Abstract:
A microwave imaging microscope and associated probe, or a read head. The probe or the read head includes a sensor unit with three fixed electrodes, preferably a stimulating electrode surrounding a sensing electrode and isolated by a grounded electrode. Circuitry couples the stimulating electrode to the probe signal variably selected in the range of 100 MHz to 100 GHz and couples the sensing electrode to a signal processor detecting in-phase and out-of-phase components of the current or voltage across the sensing electrode and the grounded electrode. A mechanical positioner moves the probe vertically towards the sample and scans it across the sample. The probe may be formed by semiconductor processing methods on a silicon chip.

Description:
FIELD OF THE INVENTION 
     The invention relates generally to electromagnetic measuring equipment and methods. In particular, the invention relates to a high-frequency probe used for mapping dielectric constant, resistivity and other electromagnetic characteristics in a sample with resolution of substantially less than a millimeter. 
     BACKGROUND ART 
     There is much interest in developing a microwave microscope that can measure one or more electrical characteristics of a sample in the gigahertz range and, by scanning the probe over the sample surface, to image the spatial variation of such characteristics. Such a microwave microscope would be very useful in the semiconductor industry for mapping resistivity and dielectric constant over the wafer, particularly during its fabrication since a microwave measurement can be non-destructive. In some instances, the thickness of a layer may be related to such electrical characteristics. The gigahertz measurement frequency corresponds to the important frequencies utilized in semiconductor devices. The probe of such a microwave microscope can also be used as a read head for nano-scale information storage on ferroelectric recording medium. 
     For integrated circuits, the imaging resolution must be on the order of less than a few microns since feature sizes are being pushed to much less. However, microwave wavelengths and waveguide dimensions are in the range of centimeters to millimeters, far greater than the desired resolution. 
     Several proposals have been made for microwave probes that have a spatial resolution much less than the wavelength of the radiation being used, using a technique called near-field. This technique allows spatial resolution less than the wavelength being used by scanning a probe very close to a sample. For example, Xiang et al. in U.S. Pat. No. 5,821,410 describe a sharpened probe tip extending through an aperture in a resonant quarter-wavelength cavity and projecting toward the sample under test. Anlage et al. in U.S. Pat. No. 5,900,618 disclose a somewhat similar microwave microscope. 
     Somewhat similar measurements can be made using a scanning capacitor measurement apparatus with a small tip electrode and the sample acting as the other electrode, such as disclosed by Williams et al. in U.S. Pat. No. 5,523,700, by Slinkman et al. in U.S. Pat. No. 5,065,103, and by Matey in U.S. Pat. No. 5,581,616 and reissued U.S. Pat. Re. 32,457. Calculations relate the measured capacitance some measurement parameters such as DC voltage with electrical characteristics of the material. This design is a non-resonant structure, thus can have a broad bandwidth of operation. The sense area of these designs however extends far from the probe electrode, and it is difficult to relate the measured impedance to the dielectric constant and resistivity of the material. 
     Kelly et al. in U.S. Pat. No. 6,825,645, incorporated herein by reference, discloses a microwave imager, which utilizes a non-resonant structure to gain a broad bandwidth of operation and further puts a grounded electrode next to the sensing electrode, which avoids the problem of a large sense area. 
     These proposals, whether using a resonant structure or a non-resonant structure, all depend upon a single electrode to stimulate the sample and to sense the electrical potential change on the sample surface. Thus, there is often a large reflected excitation signal on the electrode which has not interacted with the sample and which is larger than the sensed signal which has interacted. The reflected signal may exist exists even when no sample is present. This reflected signal is referred to as the common mode signal. In an attempt to detect a small signal emitted from a sample by amplifying the signal from the probe, the common mode signal can easily saturate a detector. A common mode cancellation circuitry can be used to cancel the common mode signal. However, such a circuitry is not always stable, and it adds another source of shot noise to the original shot noise in the common mode signal. 
     SUMMARY OF THE INVENTION 
     A microwave microscope may be scanned over a sample surface to image electromagnetic characteristics of the sample, thereby allowing sample characterization at a fixed frequency in the range of 100 MHz to 100 GHz, more preferably 500 MHz to 5 GHz, for example, 1 GHz. The microscope uses a probe and a circuitry. 
     The probe preferably includes a stimulating electrode to excite the sample, and a separate sensing electrode to sense the surface potential change. Preferably, the stimulating electrode surrounds the sensing electrode. The sensing electrode may have a sharpened tip to improve spatial resolution. The probe may also include an isolating electrode located between the stimulating electrode and the sensing electrodes. 
     Alternatively, such a probe may be used as a read head for nano-scale information storage on ferroelectrics recording medium. 
     The circuitry may include a detection circuit which compares the output signal from the sensing electrode with a reference signal from the microwave source to detect the in-phase and out-of-phase components of the output. 
     The probe may be formed by standard semiconductor techniques in a five layer structure deposited on a silicon chip. Three metal layers are separated by two dielectric layers. Microwave strip transmission lines may be formed in the top and bottom metal layers with the middle metal layer acting a ground. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a bottom plan view of an embodiment of the microwave probe of the invention. 
         FIG. 2  is a cross-sectional view of the probe tip of the probe of  FIG. 1  including the stimulating and sensing structures. 
         FIG. 3  is a schematic top view of the probe tip of  FIG. 2 . 
         FIG. 4  is a cross-sectional view of the probe tip of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of as stack structure from which the probe assembly may be fabricated. 
         FIG. 6  is a circuit diagram of an example of the circuitry which is connected to the electrical components of the microwave microscope to detect the orthogonal components of the sensed signal. 
         FIG. 7  is a schematic illustration of a microwave microscope system. 
         FIG. 8  is a schematic illustration of part of a read head system for reading a recorded disk. 
         FIG. 9  is plan view of the disk of  FIG. 8  formed with ferroelectric islands. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment of the invention includes a probe  10  illustrated in the cross-sectional view of  FIG. 1  and in the bottom plan view of  FIG. 2 , which is part of a probe assembly  12 . A probe tip  14  at the distal end of a cantilever  16  supported on a mount  18  is scanned adjacent to a sample  20  being electrically characterized. The probe tip  14  is shown in more detail in the top plan view of  FIG. 3  and the cross-sectional view of  FIG. 4 . 
     Although the more general aspects of the invention are not limited to its fabrication method, the probe  10  and the mount  18  may be formed from a silicon chip  22 , illustrated in the cross-sectional view of  FIG. 5  in an inverted orientation corresponding to its final usage. The fabrication steps follow those used in forming silicon integrated circuits and micro electromechanical system (MEMS) devices. The chip  22 , which may have a size of approximately 2 mm×4 mm, may be developed as an array of replicated chips  22  on a silicon wafer according to standard semiconductor processing techniques. A vertical structure is formed on the chip to include a top metal layer  24 , an upper dielectric layer  26 , a middle metal layer  28 , a lower dielectric layer  30 , and a bottom metal layer  32 . The top and bottom metal layers  24 ,  26  serve as transmission conductors and other purposes in opposition to the grounded middle metal layer  26  across the respective dielectric layers  26 ,  30 . The metal layers  24 ,  28 ,  32  may be sputtered aluminum or aluminum alloy. The dielectric layers  26 ,  30  may be silicon dioxide or related silica compound deposited by chemical vapor deposition (CVD). Typical thickness of all layers  24 ,  26 ,  28 ,  30 ,  32  are between 2 and 5 μm although other thicknesses may be chosen. The dielectric thickness should be thick enough to electrically isolate the metal layers at microwave frequencies and to form a strip transmission line capacitively coupled between adjacent metal layers. 
     The chip  20  is lithographically etched from its illustrated bottom side to form an electrode via to the upper metal layer  24  and an inner electrode  40  is deposited within the via with dielectric isolation to the middle and bottom metal layers  28 ,  32  but with electrical contact to the upper metal layer  24 . A tip  42  of the inner electrode  40  may be defined and sharpened by focused ion beam (FIB) milling to a radius of less than 10 μm, for example in a range of 5 to 500 nm, preferably less than 100 nm. Under the proper operating conditions, the size of the electrode tip  42  determines the resolution of the microwave microscope. The resolution is not limited by the much larger microwave wavelength. The resolution is improved by extending the electrode tip  42  to the level of the surrounding excitation electrode or beyond. 
     At the same time that the electrode via is being etched, two cross-connect vias  46  and an excitation cross-connect via  48  are etched from the bottom metal layer  32  to the top metal layer  24 . Also, when the electrode  40  is being deposited, the cross-connects vias  46 ,  48  are filled with metal with adequate sidewall isolation to the middle metal layer  28  to electrically connect the top and bottom metal layers  24 ,  32  in the respective areas. The sidewall isolation for the inner electrode  40  and the three cross-connect vias  46 ,  48  can be achieved by patterning large vias in the middle metal layer  28  immediately following its deposition. 
     The bottom side (top as illustrated) of the chip  22  is lithographically processed to remove most of the bottom metal layer  32 . The lithography leaves the cross-connect vias  46 ,  48  and their metallizations and also leaves under the eventual support  18  an instrumentation excitation strip line  50 , an instrumentation sensing strip line  52 , and associated bonding pads  54 ,  56  for contacting to the electronics. The instrumentation excitation and sensing strip lines  50 ,  52 , which are part of strip transmission lines, should be widely separated to reduce cross talk. The widths of the strip lines  50 ,  52  may be in the range to 10 to 20 μm for the stated thicknesses of the dielectric layers  26 ,  30  to act as strip transmission lines with a characteristic impedance of about 50 ohms at microwave frequencies. The bottom metal lithography further leaves an annular excitation electrode  60  surrounding the inner electrode  40  but separated from it and an interconnect  62  to the excitation cross-connect via  48 . The annular electrode  60  may have an outer diameter of between 0.1 and 40 μm and an inner diameter of about half of the out diameter. As a result, the gap between the two electrodes  40 ,  60  is substantially less than the microwave wavelength by at least a factor of ten. However, it is not required that the excitation electrode  60  be an annulus completely and continuously surrounding the sensing electrode  40 . Generally, however, improvement is improved if the excitation electrode  60  exists fully or partially in all four quadrants surrounding the sensing electrode  40   
     The bottom side as illustrated of the chip  22  is further lithographically processed to remove most of the lower dielectric layer  26  on the cantilever  46  between probe tip  14  and the cross-connect vias  48  and the support  18 . However, portions underlying the annular excitation electrode  60  and its interconnect  62  are not removed. A thin dielectric pad  64  may be deposited over and around the tip  42  of the sensing electrode  40  to protect it during usage. 
     The back side of the chip  22  is lithographically etched down to the top metal layer to remove the chip  22  away from the support  18 . The resulting cantilever  16  may have a width of between 40 and 200 μm and a length of between 100 and 500 μm. The removal of the chip  22  over the cantilever  16  causes the cantilever  16  to bend as illustrated if the top and middle metal layers  24 ,  28  have been deposited with different degrees of stress, for example, the top metal layer  24  has more tensile stress than the middle metal layer  28 . This allows the probe tip  10  supported on the inclined support to have a support area substantially horizontal to the surface being scanned. The removal of most of the bottom metal layer  32  and the lower dielectric layer  30  further simplifies the interface between the probe tip  10  and the sample  20 . 
     The top metal layer  24  is lithographically etched to define a probe excitation strip line  66  and a probe sensing strip line  68  to connect respectively the sensing cross connect  48  with the sense electrode  42  and the annular excitation electrode  60  with the excitation cross connect  46 . The top metal lithography also develops an isolation region  70  between probe strip lines  50 ,  52  and the upper metal layer  24  beneath the support. The probe strip lines  66 ,  68  act as signal lines of strip transmission lines similarly to the instrumentation strip lines  50 ,  52 . The top metal layer lithography may be performed immediately after the deposition of the top metal layer  24  onto the silicon chip. The probe excitation and sensing strip lines  66 ,  68  should be widely separated to reduce cross talk. 
     The resulting probe assembly  10  has the two strip transmission lines  54 ,  56  formed in the bottom metal layer  32  on the bottom of the support  18  connected to the two strip transmission lines  66 ,  68  formed in the top metal layer  24  in the cantilever  16 , which in turn are connected to the inner and surrounding annular electrodes  40 ,  60  at the bottom of end of the cantilever  16 . 
     The probe assembly  12  is electronically connected via the pads  54 ,  56  to a signal electronics system  80  illustrated in the circuit diagram of  FIG. 6 . A frequency generator  80  operates in the microwave band, for example, between 100 MHz to 100 GHz although 500 MHz to 5 GHz is a preferred range for simple applications. A frequency of 1 GHz will be used as an example. A microwave amplifier  84  amplifies the output of the frequency generator  82  and applies the signal to a power splitter  86 , which divides the microwave power to two and possibly three outputs. One portion, of example, having a power of 10 dBm is directed as an excitation signal to the probe assembly  12 . It passes in the reverse direction through a directional coupler  88  to be discussed later and thence to a matching network  90 , which converts the characteristic impedance in the signal electronics system  80  to that of the strip transmission lines on the probe  12 . An output  92  of the matching circuit  90  is connected to the bonding pad  54  of the instrumentation excitation strip line  50 , which eventually connects to the annular excitation electrode  60  of the probe tip  10  to thereby excite the sample  20 . A ground line  94  is connected between the ground of the instrumentation and the middle metal layer  28  of the probe assembly  12   
     An input  94  of the matching network  90  is connected to the bonding pad  56  of the instrumentation excitation strip line  52 , which is eventually connected to the sharpened tip  42  of the center electrode  40  to thus provide a sensed signal from the sample  20 . The sensed signal may have a power level in the neighborhood of −40 dBm. The sensed signal passes through another directional coupler  96  to be discussed later and a microwave amplifier amplifies the sensed signal to a level closer to that of the excitation signal, for example, by +56 dB. 
     It is possible to reverse the usage of the electrodes, that is, the center electrode  40  as the excitation electrode and the annular electrode  60  as the sense electrode. While the two modes may be considered similar, it is believed that using the sharpened inner electrode  40  as the sense electrode reduces noise. 
     A second output  102  of the power splitter  86  is used as a reference signal in a quadrature detection circuitry which allows phase-sensitive detection of the signal from the sensing electrode with respect to an unmodulated microwave signal from the frequency generator  82 . A variable phase shifter  104  adjusts the phase of the reference signal to match that of the sensed signal to account for all the phase delays introduced in the signaling and probe circuitry. A quadrature mixer  106  receives both the sensed signal from the amplifier  98  and the reference signal from the phase shifter  106 . The mixer  106  non-linearly mixes or multiplies the sensed and reference signals to provide two essentially DC or low-frequency quadrature signals on lines  108 ,  109  indicative of the real and imaginary parts (in phase and 90° out of phase) of the sensed signal as it varies during the probe scan. The two signal amplitudes are also called orthogonal components of a sinusoidal signal. The quadrature mixer  106  is well known and typically includes two 90° hybrids which split respective ones of the sensed and reference signals into two portions and introduces a 90° phase shift into one of the split portions. Two mixers receive respective pairs of the unshifted and phase-shifted portions to produce the two quadrature signals. Two amplifiers  110 ,  112  amplify the quadrature outputs of the quadrature mixer  106 , for example, by +62 dB, to produce on two output lines  114 ,  116  two signals ε′ and ε″ representative of two properties of the sample which are related as real and imaginary parts of a reflected signals produced by a complex excitation signal, for example, dielectric constant and resistivity although the invention is not limited to these two electrical characteristics. Two feedback circuits  118 ,  120  of parallel capacitive and resistive elements around the two amplifiers  110 ,  112  control the gain and the frequency response of the amplifiers  110 ,  112  and prevent them from self-oscillating at high frequency to set the bandwidth and signal level of the two outputs. 
     The quadrature detection circuitry is very powerful but the invention may utilize other detection circuitry. Amplitude and phase of the sensed signal provide equivalent information and sometimes the amplitude at a given phase is all that is required. In some situations, only the magnitude is needed, that is, the square root of the sums of the squares of ε′ and ε″, which do not need to be separately determined. In other situations, only one or the other of the quadrature outputs is required, for example, when only dielectric constant or resistivity or layer thickness is being measured. 
     Microwave microscopes typically suffer from a large difference in the power levels of the excitation and sensed signals, for example, 50 dB in the described embodiment. The large difference introduces a significant common mode problem in which the excitation signal overwhelms the sensed signal. Although the divided excitation and sense electrodes of one aspect of the invention significantly reduces the common mode interference, large portions of the probe  12  and the circuitry  80  have neighboring elements carrying excitation and sensed signals of vastly different magnitudes. Some cross talk and interference seems inevitable. Hence, it is desired to provide some common mode cancellation. A common mode cancellation circuit  130  receives a third output  132  of the power splitter  86  as a common mode reference signal. A first 90° hybrid  134  divides the common mode reference signal into two portions and introduces a 90° phase shift into one of the split portions. The split portions pass through respective voltage controlled attenuators  136 ,  138  before being recombined in a second 90° hybrid  140 , which combines the two split portions with the opposite 90° phase shift. As a result, the two phases of the common mode reference signal are separately and selectively attenuated. The second directional coupler  96  combines the so selectively attenuated common mode reference signal with the unamplified sensed signal from the probe  10 . 
     An intent is that the selectively attenuated common mode reference signal cancels any common mode signal in the sensed signal. Such a result can be accomplished by additional circuitry including two switches  142 ,  143  connected between the output lines  114 ,  116 , and two sample and hold (S/H) amplifiers  146 ,  148  having outputs controlling the two voltage control attenuators  136   138  and having associated feedback capacitive elements  150 ,  152  to limit oscillations. During set up, the probe tip  14  is positioned at a reference position relatively far away from the sample  20  and insensitive to the local variations in the sample  20 . The switches  142 ,  143  are closed to set up a feedback loop which operates to adjust the voltage controlled attenuators  136 ,  138  and the resultant fed back signal acts to cancel any sensed signal during the set up phase. After an adequate period for equilibration, the sample-and-hold amplifiers  146 ,  148  are locked and the switches  142 ,  143  are opened to put the detection circuitry into an open loop which detects only subsequent changes in the sensed signal during the scan mode. Thereby, the common mode cancellation may be used to prevent the amplifier  98  and mixer  106  from being saturated. 
     The use of separate excitation and sense electrodes and the addition of the common mode cancellation circuitry  130  greatly reduces the maximum signal levels in the mixer  106  and its pre-amplifier  98  so that more gain can be used in the pre-amplfier  98 , reducing mixer noise in the detected signal. 
     The directional coupler  88  on the input to the probe provides at least two advantages. As the probe  12  is scanned over the sample, the sample  20  reflects back a significant signal back to the exciting electrode  60  and through the power splitter  86  to the reference and common mode cancellation portions of the detection circuitry. The directional coupler  88  instead diverts the reflected excitation signal away from the power splitter  86 . Further, the reflected signal can be used as a measure of the height of the probe tip  14  from the surface of the sample  10 . For example, when the probe tip  10  is being lowered toward the sample  20  prior to lateral scanning, the reflected and diverted signal indicates when a desired height has been reached. The nulling of the common mode cancellation circuit  130  may be performed when the desired height is attained. Alternatively, the reflected signal can be used as a servo signal to maintain the probe tip  10  at a moderately fixed height above an undulating surface of the sample  20  being laterally scanned. 
     A microscope system  140  schematically illustrated in  FIG. 7  includes an XYZ mechanical scanning stage  142  which is fixed to the support  18  and under directions from a position controller  144  determines the position and movement of the probe tip  14  adjacent the test sample  20 . The z-motion is used to approach the probe tip  10  to the sample  20  and the x- and y-motions are used to scan the probe tip  10  over the surface of the sample  20 . The probe tip may contact the sample  20  if the sharpened tip  42  is protected. However, the probe tip may be operated close to but separated from the sample  20 . Alternatively to the XYZ mechanical scanning stage  142 , one of more degrees of translation may be incorporated into a movable stage supporting the sample  20 . The excitation and sense transmission lines connect the excitation and sensing electrodes  42 ,  58  in the probe  10  to the signal electronics system  80 . A signal processor  146  samples the quadrature output signals from the signal electronics system  80  and may calculate one or more electrical characterizations, such as resistivity and dielectric constant from the quadrature output signals. One- and two-dimensional images of these quantities may be obtained as the probe  10  is scanned over the surface of the sample  20 . 
     The described microwave microscope system has the ability to be operated according to different modes of speed and resolution. For example, the probe tip  10  can be positioned a substantial distance above the sample  20  and scanned at a relatively high rate. The spatial resolution is determined by the probe height and may be considerably less than the achievable resolution. However, in this mode the probe  12  may be scanned at a relatively high speed commensurate with the reduced resolution. That is, a first scan is both rough and fast, and may be used for an initial inspection of the sample, for example, determining chip boundaries, substrate, typography, or gross imperfections. Thereafter, the probe tip  10  may be lowered to be closer to the surface of the sample  20  at a position of interest determined by the first scan. At the lower position, the resolution of the microscope is increased, possibly to the degree determined by the size of the tip  42  of the sensing electrode  40 . However, in the latter mode, scanning is done at a slower rate though with higher resolution, that is, the scan is both fine and slow. 
     Many features of the microwave microscope are described in more detail by one of the inventors Zhengyu Wang in  Evanescent Microwave Probe: Applications and Implications , Stanford University PhD thesis 3781 2004, [Jun. 2] 2004. 
     Another embodiment of the invention includes a microwave disk reading system  150 , schematically illustrated in  FIG. 8 . A disk  152  having data recorded in a spiral track is loaded onto a platen  154  that rotates about a center of the disk  152  at a predetermined, typically constant, rotation rate. A YZ positioner  156  moves the probe tip  10  in the z-direction to be adjacent to the disk  154 , at which point the probe tip  10  flies along the tracks in the local x-direction, as is well known for optical and magnetic recorded disks. The y-motion in a direction transverse to the tracks is used to keep the probe tip  10  on the spiral track or to move the probe tip  10  to a different band of the disk  152 . The probe  10  includes separate excitation and sensing electrodes. The electronics may be adapted from the signal electronics system  80  but may be simpler since orthogonal signals are not always necessary for detecting the data pattern. The reflected excitation signal from the input directional coupler  88  or the orthogonal signals ε′ and ε″ may be used in part to control the YZ positioner  156  to maintain the probe tip  10  in the middle of the recorded track. 
     The disk may be recorded with information that is readable by the microwave probe and electronic control system. For example, a conventional optical CD has a track that is burned into a dielectric layer. The burned area is either free space or filled with another material having a different dielectric constant. Hence, the microwave sensor system  150  of  FIG. 8  can detect the difference in dielectric constant along the track. As a result, the disk  152  becomes the sample  20  of the previously described microscope embodiments. 
     However, recording density can be greatly increased by forming, as illustrated in the plan view of  FIG. 9 , islands  160  of distinctive material at the top surface of the disk  152 , which are arranged in a spiral track  162  in a pattern corresponding to recorded data. The islands  160  may be separated in a return-to-zero (RZ) data arrangement or contiguous islands  160  may joined into elongated islands in a non-RZ (NRZ) arrangement. The islands  160  may be formed of ferroelectric material having a high dielectric constant at microwave frequencies, such as lead zirconium titanate (PZT), over a polymeric disk exhibiting a much lower dielectric constant or even over a metal covered disk. The microwave microscope can detect with high resolution corresponding to the sensing electrode radius the areas of high dielectric constant over a base surface of lower dielectric constant or over a metal and non-dielectric surface. Such islands  160  may be formed by causing a localized phase change in a ferroelectrical layer using a thermal or electrical stimulus, thereby altering the dielectric constant in the stimulated area. Alternatively, the islands  160  may be formed of metal such as aluminum or chromium deposited on a dielectric layer such as polymer. 
     The microwave microscope and the associated probe have several advantages over prior art microwave microscopes. The stimulating and the sensing electrode are two separated electrodes, which prevents a large common mode signal from being collected and possibly saturate the detector. By reducing the common mode signal, the amplifying capability of the amplifiers can be more fully utilized. By using a probe with inherent common mode cancellation instead of a common mode cancellation circuitry, shot noise can be largely avoided. The MEMS fabrication provides fine resolution in a low-cost probe.