Electrostatic Chucks (ESCs) are essential to precision semi-conductor wafer manufacturing process. Existing chucks may be divided into two major categories, each category having its particular strengths and weaknesses.
The dielectric in a Polyimide ESC (PESC) is a strong insulator, consequently, most of the applied voltage drop is across the dielectric and creates a Coulombic chucking force. Unfortunately, the PESC working surface is super-sensitive to scratching. Further, the PESC is susceptible particle embedding, which can cause arcing between the backside of the wafer and the copper electrode on the PESC. Still further, the PESC cannot be used at high temperatures because high temperature operation can cause blisters of water moisture through the polyimide layer.
The partially conducting Ceramic ESCs (CESC) require a constant current to achieve adequate chucking force, thus CESC leakage current is greater, requiring a larger power supply than a PESC. This current-dependent chucking force, known as the Johnsen-Rahbek Effect, is small compared to the Coulombic force in the PESC.
CESCs with anodized aluminum trioxide (Al2O3), currently available only as monopolar devices, are extremely sensitive to moisture. Further, anodized aluminum has been used as an ESC insulating layer in some bipolar ESCs. However, arcing and anodization defects often cause early failure of this type of ESC.
Doped alumina has been also used for sonic CESCs for etch applications. The doped ceramic helps to control its resistivity in Johnson-Rahbeck ESC resistivity range. But the grain boundary attack on glass phases can change the surface roughness of the ESC and therefore increase the resistivity. Further, the roughened ceramic surface will cause high helium leak. An attack on the ceramic surface during a wafer-less auto-clean cycle on ceramic grain boundaries often causes the impedance of the ceramic to move from a Johnson-Rahbeck type impedance to a Coulombic type impedance.
High purity ceramic, e.g., Alumina, has been widely used as the dielectric puck layer on a surface of ESCs. It has been used either as a monopolar or a bipolar ESC. Further, it is used as a Coulombic ESC due to its high resistivity. High purity alumina, e.g., 99.7% purity or higher, may be applied as ESC dielectric puck layer either using a solid sintering ceramic or using a thermal spray coating.
Recently introduced CESCs, employing a sintered Aluminum nitride (AlN) dielectric, have poor thermal transfer characteristics. Resistivity of the ceramic material is temperature-dependent and more variable from piece to piece than the PESC. AlN has a superior thermal conductivity in comparison with alumina. Therefore, it has received wide applications as a high temperature ESC operating at 200° C. or higher. In most cases, AlN surface has mesa surface patterns to control ESC contact area with a wafer surface. The major problem of AlN is that it can generate AlF3 particles when SF6, NF3 and other F-based gases are used in the etching chamber. AlF3 is one the major particle sources in etching chamber technology. Since AlN resistivity depends on the operating temperature as a Johnsen-Rahbek ESC, selection of suitable types of AlN to maintain a workable resistivity and maintaining high plasma-resistance under high density plasma are very important.
Depending on end user requirements and installed equipment, each of the PESC or CESC will satisfactorily hold (Chuck) and release (Dechuck). Generally speaking, it doesn't matter which type of ESC will be used, the capacitance and resistivity of an ESC are the two key parameters for ESC functionality.
Mounting holes (not shown) on mounting ledge 104 enable mounting of ESC 100 onto a system.
In operation, a first voltage differential is applied across first electrode 106 and second electrode 108. The voltage differential creates an electric field, which is used to attract and hold a wafer for processing. When the processing is finished, a second voltage differential (dechucking voltage) is applied across first electrode 106 and second electrode 108 to release the wafer.
Although briefly described above, the voltage control on a conventional ESC, whether single or multi-poled is critical. In this light, many parameters of the ESC that may affect such voltage control are therefore also critical. Non-limiting parameters include resistance, capacitance, impedance and frequency phase shift. Further, the parameters may be further analyzed for each independent portion of the ESC, as opposed to analyzing the parameter of the chuck in its entirety. Non-limiting examples of which include, a specific parameter measured from one electrode to another electrode (pole-to-pole), a specific parameter measured from one electrode on the top surface to the base (pole-to-base).
In this manner, when first terminal 304 is connected to measuring terminal 308 and when second terminal 306 is connected to measuring terminal 312, a characteristic of first electrode 106 may be measured using a pole-to-base measurement. Similarly, when first terminal 304 is connected to measuring terminal 308 and when second terminal 306 is connected to measuring terminal 310, a characteristic of first electrode 106 and of second electrode 108 may be measured using a pole-to-pole measurement. Similarly, when first terminal 304 is connected to measuring terminal 312 and when second terminal 306 is connected to measuring terminal 310, a characteristic of second electrode 108 may be measured using a pole-to-base measurement.
In the above discussed conventional technique, when conventional measuring device 302 is capable of measuring resistance, the user may measure any one of the resistance of first electrode 106 from pole-to-base, the resistance of second electrode 108 from pole-to-base and the resistance of first electrode 106 and second electrode 108 from pole to pole. Similarly, when conventional measuring device 302 is capable of measuring capacitance, the user may measure any one of the capacitance of first electrode 106 from pole-to-base, the capacitance of second electrode 108 from pole-to-base and the capacitance of first electrode 106 and second electrode 108 from pole to pole. When conventional measuring device 302 is capable of measuring inductance, the user may measure any one of the inductance of first electrode 106 from pole-to-base, the inductance of second electrode 108 from pole-to-base and the inductance of first electrode 106 and second electrode 108 from pole to pole. When conventional measuring device 302 is capable of measuring impedance, the user may measure any one of the impedance of first electrode 106 from pole-to-base, the impedance of second electrode 108 from pole-to-base and the impedance of first electrode 106 and second electrode 108 from pole to pole. When conventional measuring device 302 is capable of measuring a phase delay of the frequency of the applied voltage, the user may measure any one of the phase delay of first electrode 106 from pole-to-base, the phase delay of second electrode 108 from pole-to-base and the phase delay of first electrode 106 and second electrode 108 from pole to pole.
Reliable electrical performance is of the utmost importance. Accordingly, an ESC manufacturer may perform quality assurance checks on manufactured ESC before shipping to customers. One conventional quality assurance check may include determining whether a specific parameter of a manufactured ESC is within a predetermined acceptable range, non-limiting examples of which include: the measured resistance being equal to or greater than R1Ω and equal to or less than R2Ω; the measured capacitance being equal to or greater than c1F and equal to or less than c2F; the measured impedance being equal to or greater than Z1Ω and equal to or less than Z2Ω; and the measured frequency phase shift being equal to or greater than φ1 and equal to or less than φ2. In the conventional methods, the manufacturer applies a current or voltage to the terminals as discussed above at a predetermined frequency fm. If all significant characteristics, e.g., resistance as measured by an Ohmmeter, are within the manufacturer's predetermined range of acceptability, then the ESC is determined to be acceptable.
As an example in TABLE 1 below, resistance and capacitance measurements were taken on several ESCs (p/n 718-094523-281-E). Capacitance measurements ranged from 3.478 to 3.777 nano Farads and resistance measurements ranged from 2.267 to 3.829 Meg Ohms:
Note, these devices were deemed acceptable by the manufacturer. How close the first device tested was to exceeding the maximum allowable resistance, or the second was to having too little resistance hasn't been defined.
An ESC's performance will degrade with use. If visual inspection clearly indicates ESC defects (cracked, deep scratches, residual particles) immediate replacement may be in order. It is difficult to isolate the cause when an ESC fails to perform satisfactorily, because of the sensitivity and complexity of the process control system. The present state of the art docs not provide a simple non-invasive means of detecting non-visual defects. Conventional characteristic tests, for example, duplicating the manufacturer's acceptance tests, as discussed above, may indicate that a particular ESC is acceptable even though the ESC does not perform satisfactorily.
What is needed is a reliable, non-invasive method to determine the suitability of an ESC for initial or for continued use.
It is an object of the present invention to provide a reliable, non-invasive, electrical test method for predicting satisfactory performance of ESCs, both for initial acceptance and for in-service use.
In accordance with an aspect of the present invention, a method is used to test an ESC having a front surface and a back surface and comprising a first electrode and a second electrode. The method comprises establishing predetermined acceptable limits of a parameter of the ESC within a frequency band, and measuring the parameter of the ESC within the frequency band.
In accordance with another aspect of the present invention, a method is used to establish acceptable limits of a parameter of an electrostatic chuck using a known upper limit of the parameter at a single frequency and a known lower limit of the parameter at the single frequency. The method comprises measuring the parameter of the electrostatic chuck within a frequency band to obtain a function of the parameter within the frequency band, generating a slope of the function of the parameter within the frequency band, generating an acceptable upper limit of the parameter using the known upper limit of the parameter at the single frequency and the slope of the function of the parameter, and generating an acceptable lower limit of the parameter using the known lower limit of the parameter at the single frequency and the slope of the function of the parameter.
In accordance with an aspect of the present invention, a method is used to establish acceptable limits of a parameter of an electrostatic chuck using a known upper limit of the parameter at a single frequency and a known lower limit of the parameter at the single frequency. The method comprises measuring the parameter of the electrostatic chuck within a frequency band a plurality of times to obtain a corresponding plurality of functions of the parameter within the frequency band, generating a mean function of the parameter based on the plurality of functions, generating a predetermined number of standard deviations of the parameter based on the plurality of functions, generating an acceptable upper limit of the parameter and an acceptable lower limit of the parameter using the predetermined number of standard deviations of the parameter.
Additional objects, advantages and novel features of the invention are set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
The accompanying drawings, which are incorporated in and form a part of the specification, illustrate an exemplary embodiment of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings:
Aspects of the present invention will now be described with reference to
In this manner, when first terminal 404 is connected to measuring terminal 308 and when second terminal 406. is connected to measuring terminal 312, a characteristic of first electrode 106 may be measured using a pole-to-base measurement. Similarly, when first terminal 404 is connected to measuring terminal 308 and when second terminal 406 is connected to measuring terminal 310, a characteristic of first electrode 106 and of second electrode 108 may be measured using a pole-to-pole measurement. Similarly, when first terminal 404 is connected to measuring terminal 312 and when second terminal 406 is connected to measuring terminal 310, a characteristic of second electrode 108 may be measured using a pole-to-base measurement.
As distinct from the conventional measuring device 302 of the method illustrated in
Points 502 may be used to fit a line 510, which may be considered the impendence function of a “good” working ESC. Practically speaking, not all ESCs will have the same impedance function, yet may nevertheless be considered acceptable. There are many ways to establish acceptable boundaries for ESCs in accordance with the present invention.
In one method of establishing acceptable boundaries for a parameter of ESCs in accordance with the present invention, a manufacturer may indicate that an ESC is considered acceptable if a measureable parameter is within an acceptable range. For example, as illustrated in
In the embodiment discussed above, with respect to
Another method of establishing acceptable boundaries for a parameter of ESCs in accordance with the present invention includes taking measurements from a plurality ESCs that are known to be acceptable.
Line 802, plurality of lines 804 and plurality of lines 806 may then be used to generate acceptable boundaries of impedance functions via a mean impedance function in addition to a standard deviation. For example, as illustrated in
Once acceptable boundaries for a parameter of ESCs are established, an ESC, whether new or used, may be tested in accordance with an aspect of the present invention to determine if it remains acceptable. An exemplary method of testing an ESC for acceptability will now be described with reference to
Once the parameter to be measured is determined, frequency response analyzer 402 scans a predetermined frequency band (S1006) and measures the corresponding impedances.
Similar to
In the present example, measured impedance function 1102 includes a first portion 1202, a second portion 1204 and a third portion 1206. Both first portion 1202 and third portion 1206 are disposed within lines 702 and 704 and may therefore be considered acceptable. Portion 1204 however is disposed below line 704 and therefore would be considered unacceptable. Further the slope of portion 1204 is dramatically different from the slope of line 510 and therefore would be considered unacceptable.
A portion 1208, which spans from the impedance Za as measured at frequency fa to the impedance Zθ as measured at frequency fθ, of portion 1202 lies with lines 702 and 704, but has a dramatically different slope than that of line 510. Similarly, portion 1210, which spans from the impedance Zφas measured at frequency fφ to the impedance Zψ as measured at frequency fψ, of portion 1206 has a dramatically different slope than that of line 510. As such, although the measured impedance within portions 1208 and 1210 may be acceptable, the rate of change of impedance within the corresponding frequency bands may raise concern as to suitability.
100611 Returning back to
Deviations in magnitude of a measured parameter or deviations in the rate of change of the measured parameter over a frequency band may indicate any one of a plurality of problems or potential problems, non-limiting examples of which include: an integrity breakdown in at least one part of the ESC; stress, which may lead to an integrity breakdown, in at least one part of the ESC. Further, in accordance with the present invention, a user may determine that a particular ESC that is tested, is not acceptable within a specific frequency band but is acceptable in other frequency bands. For example, returning to
Returning to
The above discussed exemplary embodiments of the present invention with reference to
Returning to
In other embodiments, the “other parameter” of step S1012 of
In other embodiments, the “other parameter” of step S1012 of
As discussed earlier, and ESC manipulates electric fields to chuck, hold and de-chuck wafers for processing. Therefore, capacitance of an ESC is a particularly important parameter to monitor and maintain. In a specific working example, capacitance is determined by measuring impedance via a frequency response analyzer, as will be discussed in more detail below.
ESC 100, for example as measured from measuring terminal 310 to measuring terminal 312, may be schematically illustrated as a parallel RC circuit as illustrated in
where
Xc=1/(ωC), and
where Xc is the capacitive resistance as measured in Ohms, ω is the driving frequency of the input signal, C is the capacitance as measured in Farads and R is the resistance as measured in Ohms.
By using the known impedance function discussed above, and by measuring the impedance of ESC 100, for example as measured from measuring terminal 310 to measuring terminal 312, with frequency response analyzer 402, the capacitance C and resistance R of ESC 100 may be determined.
With known upper and lower limits on acceptability of capacitance and resistance, for example having previously been determined by any of the example methods discussed above, then a measured impedance of ESC 100 may be used to determine whether ESC 100 is acceptable based on its resistance and capacitance. For example, a Bode plot based on the measured impedance will readily determine whether ESC 100 is acceptable.
The above discussed exemplary embodiment include testing of a bipolar ESC. Of course any type of ESC may be tested in accordance with the present invention. Further, any number of measurable parameters, any number of measurements from different measuring ports or sites, and any number of temperatures for measurements of an ESC may be used to determine acceptability in accordance with the present invention.
To tailor the method for a specific wafer production facility, accept-reject criteria may be developed for each different type/model/series of ESC used at that facility, based on the installation's equipment configuration, the test equipment selected, and applicable proprietary processes. Before placing new ESCs in service, each ESC may be tested and its initial, benchmark, measurements recorded. Subsequently, each time an ESC is tested, the measurements would be added to its record, until it fails the acceptance criteria for resistance, capacitance or any other controlled parameter, or is withdrawn from service for other reasons, e.g. unacceptable visible defects.
The foregoing description of various preferred embodiments of the invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The exemplary embodiments, as described above, were chosen and described in order to best explain the principles of the invention and its practical application to thereby enable others skilled in the art to best utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto.
This application is a divisional of, and claims any benefit associated with, U.S. Pat. application Ser. No. 12/249,215, filed Oct. 10, now U.S. Pat. No. 8,143,904 B2, 2008, the entire disclosure of which is fully incorporated herein by reference.
Number | Name | Date | Kind |
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6853953 | Brcka et al. | Feb 2005 | B2 |
20020008954 | Leeser | Jan 2002 | A1 |
20030033116 | Brcka et al. | Feb 2003 | A1 |
20050022736 | Steger | Feb 2005 | A1 |
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Number | Date | Country |
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06198532 | Jul 1994 | JP |
Entry |
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English translation of JP 06198532A, Jul. 19, 1994. |
Number | Date | Country | |
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20120153971 A1 | Jun 2012 | US |
Number | Date | Country | |
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Parent | 12249215 | Oct 2008 | US |
Child | 13372423 | US |