A DEVICE FOR MEASURING STRESSES IN VACUUM DEPOSITION COATINGS

Abstract: In measuring stresses present in material coatings deposited by sputtering, a pressure transducer (100) is used as an in-situ sensor. The transducer (100) has a flexible conductor (112) clamped about its periphery to a rigid substrate (114) and forms a variable capacitor with a relatively rigid conductor (122) across an underlying evacuated chamber (118). A mask element (26) having an aperture (28) that covers an area confined within the boundary defined by the underlying evacuated space (118) is located adjacent the flexible conductor to limit the exposed surface area to receive sputtered material coating. Electronic circuitry monitors changes in the capacitance of the sensor (100) and reflects the stress types and values.

Description: The present invention is directed to the field of devices used for measuring stress build up in vapor deposition coatings and more specifically to the area of measuring such coatings utilising a capacitive sensor. In applying metal or dielectric coatings to work pieces by the use of sputtering systems, such as the one shown in U.S. Patent No. 4,525,264 , it has been found that some degree of residual stresses remain in the coating. The detection and control of such coating stresses or "film stresses" is seen as highly desirable in order to prevent resulting failures or unwanted effects in the coatings. By being able to sense the occurrance, strength and type of internal stresses developed in vapor deposited material coatings, one may thereafter control subsequent coatings by making appropriate changes in the process. For instance, by lowering the gas pressure or raising the discharge current in a sputtering system one can reverse the stresses in subsequent coatings from tensile to compressive. One of the principal conventional ways of assessing the stress effects produced by different vapor process conditions involves the generation of individual samples of coatings at each distinct setting of the various process parameters. The individual samples receiving sputtered material coatings comprise relatively thin substrates so that residual film stresses can be determined by observation. The thin substrates have a resultant curvature based upon the type and amount of stresses within the film coating. Film stresses are determined in this method by removing the coated samples from the deposition chamber for subsequent examination. Conventionally, in-situ measurements of stress forces present in thin films prepared by sputtering techniques utilize a "cantilever" method, whereby a thin conductive substrate or plate is rigidly clamped at only one end and oriented in a position so as to receive the sputtered material. A rigidly mounted and nonmoveable reference electrode is positioned nearby the free end of the cantilevered conductive substrate so that the substrate and the electrode serve as a capacitor having a value of capacitance which varies according to the spacial distance between the two. Typically, in this prior art method, the cantilevered substrate receives a coating of material and bends either towards the rigid electrode or away from the rigid electrode depending upon the stresses present to cause such deflection. Changes in capacitance are electrically sensed and provide an indication as to the stress levels. The cantilever measurement technique provides relative measurement from one coating to the other. However, due to the mechanical instabilities present in the set up, it is only suitable for laboratory use under ideal conditions. For instance, its use would be difficult in an industrial application where the coating quality is periodically sampled to determine stress levels of coating materials being sputtered on the surface of production parts. This is especially true where vibrations are present which would cause variations in the readings. According to the present invention, there is provided a sensor for measuring stresses present in deposited coatings of materials emitted from a vapour source comprising first means defining a rigid electrical conductor having a planar surface, second means defining a relatively flexible electrical conductor having a first planar surface opposing said first means and a second planar surface for receiving said vapour deposition coatings, third means providing rigid support of said first and second means, a space defined to separate said opposing first and second means over an uninterrupted common area wherein said space between said opposing first and second means provides a variable electrical dielectric when said second means is flexed with respect to said first means, and mask means disposed adjacent said second planar surface of said second means to limit the vapour deposited material being coated on said second planar surface by said vapour source to an area that is within the boundaries defined by the uninterrupted common area of opposition in the space between said first and second means. The invention will now be described further, by way of example, with reference to the accompanying drawings,in which: Figure 1 is an exploded perspective view of the elements employed adjunct to a sensor of an embodiment of the present invention, Figure 2 is an elevational cross-section of the sensor employed in the embodiment of Figure 1, Figure 3 is a schematic diagram illustrating the associated circuitry employed with the sensor, Figure 4 is a set of waveform diagrams illustrating the relationship of a plurality of signals occurring within the circuit shown in Figure 3, and Figure 5 is a plot of sensor sensitivity versus the ratio of mask size to cavity size. The desirability of measuring stresses present in vapor deposited coatings, such as metal, is achieved through the use of the present invention in-situ of the low pressure chamber such as that described in U.S. Patent No. 4,525,264, mentioned above. The actual sensor is designated as 100 in the figures and is provided within a protective housing illustrated in Figure 1. The protective housing incorporates a base element 10 made of a heat conductive material, such as aluminum. A cooling coil 15 is provided that has an inlet 12 and an outlet 14 so as to pass coolant and dissipate heat away from the sensor. A printed circuit board 16, carrying the sensor 100, electronic circuitry 200 and appropriate heat sink conductors is mounted on the base element 10. B+ power is supplied to the printed circuit board 16 through a shielded conductor 18. The signal output from the circuitry 200 is provided on a shielded cable 20. As can be better seen in the assembled crosssection of Figure 2, a heat sink device 22 is mounted on the printed circuit board 16 so as to contact the exposed edges of the upper surface 117 of the sensor 100. The heat sink device 22 conducts excessive heat away from the sensor and to a grounded bus on the printed circuit board 16. A mask 26 is provided with a relatively small aperture 28 and is mounted above the sensor 100 so as to limit the exposure area on the exposed upper surface 117 of the sensor 100. A metal cover 30 is provided that attaches to the base element 10 so as to protect the printed circuit board from being coated with materials that are being deposited onto the sensor. An aperture 32 is defined in the cover 30 so as to provide an uninterrupted area for the coating materials impinging onto the exposed surface 117 of the sensor 100 through the mask aperature 28. It should be noted that the sensor 100 is preferrably a commercially available, variable capacitance pressure transducer such as that described in U.S. Patent No. 4,261,086. The desirability of using a commercially available pressure transducer enhances the reliability and follows for interchanging the sensor components when the coating samples are accumulated onto the exposed surface of the sensor to a thickness that is sufficient to decrease its sensitivity. As shown in Figure 2, sensor 100 is formed of a silicon semiconductor material 112 hermetically bonded to a glass dielectric material 14 along the interface 116. The silicon semiconductor material 112 is formed to define an evacuated space 118 having predetermined boundaries and, in the case of the embodiment shown in Figures 1 and 2, is a square area having a diagonal dimensions "d₂". The semiconductor silicon layer 112 is doped so as to be conductive. Therefore, the silicon layer 112 is utilized as one conductor of a sensor capacitor. The silicon layer 112 is connected via a conductive pin 128, formed in the glass substrate 114, to a conductor 17 on the printed circuit board 16. A second electrical conductor 122 is formed on the upper surface 120 of the glass dielectric substrate 114 within the evacuated space 118 to define an uninterrupted common area of overlap. The conductor 122 is connected through a conductive pin 126, formed in the glass substrate 114, to a conductor 19 on the printed circuit board 16. The thickness between the lower surface 119 of the silicon layer 112 and the exposed upper surface 117 is much less than the thickness of the glass substrate 114. Therefore the silicon layer 112 provides a relatively flexible conductor membrane opposed to the relatively rigidly mounted conductor 122 across the evacuated space 118. In the preferred embodiment, the heat sink element 22 has a relatively large aperture 24 and is mounted so as to contact the peripheral edges of the exposed surface 117 and to conduct heat away from the sensor. In the illustration shown in Figure 2, the heat sink 22 is connected to the ground conductor 17, which is in turn connected to the housing. The mask element 26 is superimposed above the heat sink 22 and is also attached to the heat sink and ground conductor 17 in order to provide additional heat protection. The aperture 28 formed in the mask 26 is shown as having a diameter "d₁" that is substantially less than d₂ dimension. The purpose of the smaller aperture 28 is to allow the vapor coating materials "S p " to be deposited on an area on the relatively flexible portion of the silicon layer 112 and away from the clamped, immovable boundaries. In Figure 5, a plot is shown of the relative sensitivity of the sensor with a d₂ chamber dimension for various d₁ sizes of mask apertures. As can be seen from the plot, the sensitivity peaks when the d₁/d₂ ratio is approximately 0.6 and has acceptable sensitivity in the range of d₁/d₂ ratios from 0.3 to 0.8. The important relationship appears to be that the area exposed to receive vapor coatings on the upper flexible conductor must be less than and substantially within the corresponding evacuated space area defined by the bonding of the upper flexible conductor to the rigid substrate. When employed within a vapor coating chamber the device shown in Figure 2 could be incorporated with a shutter (not shown) which would be used to time the exposure of the surface 117 of the sensor 100 to the coating material S p . In Figure 3, the sensor 100 is schematically represented, with the apertured mask 26, in its electrical configuration. The sensor 100 serves as a variable tuning capacitor to a sensor oscillator 220, resident in the intergrated circuit 200 shown in Figure 1. A reference oscillator 230, utilizing a reference capacitor 232, provides a reference frequency to a mixer circuit 240. The difference in frequency between the two oscillators 220 and 230 is provided at the output of the mixer 240 to a divider circuit 250. The aforementioned portion of the circuit 200 is disclosed in U.S. Patent No. 4,377,851 and provides a square wave output signal "Pwa" as shown in Figure 4. At the appropriate partial pressure present in the vapor coating chamber, the square wave signal produced as Pwa is approximately 140 Hertz that has a cycle duration of approximately 70 nanoseconds. An AND gate 260 and a one shot multivibrator circuit 270 are provided to receive the Pwa signal. The one shot multivibrator is triggered by the negative going portion of the Pwa square wave and provides a relatively high level signal "Pwb" having a known duration which is less than the full-cycle length of Pwa and greater that the half-cycle length of Pwa. Preferrably, the output period of Pwb would be on the order of 75% of the cycle length of Pwa prior to coating depositions being made on the sensor 100. The output Pwb of the one shot multivibrator 270 is provided to the second input terminal of AND gate 260 so that when both signals are relatively high, an output signal "Pwa-Pwb" is generated having a pulse duration equal to the time that both signals are in their relatively high states. The output of the AND gate 260 is provided to a detector filter 280 which accumulates the output signals and provides a rectified DC analog output level. In a preferred in-situ use environment, the sensor 100 is placed in the approximate location that work pieces would normally be positioned to receive vapor coatings. The exposed, relatively flexible surface 117 of the sensor 100 receives the sputtered coating material. Any stresses present in the sputtered coating cause corresponding changes in the capacitance of the sensor 100. An increase in the analog output of the circuit 200 results when the stresses in the layer are under tension and a decrease in the analog output results when the stresses are in compression. Through periodic sampling and adjustments to the partial pressure or sputtering voltages, the coating stresses can be changed and monitored by the in-situ sensor 100 until such time as the desired stresses are measured. Utilizing the desired parameters, the chamber can then be used for production purposes to vapor deposit material coatings on work pieces. From time to time, it may be desirable to utilize the in-situ sensor to monitor the production and verify that the stresses present in the coatings are as desired. The use of sensor 100 has been found to be highly desirable since the life of the sensor has been found to be highly accurate over several hundred cycles of tensile and compressive stress layers deposited on the exposed surface 17.

Claims: A sensor for measuring stresses present in deposited coatings of materials emitted from a vapour source comprising first means (122) defining a rigid electrical conductor having a planar surface, second means (112) defining a flexible electrical conductor having a first planar surface (119) opposing said first means (122) and a second planar surface (117) for receiving said vapour deposition coatings, third means (114) providing rigid support of said first and second means (122,112), a space (118) defined to separate said opposing first and second means (122,112) over an uninterrupted common area wherein said space (118) between said opposing first and second means (122,112) provides a variable electrical dielectric when said second means (112) is flexed with respect to said first means (122), and mask means (26) disposed adjacent said second planar surface (117) of said second means (112) to limit the vapour deposited material being coated on said second planar surface (117) by said vapour source to an area that is within the boundaries defined by the uninterrupted common area of opposition in the space between said first and second means (122,112). A sensor as claimed in Claim 1, wherein said second means and said third means are hermetically bonded around said defined space. A sensor as claimed in Claim 1 or 2, further including a circuit means electrically connected between said first and second means to measure changes in electrical capacitance across said variable dielectric which occur due to stresses present in vapour deposition material coatings on said second surface of said second means. A sensor as claimed in Claim 3, wherein said circuit includes a variable frequency oscillator which utilizes the electrical capacitance across said variable dielectric to provide an output frequency that reflects the value of electrical capacitance present between said first and second means.

Inventors: HOFFMAN, DAVID WYETH, KUKLA, CASSIMER MARION
Applicants: FORD FRANCE SOCIETE ANONYME, FORD MOTOR COMPANY, FORD MOTOR COMPANY LIMITED, FORD-WERKE AKTIENGESELLSCHAFT
Application year: 1987
Main classification: G01L9
IPC codes: G01L9, G01L1, G01N27