Measuring and testing – Coating material: ink adhesive and/or plastic
Reexamination Certificate
1998-12-10
2002-01-22
Williams, Hezron (Department: 2856)
Measuring and testing
Coating material: ink adhesive and/or plastic
C073S827000
Reexamination Certificate
active
06339958
ABSTRACT:
FIELD OF THE INVENTION
The present invention is directed to method and apparatus for testing adhesion strength between two materials using a depth sensing indentation technique.
DESCRIPTION OF RELATED ART
Thin films are very important in many applications. For example, thin films are used extensively in microelectronics applications where devices often have features of submicron size. Thin films are also used extensively in micro-mechanical applications for making devices such as microgears and accelerometers, and other applications such as for making hard disks in a hard drive and hard coating for gear boxes.
Determining the mechanical properties of thin films in these applications can be critical. For example, a thin film having a large tensile stress may delaminate causing device failure under certain conditions. The mechanical properties of a thin film material cannot be simply predicted based upon the properties of the bulk material for a number of reasons. The mechanical properties of the thin film will generally be different from that material in bulk form, and will depend upon the particular technique for forming the film, and the conditions under which the film is formed. For example, a thin film formed on a substrate at high temperature and then cooled to room temperature may exhibit either a tensile or compressive stress due to the difference in the coefficient of thermal expansion between the film and the substrate. Also a thin film may delaminate from a substrate due to stress applied by an outside source.
Depth sensing techniques, such as nanoindentaion and microindentation techniques, have been used for measuring material hardness and elastic modulus of a material. One exemplary system for performing nanoindentation hardness measurements is shown in FIG.
1
. The apparatus
1
contains a sample stage
2
on which the sample to be tested
3
is placed. The indenter tip
4
is situated above the sample
3
. As an example, the indenter tip may be a sharp Berkovich type diamond indenter tip which has a three sided pyramid tip with a known area to depth correlation. The indenter tip
4
is supported by the indentation column
5
which is moved up and down by a load application coil
6
. The apparatus also contains indentation column guide springs
7
and a capacitive displacement sensor
8
. The load application coil is connected to a current source
9
and an oscillator
10
, while the capacitive displacement sensor is connected to a lock-in amplifier
11
and an electronic displacement sensor
12
. The electronics are controlled by a computer
13
.
The prior art apparatus operates in the following manner. The computer sends a signal to lower the indenter tip
4
into the sample. The computer operator enters into the computer a desired maximum load on the indenter tip. The indenter tip can penetrate up to a certain maximum depth into the sample for a given maximum load. Of course, for each material, the maximum depth differs for a given load because each material has a different hardness. The maximum penetration depth of the indenter tip is monitored by the capacitive displacement sensor
8
. As the indenter tip
4
penetrates further into the sample, the capacitive plate attached to the indentation column
5
moves closer to the capacitive plate attached to the sample stage
2
. Therefore, the capacitance between the two plates changes. The change in capacitance is detected by the electronic displacement sensor
12
which forwards the data to the computer
13
. The computer than correlates the maximum load applied to the maximum penetration depth. The oscillator and lock-in amplifier may be used to scan the indenter tip across the sample.
Sample hardness may be calculated from nanoindentation measurements using, for example, two different methods. The first method is a depth measuring method. In this method, the penetration depth of the indenter tip for a predetermined peak (maximum) load is measured by the indentation apparatus and the contact area (e.g., the area of the sample contacted by the indenter tip) is extrapolated from the known shape and geometry of the indenter tip. Then sample hardness is calculated as a function of penetration depth.
The second method is an imaging method. In this method, the peak load exerted by the indentation apparatus is preset and the contact area is determined by an optical or electron microscopy examination. Sample hardness is determined from a ratio of the applied maximum load applied to the measured contact area.
Once hardness is calculated, the elastic modulus may be calculated from the hardness. The hardness may be calculated from a simple formula with a high error margin (See W. C. Oliver, R. Hutchins and J. B. Pethica, “Measurement of Hardness at Indentation Depths as Low as 20 nanometers,”
Nanoindentation Techniques in Materials Science and Engineering
, ASTM STP 889, P. J. Blau et al. eds., ASTM (1986) pp. 90-108, incorporated herein by reference) or from finite element analysis calculations using the unloading portion of a load displacement curve with a low error margin (See W. C. Oliver and G. M. Pharr,
J. Mater. Res.
Vol. 7, No. 6 (1992) pp. 1564-83 and T. A. Laursen and J. C. Simo,
J. Mater. Res.
Vol. 7, No. 3(1992) pp. 618-26, both incorporated herein by reference). The sample may be a bulk sample or it may be a film on a substrate. Thus, hardness of a film may be measured by nanoindentation techniques.
Despite the different calculation techniques, hardness measurements in the prior art utilize a predetermined maximum load. In other words, the user selected a certain value of the maximum load, then this maximum load was applied to the sample, the penetration depth or contact area were measured, and hardness was calculated.
In addition, the prior art adhesion testing methods require destructive testing of the adhered materials. For example, in one prior art method, a representative article containing a film adhered to a substrate would be selected during mass production and pulled apart from the substrate by a layer of glue attached to the film. However, this stud pull method requires a large testing area and is expensive and time consuming. It also cannot be used to test the adhesion strength of all articles actually used or sold to the customers. Thus, even if adequate adhesion strength between materials was measured on the test article, other articles sold to customers or used by the manufacturer could easily have inadequate adhesion strength between adhered materials. Furthermore, the stud pull test cannot be properly used for thin films, such as those thinner than a thousand microns.
A second prior art method of adhesion testing is the blister test, where a hole is made in the substrate and then liquid or air pressure is applied through the hole to bulge the film outward. This method has the same disadvantage as the above-noted stud pull test. This method is also cumbersome and destructive.
A third prior art method of adhesion strength testing is the scratch test, where the film is scratched until it delaminates from the substrate. Such a method is taught, for example, by T. W. Wu et al., MRS Symp. Proc., Vol. 130 (1989) page 117, and in
Adhesion Measurement of Thin Films, Thick Films and Bulk Coatings
, K. L. Mittal, ed., ASTM (1978), pages 134-183, both incorporated herein by reference. In this method, a indenter tip is dragged across a thin film under an increasing load until the film delaminates from the substrate. The critical load for delamination is determined from an onset of a drop in the load. However, this method damages a significant portion of the film under investigation. This method also is subject to inaccuracies, because the drop in the load may be caused by flaking of the thin film instead of by delamination of the film from the substrate. Since for certain brittle thin films the onset of flaking precedes the delamination, the adhesion strength of such thin films cannot be accurately determined by the scratch test.
SUMMARY OF THE INVENTION
One object of an embodiment of the present inv
Joo Young-Chang
Tsui Ting Y.
Advanced Micro Devices , Inc.
Cygan Michael
Williams Hezron
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