Abstract:
Measurement of the permittivity of thin films is facilitated through the use of a short cylindrical metal cavity containing parallel plates between which a specimen to be measured is placed. The use of such parallel plates contained within such a cavity is particularly advantageous when swept frequency measurement methods utilizing frequency ranges from  0  to  20  GHz are employed. A test fixture which is preferred for use in providing such a cavity is disclosed as are methods of using the test fixture.

Description:
FIELD OF THE INVENTION  
         [0001]    The field of the invention is permittivity measurement methods and devices.  
         BACKGROUND OF THE INVENTION  
         [0002]    The permittivity of a specimen is the complex ratio of the capacitance between a pair of electrodes which sandwich the specimen and that of the same pair with an air gap. Measurement of capacitance and determination of permittivity is generally accomplished through the use of a test fixture coupled to a measurement instrument such as an LCR meter, an impedance/material analyzer, or a network analyzer.  
           [0003]    Test fixtures may be classified according to the measurement technique which they employ, and thus may be classified as being a parallel plate fixture, coaxial probe, transmission line fixture, free-space fixture, or a resonant cavity fixture. Parallel plate fixtures provide two parallel plates which are essentially electrodes between which a specimen is placed for measurement. Although such fixtures have many desirable characteristics, they are not suitable for measurement using signal frequencies greater than 1.8 GHz. Although other types of fixtures are suitable for use for frequencies greater than 1.8 GHz, their use is often undesirable for other reasons.  
           [0004]    As an example, coaxial probe devices essentially transmit a signal into a specimen and examine any reflected portion of the signal picked up by the probe. Unfortunately, because of the need for the material to reflect back a significant portion of the signal, coaxial probes are generally not suitable for thin specimens (i.e. less than or equal to 1 cm). Transmission line fixtures are also problematic because they require that a specimen be shaped to fit within a transmission line such as a wave guide or a coaxial transmission line so that the effects of the specimen on a signal transmitted through the line can be examined. Free space systems broadcast a signal at a specimen through free space and examine the effect of the specimen on the signal. The use of such systems generally require that the specimen be large, flat, thin, and parallel faced, and requires tight control of the distance between an antenna to a sample. Resonant cavity fixtures are similar to transmission line fixtures in that a precisely shaped specimen is placed within a resonant cavity or a transmission line and the effects of the specimen on fields within the cavity/line are examined. As with transmission line fixtures, having to precisely shape a specimen is generally not a desirable aspect of use of the fixture.  
           [0005]    Due to the inadequacies described for non-parallel plate fixtures, it is desirable that new parallel plate fixtures which permit the use of frequencies greater than 1.8 GHz be developed.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention is directed to methods and apparatus which facilitate the measurement of the permittivity of thin films using a parallel plate device for frequencies greater than 1.8 GHz. More specifically, the use of a short cylindrical metal cavity enclosing two parallel plates/surfaces is used as a fixture for permittivity measurement of thin film. The use of such a cavity is particularly advantageous when swept frequency measurement methods utilizing frequency ranges from 0 to 20 GHz are employed.  
           [0007]    Various objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the invention, along with the accompanying drawings in which like numerals represent like components. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]    [0008]FIG. 1 is a cutaway side view of a test fixture embodying the invention.  
         [0009]    [0009]FIG. 2 is a bottom view of a test fixture embodying the invention;  
         [0010]    [0010]FIG. 3 is a top view of a test fixture embodying the invention;  
         [0011]    [0011]FIG. 4 is a side view of a test fixture embodying the invention;  
         [0012]    [0012]FIG. 5 is a side view of a test fixture embodying the invention;  
         [0013]    [0013]FIG. 6 is a perspective schematic illustration of a cavity.  
         [0014]    [0014]FIG. 7 is a graph of various curves illustrating the relationship of f r D to D/L.  
         [0015]    [0015]FIG. 8 is a graph of TM 010  where D=0.5 cm.  
         [0016]    [0016]FIG. 9 is a schematic illustrating how the test fixture of FIG. 1 may be coupled to a capacitance meter. 
     
    
     DETAILED DESCRIPTION  
       [0017]    It is contemplated that positioning a specimen between parallel plates within a metal cavity which has a resonant frequency higher than the frequency of the signal being used to measure the capacitance of the specimen will help prevent inaccuracies which are typically experienced during high frequency capacitance measurements.  
         [0018]    In FIG. 7, each curve or line indicates the relation between the product f r D of the resonant frequency, f r , in GHz and the diameter, D, in cm, and the ratio of the diameter of the cylinder, D, to the length of the cylinder, D/L. (FIG. 6 illustrates how D and L correspond to cylinder  100 .) In FIG. 7, TE stands for the transverse electric fields and means that there is no longitudinal field (electric), while TM stands for the transverse magnetic fields with no longitudinal magnetic field. The word “longitudinal” refers to the length of the cylinder. The suffixes indicates modes n, m, and p which are the integral number of circumferential turns, the ordinal number of the roots of the radial derivative of the field, and the integral number of the half wave lengths in the longitudinal direction respectively.  
         [0019]    The following equation, in which K nm =π/m th  root of the derivative (radial) of the field, and ε r  and μ r  are relative permittivity and permeability respectively, can be used to determine the resonant frequency of cylinder  100  and was used to obtains the curves of FIG. 7:  
           f   r        D     =         3   ×     10   10             ɛ   r          μ   r                      (     1     K     n                 m         )     2     +       (       D                 p       2      L       )     2                                 
 
         [0020]    In free space, ε r =μ r =1, so, for TM 010  where D=0.5 cm, p=0 and K 01 =1.306 2 , f r D=1.3×10 10 /1.306=2.971×10 10  cm-Hz=22.971 GHz-cm. As can be seen in FIG. 8, for TM 010  where D=0.5 cm, TM 010  is a straight line wherein there is no resonance in the region below the line, i.e. where f r  is &lt;4.59418×10 10  GHz. Thus, a cavity having a diameter of 0.5 cm provides resonance free measurement between 0 and 20 GHz for specimens having a dielectric constant of 5 or less.  
         [0021]    Once one realizes that resonant frequency may affect measurement accuracy for frequencies at or near the resonant frequency, and once one realizes that any such inaccuracies can be decreased or eliminated through proper cavity selection, one can choose to provide a cavity within which to test a specimen wherein the cavity has a resonant frequency outside of a range of frequencies over which the capacitance or permittivity of the specimen is to be measured. A method of obtaining the permittivity of a specimen using such a chosen cavity may include (a) placing a specimen between parallel plates within the cavity and measuring the capacitance, C, of the specimen; (b) measuring the capacitance, Co, between the plates when the fixture does not contain a specimen; and (c) computing the relative real permittivity, E r ′, and/or the relative imaginary permittivity, E r ″, by calculating the ratio between the real and/or imaginary parts of C and C 0 , or calculating loss tangent by computing the ratio of the imaginary relative permittivity to the real relative permittivity. The fixture of FIGS.  1 - 6  may be used in such a method.  
         [0022]    Referring to FIG. 1, a preferred permittivity test fixture  10  comprises a metal cylindrical cavity  100  into which a thin film specimen  200  is inserted so that the permittivity of specimen  200  may be determined. Fixture  10  comprises base plate assembly  300 , cap assembly  400 , sleeve assembly  500 , and retainer ring  600 . Base plate assembly  300  comprises cylindrical base plate  310  and female pin  320 . Cap assembly  400  comprises threaded plunger  410 , indicator  420 , and sleeve receiving portion  430 . Sleeve assembly  500  comprises externally threaded cap end  510 , internally threaded connector end  520 , indicia portion  530 , and shaft  540 . Cavity  100  (also shown in FIG. 6) is defined, when fixture  10  is assembled, by base plate specimen contacting surface  311  of base plate  300 , plunger specimen contacting surface  412  of plunger  410 , and the cylindrical side wall of shaft  540 . Base plate  310  surface  311  and plunger  410  surface  412  are the “parallel plates” between which the specimen is positioned and which are contained with cavity  100 .  
         [0023]    In the preferred embodiment, base plate assembly  300 , cap assembly  400 , and sleeve assembly  500  are each metal and conductive, with cap assembly  400  and sleeve assembly  500  being electrically coupled to each other and to a ground (line  920  in FIG. 9) of a capacitance meter (meter  900  in FIG. 9), and base plate assembly  300  is preferably metal and electrically coupled to an output signal line (line  910  of FIG. 8) of a capacitance meter (meter  900  in FIG. 9). Retainer ring  600  is preferred to be non-conductive so as to keep base assembly  400  electrically isolated from cap assembly  400  and sleeve assembly  500 . Coupling of test fixture  10  to a standard connector  930  of a capacitance meter so as to couple fixture  10  to ground and signal lines  920  and  910  is facilitated by sizing and dimensioning test fixture  10  to be coupled directly to connector  930 .  
         [0024]    Base plate assembly  300  provides the means by which specimen  200  is electrically coupled to the signal line  920  of a capacitance meter. Such a coupling is facilitated by female pin  320  which is sized, dimensions, and positioned to receive a male pin of standard connector  930  when device  10  is screwed into connector  930 . Base plate  310  should be smaller than the diameter of shaft  540  to keep base plate  310  electrically isolated from the walls of shaft  540  and thus from sleeve assembly  500 . The dimensions of base plate assembly may vary between embodiments as may its composition, but it is preferred that base plate assembly  300  comprise a single piece of beryllium-copper.  
         [0025]    Cap assembly  400  seals off the end of the cavity  100  in which specimen  200  is placed. Plunger  410  extends into shaft  540  to a point adjacent to specimen  200  such that surface  412  contacts but does not compress specimen  200 . By internally threading end  520  of sleeve assembly  500  and placing corresponding threads  411  on plunger  410 , surface  412  can be properly positioned. Indicator  420  can be used in conjunction with indicia  530  to determine when surface  412  is properly positioned. Proper positioning will likely be related to both the desired resonant frequency of chamber  100  and the thickness of specimen  200 . Although the dimensions and composition of cap assembly  400  may vary between embodiments, it is preferred that cap assembly  400  comprise a single piece of brass and that surface  412  of plunger  410  have a diameter of 0.5 cm.  
         [0026]    Sleeve assembly  500  is preferred to be internally threaded on end  520 , and externally threaded on end  510 . The threads  511  on end  510  facilitate coupling fixture  10  (as previously discussed) to standard connector  930  of capacitance meter  900 . For embodiments intended to be coupled to different types of connectors, end  510  and/or threads  511  may be modified or replaced by structures which facilitates coupling fixture  10  to such connectors. The dimensions and composition of sleeve  500  may also vary between embodiments, but sleeve  500  is preferred to comprise a single piece of brass and is preferred to have end  510  fit within a standard 3.5 mm female connector.  
         [0027]    Retainer ring  600  is preferably formed from a single piece of non-conductive material such as TEFLON® and to be non-movably fixed within shaft  540  of sleeve assembly  500 . Base plate  310  of base plate assembly  300  is preferably non-moveably and adhesively coupled to surface  610  of retainer ring  600 . Female pin  320  of base assembly  300  extends through retainer ring  600  to provide an external electrical connection point to base plate  310 .  
         [0028]    It is contemplated that a particular embodiment of fixture  10  may be designed for a single thickness of dielectric material and a corresponding length of cavity  100 . Referring to FIG. 4, an indicia portion  530  of sleeve assembly  500  may interact with indicator  420  of cap assembly  400  to indicate when surface  412  is positioned correctly. Alternatively, indicia portion  530  of sleeve assembly  500  may facilitate the use of fixture  10  for various lengths of cavity  100  by providing a scale such as that shown in FIG. 5.  
         [0029]    A typical method of using device  10  may comprise: (a) removing cap assembly  400  from sleeve assembly  500 ; (b) inserting a specimen  200  to be measured into shaft  540  so that it is positioned on surface  310  of base assembly  300 ; (c) re-coupling cap assembly  400  to sleeve assembly  500  by inserting plunger portion  410  into shaft  540  and screwing cap assembly  500  onto sleeve assembly  500  until indicator  420  and indicia portion  530  indicate that surface  412  is adjacent to specimen  200 ; (d) attaching device  10  to a connector  930  of a test instrument  900  by screwing sleeve assembly  500  into connector  930  until female pin  320  is properly coupled to a male signal pin of connector  930  (attachment to connector  930  may also be accomplished prior to insertion of specimen  200 ); measuring the capacitance of specimen  200 ; (e) removing specimen  200  and by removing and replacing cap assembly  400 ; (f) measuring the capacitance of the free space of cavity  100  (this measurement may be done prior to measuring the capacitance of specimen  200  as well); (g) computing the permittivity of specimen  200  by computing the ratio of the appropriate portions of the capacitance of the specimen and the capacitance of the empty cavity  100 .  
         [0030]    Thus, specific embodiments and applications of permittivity test fixtures have been disclosed. It should be apparent, however, to those skilled in the art that many more modifications besides those already described are possible without departing from the inventive concepts herein. The inventive subject matter, therefore, is not to be restricted except in the spirit of the appended claims. Moreover, in interpreting both the specification and the claims, all terms should be interpreted in the broadest possible manner consistent with the context. In particular, the terms “comprises” and “comprising” should be interpreted as referring to elements, components, or steps in a non-exclusive manner, indicating that the referenced elements, components, or steps may be present, or utilized, or combined with other elements, components, or steps that are not expressly referenced.