Orthogonal microwave imaging probe

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.

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.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

An embodiment of the invention includes a probe10illustrated in the cross-sectional view ofFIG. 1and in the bottom plan view ofFIG. 2, which is part of a probe assembly12. A probe tip14at the distal end of a cantilever16supported on a mount18is scanned adjacent to a sample20being electrically characterized. The probe tip14is shown in more detail in the top plan view ofFIG. 3and the cross-sectional view ofFIG. 4.

Although the more general aspects of the invention are not limited to its fabrication method, the probe10and the mount18may be formed from a silicon chip22, illustrated in the cross-sectional view ofFIG. 5in 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 chip22, which may have a size of approximately 2 mm×4 mm, may be developed as an array of replicated chips22on a silicon wafer according to standard semiconductor processing techniques. A vertical structure is formed on the chip to include a top metal layer24, an upper dielectric layer26, a middle metal layer28, a lower dielectric layer30, and a bottom metal layer32. The top and bottom metal layers24,26serve as transmission conductors and other purposes in opposition to the grounded middle metal layer26across the respective dielectric layers26,30. The metal layers24,28,32may be sputtered aluminum or aluminum alloy. The dielectric layers26,30may be silicon dioxide or related silica compound deposited by chemical vapor deposition (CVD). Typical thickness of all layers24,26,28,30,32are 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 chip20is lithographically etched from its illustrated bottom side to form an electrode via to the upper metal layer24and an inner electrode40is deposited within the via with dielectric isolation to the middle and bottom metal layers28,32but with electrical contact to the upper metal layer24. A tip42of the inner electrode40may 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 tip42determines 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 tip42to the level of the surrounding excitation electrode or beyond.

At the same time that the electrode via is being etched, two cross-connect vias46and an excitation cross-connect via48are etched from the bottom metal layer32to the top metal layer24. Also, when the electrode40is being deposited, the cross-connects vias46,48are filled with metal with adequate sidewall isolation to the middle metal layer28to electrically connect the top and bottom metal layers24,32in the respective areas. The sidewall isolation for the inner electrode40and the three cross-connect vias46,48can be achieved by patterning large vias in the middle metal layer28immediately following its deposition.

The bottom side (top as illustrated) of the chip22is lithographically processed to remove most of the bottom metal layer32. The lithography leaves the cross-connect vias46,48and their metallizations and also leaves under the eventual support18an instrumentation excitation strip line50, an instrumentation sensing strip line52, and associated bonding pads54,56for contacting to the electronics. The instrumentation excitation and sensing strip lines50,52, which are part of strip transmission lines, should be widely separated to reduce cross talk. The widths of the strip lines50,52may be in the range to 10 to 20 μm for the stated thicknesses of the dielectric layers26,30to 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 electrode60surrounding the inner electrode40but separated from it and an interconnect62to the excitation cross-connect via48. The annular electrode60may 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 electrodes40,60is substantially less than the microwave wavelength by at least a factor of ten. However, it is not required that the excitation electrode60be an annulus completely and continuously surrounding the sensing electrode40. Generally, however, improvement is improved if the excitation electrode60exists fully or partially in all four quadrants surrounding the sensing electrode40

The bottom side as illustrated of the chip22is further lithographically processed to remove most of the lower dielectric layer26on the cantilever46between probe tip14and the cross-connect vias48and the support18. However, portions underlying the annular excitation electrode60and its interconnect62are not removed. A thin dielectric pad64may be deposited over and around the tip42of the sensing electrode40to protect it during usage.

The back side of the chip22is lithographically etched down to the top metal layer to remove the chip22away from the support18. The resulting cantilever16may have a width of between 40 and 200 μm and a length of between 100 and 500 μm. The removal of the chip22over the cantilever16causes the cantilever16to bend as illustrated if the top and middle metal layers24,28have been deposited with different degrees of stress, for example, the top metal layer24has more tensile stress than the middle metal layer28. This allows the probe tip10supported on the inclined support to have a support area substantially horizontal to the surface being scanned. The removal of most of the bottom metal layer32and the lower dielectric layer30further simplifies the interface between the probe tip10and the sample20.

The top metal layer24is lithographically etched to define a probe excitation strip line66and a probe sensing strip line68to connect respectively the sensing cross connect48with the sense electrode42and the annular excitation electrode60with the excitation cross connect46. The top metal lithography also develops an isolation region70between probe strip lines50,52and the upper metal layer24beneath the support. The probe strip lines66,68act as signal lines of strip transmission lines similarly to the instrumentation strip lines50,52. The top metal layer lithography may be performed immediately after the deposition of the top metal layer24onto the silicon chip. The probe excitation and sensing strip lines66,68should be widely separated to reduce cross talk.

The resulting probe assembly10has the two strip transmission lines54,56formed in the bottom metal layer32on the bottom of the support18connected to the two strip transmission lines66,68formed in the top metal layer24in the cantilever16, which in turn are connected to the inner and surrounding annular electrodes40,60at the bottom of end of the cantilever16.

The probe assembly12is electronically connected via the pads54,56to a signal electronics system80illustrated in the circuit diagram ofFIG. 6. A frequency generator80operates 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 amplifier84amplifies the output of the frequency generator82and applies the signal to a power splitter86, 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 assembly12. It passes in the reverse direction through a directional coupler88to be discussed later and thence to a matching network90, which converts the characteristic impedance in the signal electronics system80to that of the strip transmission lines on the probe12. An output92of the matching circuit90is connected to the bonding pad54of the instrumentation excitation strip line50, which eventually connects to the annular excitation electrode60of the probe tip10to thereby excite the sample20. A ground line94is connected between the ground of the instrumentation and the middle metal layer28of the probe assembly12

An input94of the matching network90is connected to the bonding pad56of the instrumentation excitation strip line52, which is eventually connected to the sharpened tip42of the center electrode40to thus provide a sensed signal from the sample20. The sensed signal may have a power level in the neighborhood of −40 dBm. The sensed signal passes through another directional coupler96to 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 electrode40as the excitation electrode and the annular electrode60as the sense electrode. While the two modes may be considered similar, it is believed that using the sharpened inner electrode40as the sense electrode reduces noise.

A second output102of the power splitter86is 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 generator82. A variable phase shifter104adjusts 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 mixer106receives both the sensed signal from the amplifier98and the reference signal from the phase shifter106. The mixer106non-linearly mixes or multiplies the sensed and reference signals to provide two essentially DC or low-frequency quadrature signals on lines108,109indicative 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 mixer106is 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 amplifiers110,112amplify the quadrature outputs of the quadrature mixer106, for example, by +62 dB, to produce on two output lines114,116two 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 circuits118,120of parallel capacitive and resistive elements around the two amplifiers110,112control the gain and the frequency response of the amplifiers110,112and 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 probe12and the circuitry80have 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 circuit130receives a third output132of the power splitter86as a common mode reference signal. A first 90° hybrid134divides 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 attenuators136,138before being recombined in a second 90° hybrid140, 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 coupler96combines the so selectively attenuated common mode reference signal with the unamplified sensed signal from the probe10.

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 switches142,143connected between the output lines114,116, and two sample and hold (S/H) amplifiers146,148having outputs controlling the two voltage control attenuators136138and having associated feedback capacitive elements150,152to limit oscillations. During set up, the probe tip14is positioned at a reference position relatively far away from the sample20and insensitive to the local variations in the sample20. The switches142,143are closed to set up a feedback loop which operates to adjust the voltage controlled attenuators136,138and 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 amplifiers146,148are locked and the switches142,143are 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 amplifier98and mixer106from being saturated.

The use of separate excitation and sense electrodes and the addition of the common mode cancellation circuitry130greatly reduces the maximum signal levels in the mixer106and its pre-amplifier98so that more gain can be used in the pre-amplfier98, reducing mixer noise in the detected signal.

The directional coupler88on the input to the probe provides at least two advantages. As the probe12is scanned over the sample, the sample20reflects back a significant signal back to the exciting electrode60and through the power splitter86to the reference and common mode cancellation portions of the detection circuitry. The directional coupler88instead diverts the reflected excitation signal away from the power splitter86. Further, the reflected signal can be used as a measure of the height of the probe tip14from the surface of the sample10. For example, when the probe tip10is being lowered toward the sample20prior to lateral scanning, the reflected and diverted signal indicates when a desired height has been reached. The nulling of the common mode cancellation circuit130may be performed when the desired height is attained. Alternatively, the reflected signal can be used as a servo signal to maintain the probe tip10at a moderately fixed height above an undulating surface of the sample20being laterally scanned.

A microscope system140schematically illustrated inFIG. 7includes an XYZ mechanical scanning stage142which is fixed to the support18and under directions from a position controller144determines the position and movement of the probe tip14adjacent the test sample20. The z-motion is used to approach the probe tip10to the sample20and the x- and y-motions are used to scan the probe tip10over the surface of the sample20. The probe tip may contact the sample20if the sharpened tip42is protected. However, the probe tip may be operated close to but separated from the sample20. Alternatively to the XYZ mechanical scanning stage142, one of more degrees of translation may be incorporated into a movable stage supporting the sample20. The excitation and sense transmission lines connect the excitation and sensing electrodes42,58in the probe10to the signal electronics system80. A signal processor146samples the quadrature output signals from the signal electronics system80and 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 probe10is scanned over the surface of the sample20.

The described microwave microscope system has the ability to be operated according to different modes of speed and resolution. For example, the probe tip10can be positioned a substantial distance above the sample20and 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 probe12may 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 tip10may be lowered to be closer to the surface of the sample20at 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 tip42of the sensing electrode40. 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 inEvanescent Microwave Probe: Applications and Implications, Stanford University PhD thesis 3781 2004, [Jun. 2] 2004.

Another embodiment of the invention includes a microwave disk reading system150, schematically illustrated inFIG. 8. A disk152having data recorded in a spiral track is loaded onto a platen154that rotates about a center of the disk152at a predetermined, typically constant, rotation rate. A YZ positioner156moves the probe tip10in the z-direction to be adjacent to the disk154, at which point the probe tip10flies 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 tip10on the spiral track or to move the probe tip10to a different band of the disk152. The probe10includes separate excitation and sensing electrodes. The electronics may be adapted from the signal electronics system80but may be simpler since orthogonal signals are not always necessary for detecting the data pattern. The reflected excitation signal from the input directional coupler88or the orthogonal signals ε′ and ε″ may be used in part to control the YZ positioner156to maintain the probe tip10in 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 system150ofFIG. 8can detect the difference in dielectric constant along the track. As a result, the disk152becomes the sample20of the previously described microscope embodiments.

However, recording density can be greatly increased by forming, as illustrated in the plan view ofFIG. 9, islands160of distinctive material at the top surface of the disk152, which are arranged in a spiral track162in a pattern corresponding to recorded data. The islands160may be separated in a return-to-zero (RZ) data arrangement or contiguous islands160may joined into elongated islands in a non-RZ (NRZ) arrangement. The islands160may 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 islands160may 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 islands160may 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.