Further areas of applicability will become apparent from the description provided herein. It should be understood that the description and specific examples are intended for purposes of illustration only and are not intended to limit the scope of the present disclosure.



DRAWINGS
The drawings described herein are for illustration purposes only and are not intended to limit the scope of the present disclosure in any way.
In order that the invention may be well understood, there will now be described various forms thereof, given by way of example, reference being made to the accompanying drawings, in which:
FIG. 1 is an exploded view of a layered heater constructed in accordance with the teachings of the present disclosure;
FIG. 2 is a cross-sectional view of a heater assembly including a layered heater and a heating target and constructed in accordance with the teachings of the present disclosure;
FIG. 3 is a cross-sectional view of a variant of a heater assembly including a layered heater and a heating target and constructed in accordance with the teachings of the present disclosure;
FIG. 4 is a flow diagram of a method of forming a heater assembly for use in semiconductor processing; and
FIG. 5 is a flow diagram of a method of forming another heater assembly for use in semiconductor processing.



Corresponding reference numerals indicate corresponding parts throughout the several views of the drawings.
DETAILED DESCRIPTION
The following description is merely exemplary in nature and is not intended to limit the present disclosure, application, or uses.
Referring to FIG. 1, a layered heater 10 constructed in accordance with the teachings of the present disclosure includes an application substrate 12, a heater substrate 14, a first dielectric layer 16 formed on the heater substrate 14, a resistive heating layer 18 formed on the first dielectric layer 16, and a second dielectric layer 20 formed on the resistive heating layer 18. The first dielectric layer 16, the resistive heating layer 18 and the second dielectric layer 20 are formed by a layered process, such as thick film, thin film, thermal spray and sol gel.
Referring to FIG. 2, the layered heater 10 is joined to a heating target 22 to form a heater assembly 25. For example, the heating target 22 may be a chuck top of a heated electrostatic chuck for semiconductor processing. It should be understood, however, that the heating target 22 may be any application and thus the description and illustration of a chuck for semiconductor processing shall not be construed as limiting the scope of the present disclosure. The application substrate 12 and the heater substrate 14 are made of different materials and are joined by brazing to form a composite chuck.
A brazing layer 24 is formed between the application substrate 12 and the heater substrate 14. The brazing material may be a sliver brazing material. Other joining processes, such as welding, soldering, diffusion bonding, epoxying, vulcanizing, may be used to join the application substrate 12 and the heater substrate 14 without departing from the scope of the present disclosure. Similarly, the application substrate 12 may be joined to the heating target 22 by any joining methods such as brazing, welding, soldering, diffusion boding, epoxying, vulcanizing.
The application substrate 12 includes a material having a coefficient of thermal expansion (CTE) that closely matches the CTE of the heating target 22. Alternatively, the application substrate 12 may be an integral part of the heating target to which the heat from the layered heater is transferred. In one form, the heater substrate 14 includes a material having a CTE that matches the CTE of the first dielectric layer 16. In another form, the heater substrate 14 includes a material having a CTE that matches the CTE of the resistive heating layer 18. In other words, the material of the application substrate 12 depends on the materials of the heating target 22, whereas the material of the heater substrate 14 depends on the materials of the first dielectric layer 16 and/or the resistive heating layer 18.
For example, when the first dielectric layer 16 includes alumina ceramic, the heater substrate 14 may be made of alumina nitride or molybdenum. The application substrate 12 may include a material having a CTE that can easily match the CTE of most suitable materials for the heating target 22, regardless of the materials of the first dielectric layer 16 and the heater substrate 14. The application substrate 12 may include austenitic stainless steel, which has a CTE matching a wider selection of materials. Therefore, the layered heater 10 can be relatively easily joined to the heating target 22.
The layered heater 10 may be a layered heater formed by thick film, thin film, thermal-spray, and sol-gel process. The resistive heating layer 18 may be formed by applying a resistive layer throughout the first dielectric layer 14, followed by a laser removal process to form a circuit pattern.
In still another form, the resistive heating layer 18 is formed of a material having sufficient temperature coefficient of resistance such that the heating layer 18 functions as both a heater and a temperature sensor, commonly referred to as “two-wire control.” Such heaters and their materials are disclosed, for example, in U.S. Pat. No. 7,196,295 and pending U.S. patent application Ser. No. 11/475,534, which are commonly assigned with the present application and the disclosures of which are incorporated herein by reference in their entirety.
Referring to FIG. 3, a heater assembly 30 has a structure similar to that of the heater assembly 10 of FIG. 2 except that the layered heater further includes a bond coat layer 32 and a topcoat 34. The bond coat layer 32 is applied on the heater substrate 14. The topcoat 34 is applied on the second dielectric layer 20.
While two substrates are described in the present disclosure to form a composite substrate, more than two substrates may be used to form a composite substrate, which provides a gradual transition over multiple substrates in terms of coefficient of thermal expansion.
Referring to FIG. 4, a method 40 of forming a heater assembly 25 for use in semiconductor processing includes bonding an application substrate 12 to a heating target 22 in step 42. When the application substrate 12 is an integral part of the heating target 22, this step is eliminated. The heater substrate 14 is then thermally secured to the application substrate 12 in step 44. Thermally securing may include brazing, welding, soldering, diffusion bonding, epoxying, vulcanizing at a first temperature. A layered heater is then applied to the heater substrate 14 after the application substrate 12 is joined to the heating target 22.
The application of the layered heater on the heater substrate 14 includes applying a first dielectric layer 16 on the heater substrate 14 in step 46. A resistive heating layer 18 is then applied on the first dielectric layer 16 in step 48. The resistive heating layer 18 may be applied to form a circuit pattern when applied on the first dielectric layer 16. Alternatively, the resistive heating layer 18 may be applied by forming a continuous layer on the entire surface of the first dielectric layer 16, followed by a laser removal process to form the desired circuit pattern. Finally, a second dielectric layer 20 is applied on the resistive heating layer 18 in step 50. The method 40 ends in step 52.
Referring to FIG. 5, a method 60 of forming a heater assembly 30 for use in semiconductor processing is similar to the method 40 of FIG. 4 except for the steps of applying a bond coat layer and a topcoat. More specifically, the method 60 includes bonding the application substrate 12 to the heating target 22 in step 62. The heater substrate 14 is then thermally secured to the application substrate 12 in step 64. Thermally securing may include brazing, welding, soldering, diffusion bonding, epoxying, vulcanizing at a first temperature. Thereafter, a layered heater is applied to the heater substrate 14 after the heater substrate 14 is joined to the application substrate 12.
The application of the layered heater to the heater substrate 14 includes applying a bond coat layer 32 on the heater substrate 14 in step 66. A first dielectric layer 16 is then applied on the bond coat layer 32 in step 68. A resistive heating layer 18 is applied on the first dielectric layer 16 in step 70. A second dielectric layer 20 is applied on the resistive heating layer 18 in step 72. A topcoat 34 is applied over the second dielectric layer 20 in step 74. Finally, portions of the topcoat 34 are removed to achieve a predetermined surface flatness in step 76. The method 60 ends in step 78.
In the methods 40 and 60 of the present disclosure, brazing is used to effectively and reliably join the heater substrate 14 to the application substrate 12. The application substrate 12 is made of a material having a CTE that matches the CTE of the material of the heating target 22. The heater substrate 14 is made of a material having a CTE that matches the CTE of the material of the first dielectric layer 16. The composite substrate enables the layer heater to be applied to a heating target 22 having a CTE that does not match (i.e., is significantly different from) the CTE of the heater substrate 14.
Moreover, the brazing process, which requires a relatively high temperature, is performed before the various functional layers of the layered heater are applied to the heater substrate 14. Therefore, the layered heater is not subjected to the undesirable high temperature during the brazing process and thus can maintain its integrity.
The present method enables better matching of the CTE of the layered heater to any heating target 22 by using an application substrate. The present method also ensures the integrity of the layered heater by brazing the heater substrate to the application substrate before the different functional layers of the layered heater are formed on the heater substrate. Therefore, the present method can improve physical/material characteristics of the heater assembly such as machinability, surface roughness, surface hardness, chemical compatibility, thermal conductivity, electrical conductivity, emissivity, appearance, cost, etc.
The description of the disclosure is merely exemplary in nature and, thus, variations that do not depart from the substance of the disclosure are intended to be within the scope of the disclosure. Such variations are not to be regarded as a departure from the spirit and scope of the disclosure.

## Claims

1. A method of forming a heater assembly for use in semiconductor processing comprising the following steps in sequence:
thermally securing a heater substrate to an application substrate using a joining process selected from the group consisting of brazing, welding, soldering, diffusion bonding, epoxying, and vulcanizing;
applying a layered heater to the heater substrate after the heater substrate is secured to the application substrate, the application of the layered heater comprising:
applying a first dielectric layer onto the heater substrate;
applying a resistive heating layer onto the first dielectric layer; and
applying a second dielectric layer onto the resistive heating layer,

wherein the heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of at least one of the first dielectric layer and a coefficient of thermal expansion of the resistive heating layer.

2. The method according to claim 1, wherein the heater substrate is brazed to the application substrate.

3. The method according to claim 2, wherein a silver brazing material is used.

4. The method according to claim 1, wherein each of the layers of the layered heater are applied by a thermal spray process.

5. The method according to claim 4, wherein a circuit pattern is formed in the resistive heating layer by a laser removal process.

6. The method according to claim 1, wherein the first dielectric layer is an alumina material, the heater substrate is a molybdenum material, and the application substrate is an austenitic stainless steel material.

7. The method according to claim 1 further comprising applying a bond coat layer to the heater substrate prior to applying the first dielectric layer.

8. The method according to claim 1 further comprising applying a topcoat layer over the second dielectric layer and subsequently removing portions of the topcoat with a process to achieve a predetermined surface flatness.

9. A method of forming a heater assembly for use in semiconductor processing comprising the following steps in sequence:
brazing a heater substrate to an application substrate;
applying a layered heater to the heater substrate after the heater substrate is secured to the application substrate, the application of the layered heater comprising:
applying a bond coat layer by a thermal spray process onto the heater substrate;
applying a first dielectric layer by a thermal spray process onto the bond coat layer;
applying a resistive heating layer by a thermal spray process onto the first dielectric layer; and
applying a second dielectric layer by a thermal spray process onto the resistive heating layer,

wherein the heater substrate defines a material having a coefficient of thermal expansion that is matched to a coefficient of thermal expansion of at least one of the first dielectric layer and a coefficient of thermal expansion of the resistive heating layer.

10. The method according to claim 9, wherein a silver brazing material is used.

11. The method according to claim 9, wherein a circuit pattern is formed in the resistive heating layer by a laser removal process.

12. The method according to claim 9, wherein the first dielectric layer is an alumina material, the heater substrate is a molybdenum material, and the application substrate is an austenitic stainless steel material.