WO2007124202A2

WO2007124202A2 - Ac coupled voltage transducer for an rf sensor

Info

Publication number: WO2007124202A2
Application number: PCT/US2007/062782
Authority: WO
Inventors: Terry R. Turner; Enlian Lu
Original assignee: Turner Enterprises & Associates, Lp
Priority date: 2006-04-20
Filing date: 2007-02-26
Publication date: 2007-11-01
Also published as: WO2007124202A3

Abstract

An AC coupled voltage transducer for an RF sensor, the apparatus includes an RF power source, an RF current carrier and a voltage transducer, wherein the voltage transducer is adapted to form a closed surface around a portion of the RF current carrier and shielding the RF sensor.

Description

AC COUPLED VOLTAGE TRANSDUCER FOR AN RF SENSOR FIELD

[0001] This invention pertains to the design of semiconductor fabrication methods and systems, and more particularly to an AC coupled voltage transducer for an RF sensor. BACKGROUND

[0002 ] Plasma etching and deposition processes have become the dominant pattern transfer means used in semiconductor manufacturing over the past 20 years. Plasma-based etching allows the attainment of both selective and anisotropic processing in subtractive steps (as shown in prior art FIGURES IA -ID); plasma-based deposition allows the attainment of film stoichiometry and morphology control in additive steps. In both plasma-based etching and deposition, electromagnetic energy rather than thermal energy creates energetic atoms and radicals. In addition, plasma-substrate voltage differences can be set up, thereby creating charged-particle fluxes for anisotropy control in etching and film morphology control in deposition.

[0003] While plasma-based processing enables relatively low-temperature, versatile etching and deposition, its use requires careful consideration of the full process flow and of device and circuit layout. This is necessary because plasma-based processing can produce damage that can be exacerbated or mitigated by subsequent processing and by device and circuit design. A basic cause of damage plasma etching or deposition is due to current flow due to charging or induced EMFs.

[0004 ] A fundamental principle employed in most plasma based processes is disassociation of a feed gas by the application of radio frequency (RF) power. Screening designed experiments, intended to establish process sensitivity to set points, commonly indicate that the RF power setting is a major, if not the primary, contributor to plasma processing results.

[0005] Referring to FIGURE 2, a typical RF power delivery network 10 consists of an RF power generator 12 (with nominal 50 ohm output impedance); a coaxial cable 14 to conduct the power from the generator 12; an impedance matching network 16 designed to match the load impedance to the generator's output impedance and ultimately the RF plasma load 18 (chamber and all confined within it), as shown in prior art FIGURE 2. [0006] Although coaxial cables vary in characteristics, assuming the proper current carrying capability has been chosen the only remaining significant variable is length, which should be integer multiples of a quarter wavelength of the frequency of operation. The impedance matching network consists of deceptively simple capacitive and inductive components organized in such a manner as to optimize the delivery of RF power from the generator to the load.

[0007 ] However, casual inspection of the matching network yields the requirement for cooling which is caused by heat generated due to RF power absorption by the non-ideal capacitive and inductive components. The simple fact that cooling is required due to power absorption within the matching network (a series component between the generator and the load) provides the obvious conclusion that not all the RF power supplied by the generator actually reaches the RF load.

[0008] Further inspection of the components comprising the RF power delivery path, yields the observations that as components, especially the matching network, actually behave like variable attenuators in series with the load. That is to say that as these components are heat cycled, age, wear, degrade, become damaged or replaced, the amount of power absorbed by them changes, thereby causing the actual delivered power to the RF load to vary. This variation in the delivered RF power translates into process result variation. Often, this RF power variation exceeds the tolerances of the process specification. [ 0009] Unfortunately, these RF power variations also result in wafer scrap events. Unlike most other process set points, RF power is controlled from the RF generator rather than from the point of use (RF load, a.k.a. chamber) thereby allowing the variable attenuation mechanism to become an uncontrolled process variable.

[ 0010] Solution of this control architectural problem involves first making point of use measurements of the delivered RF power. Although there are several options for actually monitoring the RF delivered power, they all rely on making measurements of at least the RF voltage and current. Problems abound at the frequencies and power levels commonly found in semiconductor and flat panel manufacturing.

[ 0011] Numerous companies over the past 15 years have created RF sensors for various applications including closed loop delivered power control. However, the problem of accuracy has proved to be a major source of aggravation for the proliferation of this technology. Although the subject of overall accuracy is much broader than this document will attempt to cover, one significant aspect is the intrinsic ability of any RF sensor to function appropriately at the extreme conditions of an RF open and short. [ 0012 ] Failure of this seemingly simple test results in the near inability to properly calibrate the instrument to a manufacturing acceptable level. One of the most common failure modes occurs when the magnetic field induces a signal on the voltage transducer for the short condition. Assuming proper attention has been paid to the creation of the shorting load, the output of the voltage transducer should be at or near zero for the condition of an RF short. All too often this is not the case and causes significant changes in the overall packaged sensor as attempts are made to shield or sufficiently attenuate the magnetic field reaching the voltage transducer.

[0013] The prior art is deficient in the application of a system, apparatus or method to shield or sufficiently attenuate a magnetic field from reaching the voltage transducer. [0014 ] For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art an AC coupled transducer for an RF sensor capable of attenuating a magnetic filed from permeating the voltage transducer.

SUMMARY

[0015] The above-mentioned shortcomings, disadvantages and problems are addressed herein, which will be understood by reading and studying the following specification. [0016] The disclosed subject matter has been made in view of the above circumstances and has as an aspect an AC coupled shielded voltage transducer.

[0017 ] A further aspect of the disclosed subject matter includes an AC coupled voltage transducer forming a closed surface with respect to the RF current carrier. [ 0018] Additional aspects and advantages of the present disclosure will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosed subject matter. The aspects and advantages of the present disclosure will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

[ 0019] To achieve these and other advantages and in accordance with the purpose of the disclosed subject matter, as embodied and broadly described, the disclosed subject matter can be characterized according to one aspect the invention includes an AC coupled voltage transducer for an RF sensor, including an RF power source, an RF current carrier an a voltage transducer, wherein the voltage transducer is adapted to form a closed surface around a portion of the RF current carrier.

[ 0020 ] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only are not restrictive of the present disclosure, as claimed.

BRIEF DESCRIPTIONS OF THE DRAWINGS

[0021] The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrates several embodiments of the present disclosure and together with the description, serve to explain the principles of the present disclosure. [0022 ] FIGURES 1A-1D depict a prior art schematic of a deposition process; [0023] FIGURE 2 depicts a prior art schematic of an RF generator and plasma load configuration;

[ 0024 ] FIGURE 3 depicts an embodiment of an AC coupled voltage transducer of the disclosed subject matter;

[ 0025] FIGURE 4 illustrates a 3-D view of the AC coupled voltage transducer of the disclosed subject matter; and

[ 0026] FIGURE 5 depicts schematic diagram of a variable impedance load testing apparatus of the disclosed subject matter.

DETAILED DESCRIPTION

[0027 ] In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments, which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. [0028] Furthermore, apparatus, systems, and methods of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawings and by reading the detailed description that follows.

[0029] The following detailed description is, therefore, not to be taken in a limiting sense. The disclosed subject matter teaches and contemplates a simpler and more elegant solution to the problem of magnetic field perturbation of the voltage transducer output by a design, which obeys Gauss's law for magnetic fields. Before turning to Gauss's law, a review of the background principles and equations will be presented.

[0030] A magnetic field is usually denoted by the symbol B, called the magnetic flux density or magnetic induction. Lorentz transformation for an electric field E of a moving electric charge (for example, electric field of an electron moving in a conducting wire) results in the following equation: v \ --,- E c-

which, is the "magnetic field" and denoted by B. [0031] Substituting into the definition of the magnetic field results in the equation:

1 B = W ~ E c-

resulting in the electric field equation:

E = ,, r = 10 c , r

4 Tf₀ I- r- which, results in the Biot-Savart Law equation:

B = v x -- r

4 Vi J "

The Lorentz force on a stationary wire carrying moving charges (i.e. current) is therefore:

F = iBL where

F = force ;

B = flux density;

L = length of wire; and i = current in wire (ampere)

[0032 ] A person of ordinary skill in the art will readily appreciate, after a simplification of the above equations, that the electromotive force (EMF) created by a conductor placed in a changing magnetic field according to Faraday's law is a follows:

EMF = - (dφ/dt) .

[0033] An application of the previously detailed magnetic flux equation and its interrelationship with the EMF equation results in the magnetic flux being defined as:

A further application of Gauss's law for a magnetic flux results in the following equation:

JS B - dS = O [0034 ] This equation verifies that magnetic flux acting upon a closed surface yields an EMF of 0 volts. Hence, if the voltage transducer is fabricated as a closed surface around the RF current carrier so as to create a capacitor, the integrity of the intrinsic transducer output can be maintained.

[0035] FIGURE 3 depicts a schematic diagram of one aspect of the disclosed subject matter. In FIGURE 3, the apparatus 20 solves the RF sensor problem by surrounding RF current carrier 22 with a capacitor 24. Capacitor 24 may be formed from a conductive tube surrounding the primary RF current carrier 22 and separated from the current carrier by a suitable dielectric material 26. FIGURE 4, depicts a three-dimensional rendering of the schematic diagram of FIGURE 3, and further displays a first dielectric region 28. [0036] Suitable materials for the conductive tube 22 include but are not limited to brass or silver coated copper. Initially the height of the tube as measured along the primary RF current path may be adjusted to regulate the transducer output signal amplitude. In addition a capacitive divider network (not shown) may be employed to finely adjust the transducer signal or modify the system's frequency response.

[ 0037 ] Furthermore, the conductive tube used to create the voltage transducer performs the addition function and utility of shielding an inductively coupled current monitor placed in proximity to the tube from error inducement by an electric field or EMF (this is an error signal for the open load condition test).

[ 0038 ] In one embodiment of the disclosed subject matter, as seen in FIGURES 3 and 4, empirical tests and observations have revealed that the addition a top flange 28 to the conductive tube 22 serving to cover the inductively coupled current monitor from the top and reducing errors created by fringing electric fields.

[ 0039] An RF voltage transducer fabricated in this closed surface fashion can be used for either frequency discriminating or non- frequency discriminating RF sensors. [ 0040] Although not specific to the novelty of the disclosure, the materials used in the model shown in FIGURES 3 and 4 were brass for the primary RF current carrier and voltage transducer with a polyimide based dielectric used for separation. A person of ordinary skill in the art will appreciate that any conductive material acting as a closed surface to the RF current carrier will reduce, if not eliminate, the deleterious effects of stray magnetic and electromotive fields.

[ 0041] All references, including publications, patent applications, and patents, cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. [0042 ] In an alternate embodiment of the disclosed subj ect matter (as shown in FIGURE 5, the typical RF power delivery network (as described in prior art FIGURE 1, consists of an RF power generator (with nominal 50 ohm output impedance); a coaxial cable to conduct the power from the generator; an impedance matching device designed to match the load impedance to the generator's output impedance and ultimately the RF load (chamber and all confined within it). Since the output impedance of the generator and indeed the coaxial transmission line are well know and non-varying, devices have been developed to allow for testing of these components.

[0043] These devices typically consist of some power measurement means and what is commonly referred to as a "50 ohm load". These "loads" are specifically designed to match with the generator and cable output impedance as to affect optimal power transfer and thereby testing of the respective component functionality. It has always been common practice to have these 50 ohm loads available for periodic testing as well as diagnostic applications after process excursions. Modern 50 ohm loads are often cart mounted with the power measurement technology necessary to verify generator and cable functionality incorporated into the cart.

[ 0044 ] Unfortunately, the remaining component of the RF delivery network, the impedance matching device, by design, does not have standardized output impedance. As such, it has not been possible to test the entire RF delivery network or even just the impedance matching device portion of the RF delivery network without removing it to a test bench where even then testing at power was difficult.

[ 0045] Hence it has become common practice over the years to suspect this impedance matching device whenever process shifts occurred resulting in undesirable outcomes and to simply replace it rather than suffer the expensive of down time while the device was sent out for testing. However, this procedure is not without expense, especially if changing the impedance matching device does not cure the tool condition creating the process excursion and further diagnostics must be performed on other tool subassemblies. [ 0046] The basic problem is the need for a variable non-50 ohm load which could easily be attached in the manufacturing environment (similar to the traditional 50 ohm loads), but rather to the entire RF delivery network including the impedance matching device and verify proper functionality of the entire network quickly and with less expense. [ 0047 ] United States Pat. No. 4,147,207 teaches in the prior art that the most commonly use 50 ohm load design, which, is still in use as of this filing. United States Pat. No. 5,801,598 teaches a novel approach to 50 ohm loads but is more appropriate for RF frequencies above those commonly found in microelectronics manufacturing. Neither of these addresses the teachings of the present invention or are appropriate for non-50 ohm environments.

[0048] FIGURE 5 depicts a schematic of the non-50 ohm load 40. As illustrated, the load is derived from the combination of 50 ohm 54 and 50, non-50 ohm 46 and 42 real components and the electrical length characteristics of specifically cut coaxial cable, which when combined with the non-50 ohm 46 and 42 real component presents a reactive non-50 ohm load similar to that of a microelectronics manufacturing process chamber during operation.

[ 0049] In order to complete the diagnostic capability a suitable non-50 ohm RF sensor 58 is capable of being integrated (and contemplated by the disclosed subject matter) prior to the real component load box. By operating the switches 60, 62, 64, 66, 68, 70, 72 and 74 on the front end of the apparatus 500 the electrical length may be varied by up to 16 positions. When combined with the also variable real component 42, 44, 46, 48, 50, 52, 54 and 56 at the rear of the load apparatus a total of 64 points at various locations on the Smith chart may be presented to the RF delivery network under test. Performance of the network under test is verified by comparing the measurements made with the embedded RF power sensor to the forward power measurements made on the output of the RF generator. [ 0050 ] The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms "comprising," "having," "including," and "containing" are to be construed as open-ended terms (i.e., meaning "including, but not limited to,") unless otherwise noted.

[ 0051 ] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein.

[ 0052 ] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the disclosed subject matter unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the disclosed subject matter. [0053] Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.

Claims

1. An AC-coupled voltage transducer for an RF sensor, the system comprising: an RF power source; an RF current carrier; and a voltage transducer, wherein said voltage transducer is adapted to form a closed surface around a portion of said RF current carrier.

2. The voltage transducer of Claim 1, wherein said closed surface of said voltage transducer includes a conductive carrier.

3. The voltage transducer of Claim 1, wherein said voltage transducer closed surface is adapted to form a tubular structure.

4. The voltage transducer of Claim 3, wherein a portion of said RF conductive carrier passes through said tubular structure's interior.

5. The voltage transducer of Claim 1, further including a conductive divider network.

6. The voltage transducer of Claim 1, wherein said closed surface of said voltage transducer forms a shield to at least one of a magnetic field or an electromotive field.

7. The voltage transducer of Claim 6, said shield to at least one of a magnetic field or an electromotive force is comprised of multiple conductive layers.

8. The voltage transducer of Claim 1, further comprising a first dielectric region disposed between said RF current carrier and said closed surface of said voltage transducer.

9. The voltage transducer of Claim 8, further comprising a second dielectric region disposed between said RF current carrier and said closed surface of said voltage transducer.

10. The voltage transducer of Claim 9, wherein the dielectric region further includes a material possessing dielectric properties essentially similar to dielectric properties of polyimide.

11. The voltage transducer of Claim 2, wherein said conductive carrier is comprised of at least one of a silver coated copper and a brass conductor.

12. The voltage transducer of Claim 2, wherein said voltage transducer closed surface is adapted to form a tubular structure

13. The voltage transducer of Claim 12, wherein a portion of said RF conductive carrier passes through said tubular structure's interior.

14. The voltage transducer of Claim 13, wherein said closed surface of said voltage transducer forms a shield to at least one of a magnetic field or an electromotive field.

15. The voltage transducer of Claim 2, wherein said closed surface of said voltage transducer forms a shield to at least one of a magnetic field or an electromotive field.

16. The voltage transducer of Claim 15, said shield to at least one of a magnetic field or an electromotive force is comprised of multiple conductive layers.

17. The voltage transducer of Claim 15, further including a conductive divider network.

18. The voltage transducer of Claim 13, further comprising a first dielectric region disposed between said RF current carrier and said closed surface of said voltage transducer.

19. The voltage transducer of Claim 18, further comprising a second dielectric region disposed between said RF current carrier and said closed surface of said voltage transducer.

20. The voltage transducer of Claim 19, wherein the dielectric region further includes a polyimide based dielectric.

21. A method for transducing AC-coupled voltage with an RF sensor, comprising the steps of: generating RF power using an RF power source; supplying an RF current using an RF current carrier; and transducing AC-coupled voltage using a voltage transducer, said voltage transducer forming a closed surface around a portion of said RF current carrier.

22. The voltage transducing method of Claim 21, further comprising the step of using said closed surface of said voltage transducer as a conductive carrier.

23. The voltage transducing method of Claim 21, further comprising the step of using said voltage transducer closed surface as a tubular structure.

24. The voltage transducing method of Claim 23, further comprising the step of using a portion of the RF conductive carrier as a tubular structure interior.

25. The voltage transducing method of Claim 21, further comprising the step of operating said AC-coupled voltage transducer as portion of a conductive divider network.

26. The voltage transducing method of Claim 21, further comprising the step of using said closed surface of the voltage transducer as a shield to at least one of a magnetic field or an electromotive field.

27. The voltage transducing method of Claim 26, further comprising the step of shielding at least one of a magnetic field or an electromotive force using multiple conductive layers.

28. The voltage transducing method of Claim 21, further comprising the step of using a first dielectric region disposed between the RF current carrier and the closed surface of the voltage transducer.

29. The voltage transducing method of Claim 28, further comprising the step of using a second dielectric region disposed between the RF current carrier and the closed surface of the voltage transducer.

30. The voltage transducing method of Claim 29, further comprising the step of using said dielectric region further as a region comprising a polyimide based dielectric.

31. The voltage transducing method of Claim 30, further comprising the step of using said conductive carrier with at least one of a silver coated copper and a brass conductor.

32. The voltage transducing method of Claim 30, further comprising the step of using said voltage transducer closed surface in the form of a tubular structure

33. The voltage transducing method of Claim 30, further comprising the step of passing a portion of the RF conductive carrier through a tubular structure interior.

34. The voltage transducing method of Claim 33, further comprising the step of using said closed surface of the voltage transducer as a shield to at least one of a magnetic field or an electromotive field.

35. The voltage transducing method of Claim 34, further comprising the step of using said closed surface of the voltage transducer as a shield to at least one of a magnetic field or an electromotive field.

36. The voltage transducing method of Claim 35, further comprising the step of shielding at least one of a magnetic field or an electromotive force using multiple conductive layers.

37. The voltage transducing method of Claim 35, further comprising the step of operating a conductive divider network in association with said voltage transducer.

38. The voltage transducing method of Claim 33, further comprising the step of disposing a first dielectric region between said RF current carrier and said closed surface of said voltage transducer.