Abstract:
The present invention is a method to eliminate the influence of clamp dimensional changes on the displacement measurement during the measurement of the coefficient of thermal expansion (CTE) of samples in tension. In a first embodiment clamp dimensional changes can be eliminated by making clamps from a material with near zero CTEs. In another embodiment clamp dimensional changes can be reduced by minimizing the CTE of the clamp material. Finally clamp dimensional changes are taken into account. This is achieved directly by subtracting a prerecorded baseline from the measurements; or by determining the CTE measurement for various sample lengths and obtaining the slope of a straight line through the points on a MD (measured displacement)/DT (temperature range of displacement measurement) versus sample length plot. The slope is the corrected CTE. Clamp dimensional changes can also be taken into account indirectly by obtaining a clamp displacement contribution (CDC) factor which allows to correct measured displacements to result the correct CTE.

Description:
[0001]    This application claims priority from Provisional Applications bearing Serial Nos. 60/058,648 filed Jul. 11, 1997 and 60/059,891 filed Sep.24, 1997. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Technical Field Of The Invention  
           [0003]    The present invention is directed to increasing the accuracy of the measurement of thermal expansion coefficients (CTE) performed on thin film samples. More specifically, in accordance with the present invention, the CTE increased measurement accuracy is obtained using any of the following procedures: (a) by measuring the baseline using the same set-up used in the measurement; (b) by identifying a correction factor; (c) by using specific, low expansion clamps; or (d) by extracting the CTE from measurements of samples with various lengths.  
           [0004]    2. Description of the Related Art  
           [0005]    The testing of plastics is generally carried out for the same reasons as apply to the testing of other materials, namely to determine their suitability for a particular application, for quality control purposes or to obtain a better understanding of their behavior under various conditions. It is necessary for a manufacturer to be able to measure its performance with relation to other materials and thus be in a position to assess the market likely to be available to it.  
           [0006]    The physical testing of plastics must be standardized if the comparison of physical data from two or more different sources is to have any meaning. The physical testing of plastics can be classified generally as (a) dimensional; (b) thermal; (c) mechanical; (d) electrical; and (e) optical. The results of physical tests carried out on a material often depend upon the ambient conditions of temperature, humidity, the size, shape and method of preparation of the test pieces and the techniques of measurement employed.  
           [0007]    Testing of the thermal properties of plastics is important to give the plastic user some idea of the range of safe temperatures at which the plastic can be used. Some of the important thermal properties are thermal conductivity, coefficient of thermal or linear expansion, specific heat, softening point, heat distortion temperature and mold shrinkage.  
           [0008]    The present invention relates to accurately measuring the CTE as this test is now advantageously used to test thin samples of polymers such as polyimide, and to a lesser extent, epoxy resins used in computer applications.  
           [0009]    CTE&#39;s of materials are typically measured using a thermal mechanical analyzer (TMA). A TMA is an instrument used in a penetration type test which is capable of measuring the displacement of a measuring probe with great accuracy. The probe is in contact with a sample to be measured and detects changes in sample dimensions in probe direction and transmits results to a displacement detection unit. Test sample dimensions can change by the application of temperature or weight to the sample.  
           [0010]    These dimensional changes may be time dependent. In the case of CTE measurements, the contact with the sample must be chosen so that the sample dimension does not change as a consequence of the probe; or if the sample dimension does change, such change is minimal and reproducible.  
           [0011]    A typical TMA is depicted in FIG. 1A (prior art) comprising a probe ( 1 ), sample holder ( 2 ), heater ( 3 ), micrometer ( 4 ), thermocouple ( 5 ), differential transformer ( 6 ), core ( 7 ), force generator ( 8 ). In a typical use, the plastic sample to be tested is held between sample holder ( 2 ) and probe ( 1 ). A force is applied to the sample, and the resulting changes in sample length are detected by the independently connected differential transformer ( 6 ) and core ( 7 ). The TMA used to obtain more accurate CTE values pursuant to the present invention is depicted in FIG. 1B (prior art). The unit comprises a loading generator ( 10 ), a core ( 11 ), differential detector ( 12 ), micrometer ( 13 ), sample cylinder ( 14 ), outer tube ( 15 ), probe ( 16 ), sample ( 17 ), and furnace ( 18 ). The TMA module ( 19 ) provides a signal ( 20 ), a temperature signal ( 21 ) and heater power ( 22 ). Also included in this apparatus is a means for processing software ( 23 ) used to provide certain instructions to the apparatus during operation as hereinafter described.  
           [0012]    In this embodiment, sample ( 17 ) is set between sample cylinder ( 14 ) and probe ( 16 ). The independent sample cylinder ( 14 ) and probe ( 16 ) are connected to differential transformer ( 12 ) and its core ( 11 ) respectively. The top of probe ( 16 ) is also connected to a first end of balance arm ( 23 ). The other end of balance arm ( 23 ) is connected to electric loading generator ( 10 ) which controls the load according to CPU guidance. Differential detector ( 12 ) detects the movement of probe ( 16 ) as the sample ( 17 ) length changes, and outputs this as a TMA signal ( 21 ). Recorded signals are time, temperature, dimensional change and load.  
           [0013]    There are a variety of probes that are used in the TMA depending upon what test is being conducted. The probes are configured for expansion, volume expansion, compression and penetration. In addition, accessories for tension, three point bending, and cubical expansion are available. When combined with a dynamic loading program, a variety of applications are possible.  
           [0014]    Presently, several ways of accomplishing contact of the probe to a sample exist. However, if the sample is a thin film, there is essentially only one way to guarantee continuous contact between the probe and the film while at the same time not bending the film.  
           [0015]    In the method of the present invention, sample ( 30 ) is held by two clamps ( 31 ) and ( 32 ) as shown in FIGS. 2A and 2B (prior art front and side views respectively). FIGS. 2A and 2B show that top clamp ( 31 ) is connected to probe ( 33 ) while bottom clamp ( 32 ) is stationary or fixed to sample holder ( 34 ). Probe ( 33 ) and stationary sample holder support ( 34 ) are made from a material (such as quartz, having a CTE of 0.5 ppm/° C. which possesses a very small expansion coefficient so as not to interfere with the expansion of the sample.  
           [0016]    Using the set up described above and depicted in FIGS. 2A and 2B, sufficient tensile force is applied to the probe so that the sample film ( 30 ) is under light tension. The length of the sample between the clamps ( 31 ) and ( 32 ) is recorded. For CTE measurements the sample is heated (cooled) and the probe displacement with temperature is measured. FIG. 3 depicts a side view of how the measurement is determined. It is has been determined that it is absolutely essential in such testing procedure that clamps ( 41 ) and ( 42 ), used to hold sample ( 40 ), not contribute to the measured displacement. This requirement is specifically stated in the user manual of such an instrument; (see:  Seiko TMA Users Manual,  Appendix-A, A- 3 ).  
           [0017]    A computer analysis software program collects displacement and temperature values. Often, time values are collected. Various software routines are used to extract and calculate the CTE for the particular sample tested. The CTE is computed as the difference of measured displacement D 1 , at temperature T 1 , and displacement D 2 , at temperature T 2 , divided by the difference between T 1  and T 2  and is exemplified by the expression: CTE=(D 1 −D 2 )/(T 1 −T 2 ) [units of ° C. −1 ]; wherein [D 1 −D 2 ] is called the measured overall displacement (MD) (also referred to as (OD)); and T 1 −T 2  is called the temperature range (ΔT).  
           [0018]    In order to obtain accurate values, the TMA must be well calibrated. This calibration includes collections of baselines and other instrument parameters which are conveniently incorporated into the software to obtain accurate data during measurements performed on samples.  
           [0019]    In the electronics industry the matching of CTEs of dissimilar materials is of great importance because mismatch of the CTE of the various materials comprising an electronic device such as a memory or logic chip or an electronic package leads to stresses which influence the durability and performance of the device. Of particular importance is the measurement of materials with CTEs in the vicinity of the CTE of silicon (2.6 ppm/° C.).  
           [0020]    In order to provide for the demands of the electronic industry, the chemical industry has responded by synthesizing polymeric materials which exhibit CTEs that match the 2.6 ppm/° C. value of silicon noted above. While silicon can be measured using compression mode CTE measurements, the CTE of these newly developed polymer films can only be measured directly in the tension mode. The reason for this is that in many polymer films, the CTE is a function of film thickness. Thus, thicker films which could be measured by other means will not give the required CTE values.  
           [0021]    The thickness of the samples used in accordance with the present invention is in the range of between about 1 mm to 1 μm, preferably between about 150 μm and 5 μm  
           [0022]    Labaratory measurements have established that CTEs measured with the above-described tension mode do not give accurate results. Errors for materials such as silicon, were off by as much as 480%, even with a well calibrated instrument. Errors also varied with sample length.  
           [0023]    It is an object of the present invention to provide a method which allows accurate measurements of the CTE of thin films using the tensile mode. In particular, the present invention describes methods using the tensile mode that allow accurate measurement of the CTE of films with low expansion coefficients, that is between about −25 and +50 ppm/° C.  
         SUMMARY OF THE INVENTION  
         [0024]    The predicate for the present invention is the discovery that in the course of determining the CTE of materials using a TMA. the clamps used in the TMA apparatus to hold the sample, contribute to the overall observed dimensional change of the sample. This dimensional change phenomenon is attributable to the fact that the clamps expand with heating and contract with cooling. The absolute values of the clamp displacement contribution to the overall value of the CTE of a sample is small. The accuracy of the CTE for samples with large CTE&#39;s is therefore not greatly influenced by the clamp displacement contribution.  
           [0025]    However, clamp displacement can be a considerable portion of the dimensional change observed for samples with small CTEs. The present invention, as disclosed herein, eliminates, minimizes, or adjusts for the clamp expansion in order to obtain accurate CTEs.  
           [0026]    The present invention discloses a plurality of ways to eliminate the influence of clamp dimensional changes on the displacement measurement during the measurement of the CTE of samples in tension using the TMA apparatus.  
           [0027]    In one embodiment, clamp dimensional changes are eliminated by making clamps from a material with a CTE at or close to zero.  
           [0028]    In a second embodiment, clamp dimensional changes, i.e. expansion or contraction, during the process are accounted for by fabricating the clamps from a material with a known CTE, thereby eliminating the influence of the CTE of the clamp material. Alternatively, the clamps can be fabricated from a material that possesses a CTE that is identical for heating and cooling.  
           [0029]    In another embodiment, the clamps are fabricated from a material having a CTE that is linear in the range of operation. In this embodiment, actual clamp dimensional changes during the measurement procedure are taken into account. This is achieved by subtracting a predetermined and prerecorded baseline (value) from the test measurement results; or it is achieved by conducting the CTE measurement for various sample lengths and obtaining the slope of a straight line through points on a MD (measured displacement) / ΔT (temperature range of displacement measurement) vs. sample length plot. The slope is the corrected CTE.  
           [0030]    Finally, clamp dimensional changes can also be taken into account indirectly by obtaining a clamp contribution factor (CDC) which allows one to correct measured displacements to result in the correct CTE.  
           [0031]    The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0032]    Further objects, features, and advantages of the present invention will become apparent from a consideration of the following detailed description of the invention when read in conjunction with the figures comprising the drawings, in which:  
         [0033]    [0033]FIG. 1A is a general depiction of a prior art TMA.  
         [0034]    [0034]FIG. 1B is a more specific depiction of an existing TMA apparatus.  
         [0035]    [0035]FIG. 2A is a sideview schematic of the tension mounted sample for the measurement of the CTE of a thin sample in a TMA.  
         [0036]    [0036]FIG. 2B) is a front view schematic of the tension mounted sample for the measurement of the CTE of a thin sample in a TMA  
         [0037]    [0037]FIG. 3 is a schematic of the TMA set up to measure the CTE in tension.  
         [0038]    [0038]FIG. 4 is a graph depicting a baseline measured with a quartz sample using commercial clamps.  
         [0039]    [0039]FIG. 5 is a graph showing the CTE measurement of the clamps in compression showing clamp expansion during heating, and shows a hysteresis between heating and cooling.  
         [0040]    [0040]FIG. 6 is a plot of overall displacement, OD (in centimeters), divided by the temperature range over which the measurement was taken, (ΔT), versus the sample length.  
         [0041]    [0041]FIG. 7 shows a plot of measured displacement, MD (in 100 micrometers), divided by the temperature range over which the measurement was taken, (ΔT), versus the BPDA-PDA sample length in centimeters for a sample with unknown CTE.  
         [0042]    [0042]FIG. 8 shows the displacement vs temperature curve for a strip of quartz in tension after baseline correction.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0043]    A first embodiment of the invention eliminates or minimizes the expansion of the clamps used in a TMA apparatus such as is depicted in FIG. 1B. The material from which the clamps are fabricated has a coefficient of thermal expansion between about 0 and 20 ppm/° C. The object of the present invention is achieved by making the clamps out of a very low CTE material such as quartz or Invar which is an alloy having about 63.8% Fe, 36% Ni and 0.2 C. Other materials which may conveniently be used are ceramic, stainless steel, molybdenum, platinum, silver and aluminum. When such material(s) is used, the measured CTE is virtually the actual CTE of the measured material. In general, under ideal conditions, a clamp material is chosen which possesses a CTE value that is zero during heating and cooling.  
         [0044]    Within the purview of the first embodiment, and as a related feature of that aspect of the invention, the clamps in the TMA apparatus are made from materials with known CTE values. In this feature, a clamp material is chosen which possesses a linear CTE over the temperature range of the measurements taken.  
         [0045]    [0045]FIG. 5 shows the CTE behavior of currently used clamps which exhibit a transition thus having dimensions that are not identical during heating and cooling. It is preferable to use a clamp material which possesses a small CTE.  
         [0046]    Referring to the drawings, the clearly visible metal phase transition is responsible for the difference in the heating baseline and the cooling baseline found in FIG. 4. Clamps made from such materials with the specified phase transition are not desirable. FIG. 5 depicts the difference between heating and cooling which is due to a metal phase transition in the clamps.  
         [0047]    In a second embodiment of the invention, the influence of the clamp expansion is taken into account during the analysis of the raw data. Optionally, this can be done externally or internally.  
         [0048]    In the external case, the user measures a baseline and prepares a functional relationship as shown in FIG. 4, which is used to correct each subsequent measurement. Preferably, the user-measured baseline becomes part of an internal baseline correction routine.  
         [0049]    In the “internal” case, the data is analyzed by the TMA manufacturer and included in the software provided with the machine prior to the sale to the user. For example, the baseline is measured by the manufacturer using silicon or quartz. The resulting information is stored in the software and is automatically subtracted when the CTEs are calculated.  
         [0050]    In another embodiment of the invention, the user or equipment manufacturer determines a clamp displacement contribution (CDC) factor. Once determined, this factor is used to correct all measured displacement values or to correct the overall displacement measured in a given temperature range. The CDC factor can be used subsequently to modify the CTE measurement by the user; or it can become part of an internal correction applied to all CTE measurements obtained by tensile mode. In the latter case the user does not need to be aware of the procedure.  
       EXAMPLE 1  
       [0051]    The method of present invention for increasing the accuracy in determining the coefficient of thermal expansion of a silicon sample material using a thermal mechanical analyzer apparatus (TMA) was employed on a sample having dimensions of 20 mm length, 3 mm width and 1 mm thickness. The TMA was of the type depicted in FIG. 1B and possessed a probe, sample holder, heater, micrometer, thermocouple, differential transformer, core, force generator, analysis software and had top clamp and bottom clamp elements. The clamps were made from stainless steel; the CTE behavior is shown in FIG. 6. The silicon sample to be measured was held between the top and bottom clamps, applying a tensile force to said top clamp so that said sample film is under light tension, i.e. about 10 g., and the displacement was measured.  
         [0052]    With respect to the tensile force applied to the sample, the load needs to be chosen such that the sample will not deform within the given temperature range. In general, the load applied is between about 2g. and about 100g., preferably between about 10g. and 20g. This range is preferred because it has been found to be the range at which the force is sufficiently large not to cause displacement by vibrations and is not large enough to cause permanent sample deformation. A specific example is that a sample having a thickness of 10μis placed under a load of 10g.  
         [0053]    Thereafter the silicon sample was heated and cooled and the displacement of said sample was measured. In a separate operation, the silicon sample length was changed and the measurement steps were repeated. Taking the data generated from both experiments, the overall displacement over the temperature range was recorded and plotted against the sample length.  
         [0054]    More particularly, as noted, the CTE measurement is performed for at least two sample lengths (L 1 , L 2 , etc. . . . ) of a material over an identical temperature range (ΔT). The measured overall displacement (MD 1 , MD 2 , etc. . . . ) divided by the temperature range (ΔT), for which the displacement was measured, is plotted against the sample length (L 1 , L 2 , etc . . . . ). The slope of the straight line through the points of this plot is the correct CTE of the sample. The slope from this operation is depicted in FIG. 6. FIG. 6 is a plot of 100*(OD) /(ΔT) versus the silicon strip length in cms. The slope can also be obtained by linear regression analysis. The slope as determined from the plot in FIG. 6 obtained pursuant to this Example 1 record the CTE of silicon as 2.4 ppm/° C. This measured result is substantially identical to the literature value of silicon which is 2.6 ppm/° C.  
       EXAMPLE 2  
       [0055]    In another example of an embodiment of the present invention, using the apparatus and procedure described above in Example 1, a sample BPDA-PDA strip with an unknown CTE was measured using sample length variations. Again, MD/ΔT values are measured for a variety of sample lengths and plotted against the latter. FIG. 7 shows a plot of measured displacement, MD (in 100 micrometers), divided by the temperature range over which the measurement was taken, (ΔT), versus the BPDA-PDA sample length in centimeters for a sample with unknown CTE. FIG. 7 also shows the CTE values that were obtained for each sample length using the prior art instruments/procedure without any corrections. These are indicated for each sample length. This demonstrates that the errors in CTE values can exceed 100% in the example at hand.  
       EXAMPLE 3  
       [0056]    Another method was used wherein a baseline was measured using a quartz standard and the current stainless steel clamps. Using the apparatus and procedure described above in Example 1, a quartz sample, 0.8 mm thick, 3 mm wide and 20 mm long, was mounted in the clamps and loaded into the TMA and a load of 10 g. was applied. The sample was heated at a low heating rate (2° C./min to insure that the sample was in thermal equilibrium with the surroundings and was cooled at the same low rate. It is noted that heating rate is not significant when samples are only 10 μm thick as long as the rate is below 2° C./min. The measurement of this standard was continued until subsequent runs were identical. The displacement versus temperature curve of the last run was stored to be used as a baseline. FIG. 8 shows the displacement vs temperature curve for a strip of quartz in tension after baseline correction. There is a 0.2μdifference start and end; and between low and high temperature.  
         [0057]    This baseline is also conveniently described by a polynomial and a TMA signal is output after these effects are deducted, is used. A sample with unknown CTE and of the same length as the previously measured standard is now measured over the same range of temperature. The previously recorded baseline is subtracted from the displacement versus temperature plot of the unknown sample. Prior art methods to extract the CTE from this corrected plot are used to obtain the correct CTE value as shown in FIG. 8.  
       EXAMPLE 4  
       [0058]    From the length variation measurements described in Example 3 above, the clamp displacement contribution (CDC) can be extracted. As shown hereinafter, the CDC factor can be used to obtain the correct CTE of unknown samples.  
         [0059]    The straight line in the MD/ΔT versus L (observed displacement vs. sample length) plot of FIG. 6 is described by: 
         
       MD/ΔT=CTE*L+CDC 
     
         [0060]    From this equation it is apparent that several methods exist to obtain the CDC factor. The CDC factor can be used to correct the displacement data and to obtain the correct CTE for any given sample.  
         [0061]    One way to obtain the CDC factor is by determining the intersection of the line in the MD/ΔT versus L plot, with the MD/ΔT axis, i.e., the point at which L= 0 . The value of MD/ΔT is also the value of the CDC.  
         [0062]    Another way to obtain the CDC value is by measuring a standard with a linear CTE having a known value, wherein the measurement is taken in the tensile mode. After the length L is recorded, measurements are run until reproducible results are achieved. The measured sample displacement (MD) in the temperature range of interest is divided by said temperature range (ΔT. From the resulting value, the product of the CTE of the standard and L is subtracted to obtain the Clamp Displacement Contribution (CDC) factor according to the equation: 
         ( MD/ΔT )− CTE*L=CDC   
         [0063]    Corrected CTE&#39;s for samples with unknown CTE can be obtained with the knowledge of the CDC factor using the following calculation.  
         [0064]    The MD of the unknown sample is divided by the DT in which the MD was measured. The CDC factor is subtracted and the resultant value is divided by sample length, L, according to: 
         ( MD/ΔT )− CDC )/ L =corrected CTE of unknown sample 
         [0065]    Clamp correction factors can also be calculated from runs with a sample having known CTE. The equation used is then: 
           CDC= ( CTE (known)− CTE (measured))* L/ 1000 
         [0066]    CDC factors were calculated from the experiments using a fused quartz standard. The CDC values for various temperature ranges are given in Table 1 hereinafter. It can be seen that the variation of CDC factors is between 4 - 5%. This indicates that this method will give CTE values with an acceptable accuracy of below 5% for samples with a CTE of above 2 ppm/° C. as long as the sample length is about 20 mm. For shorter samples, the error will increase.  
         [0067]    This method can be incorporated into the software of the instrument to obtain corrected CTE&#39;s of unknown samples. The results of the experiments are set forth in Table 1.  
                                     TABLE 1                           CLAMP DISPLACEMENT CONTRIBUTION FACTORS            Temperature Range   Run 6   Run 5   Run 4   Run 3               Heating ° C.                        40-115   —   0.06315   0.07080   0.07124        30-115   0.05955   —   —   —       125-200   0.06083   0.05298   0.05189   0.06333       200-300   0.06407   0.06242   0.05850   0.06762       300-400   0.06476   0.06293   0.06054   0.06906       400-480       0.05403   0.05297   0.06795       Cooling        40-100   0.06578   0.06966   0.07217   0.06573       100-200   0.06621   0.06749   0.06251   0.06954       200-300   0.07038   0.07227   0.06625   0.07310       300-400   0.07266   0.07652   0.07392   0.08112       400-480   —   0.07923   0.07778   0.09537                  
 
         [0068]    While the present invention has been described with respect to preferred embodiments, numerous modifications, changes, and improvements will occur to those skilled in the art without departing from the spirit and scope of the invention. The  
         [0069]    Thus, while there have been shown and described and pointed out fundamental novel features of the invention as applied to currently preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the method and apparatus illustrated, and in their operation, may be made by those skilled in the art without departing from the spirit of the invention. In addition it is to be understood that the drawings are not necessarily drawn to scale but that they are merely conceptual in nature. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended herewith.