WO1998057119A1 - Improved stylus profiler and array - Google Patents

Abstract

In an optical lever sensor embodiment (140) a laser (142) reflects off a stylus arm (102) of a sensor assembly (100) having a tip (104) which has its position and displacement detected and measured with a beam reflected to a rotatable mirror/position transducer (144). The beam is further reflected onto PSD, i.e. position sensitive detector (146). The output of PSD (146) is fed into a controller (150) connected to CPU (152). Controller (150) is further connected to feed back a drive voltage to rotatable mirror/position transducer (144) in response to output therefrom.

Description

IMPROVED STYLUS PROFILER AND ARRAY

RACTΠROTTNTO OF THF, TNVK TTON This invention relates in general to instruments for scanning samples or specimens and in particular to a profiler with improved characteristics.

Profiling instruments were first developed for the purpose of characterizing surfaces in terms of roughness, waviness and form. In recent years, they have been refined for making precise measurements on semiconductor materials and devices. Profiling instruments are also used outside the semiconductor industry, for example, for scanning and sensing optical discs, flat panel displays and other devices. Stylus profilometers or profilers for use in the above- mentioned applications have been available from Tencor Instruments of Milpitas, California and other manufacturers .

As the semiconductor industry progresses to smaller feature geometries with each new generation of products, there has been an increasing need for scanning instruments that can repeatedly scan samples to a very fine resolution. Hence, profilers are increasingly used for the rapid, non-destructive measurement of features with submicrometer scale lateral and nanometer scale vertical dimensions. Several important measurement applications require the measurement of features whose lateral dimensions are on the order of 0.1 μm. These measurements require a lateral resolution of 0.01 μm. A very sharp stylus must be used to make this type of measurement. At the same time, the wafers and individual die on which these features reside continue to increase in size. Certain measurement applications require that a measurement must be performed over a large percentage of the wafer diameter. Consequently, a profiler must be able to make measurements of spatial features varying from 0.01 μm to 0.1 m - a lateral dynamic range of 10⁷. This requirement places severe constraints on the scanner, stylus assembly and transducer used in advanced profilers. Maintaining a very sharp stylus requires that one profile at very low tip-surface interaction forces (less than about 0.1 mg) . Maintaining low interaction forces is facilitated through the use of soft (low stiffness) flexures in the stylus assembly. The effective mass, stiffness and damping of the stylus assembly influence the dynamic forces realized during profiling. The dynamic forces can become problematic when the profiler is operated at fast scan speeds and small interaction force. These forces can limit the throughput of a profiler. It is therefore desirable to provide stylus profilometers or profilers where the above-described problems are avoided or are alleviated.

SUMMARY OF THF, INVENTION One aspect of the invention is based on the observation that low interaction forces (e.g. contact forces) and high throughput can be achieved by the use of a small stylus assembly. If this small stylus assembly also has low-mass, high resonance frequency and is moderately damped, throughput of the system can be increased even at small interaction forces. Low-mass stylus assemblies reduce the inertia loading of the tip on the surface relative to more massive assemblies. Low-mass stylus assemblies also enable faster control systems to be used for maintaining a constant tip- surface interaction force. A low-mass stylus probe assembly can also be moved by an XY stage at a higher speed without phase lag. All three of these characteristics are beneficial for increasing scan speed.

Thus, one aspect of the invention is directed towards a profiler for inspecting microstructures , comprising an elongated stylus arm having a notch therein, said notch defining a flexure pivot, said arm having a stylus tip for interacting with a specimen by an interaction force; means for causing relative motion between the tip and the specimen so that the tip measures a specimen, wherein the interaction force between the tip and the specimen causes the tip to rotate about the notch; and stylus displacement measuring means to provide a position signal to indicate position of the stylus tip relative to the specimen when the tip moves over the specimen.

Another aspect of the invention is directed towards a method for controlling stylus force in a profiler, the method comprising the step of providing an elongated stylus arm having a notch therein, said arm having a stylus tip for interacting with a specimen by an interaction force, and causing relative motion between the tip and the specimen so that the interaction force causes the tip to rotate about the notch. The method further includes measuring the specimen.

In the profiler currently marketed by Tencor

Instruments, the displacement of the stylus arm is measured using a capacitance gauge. Another aspect of the invention is based on the observation that, for some applications, it may be desirable to use an interferometer or optical lever sensor to measure the displacement of the profiler stylus arm instead of the capacitance gauge. Thus, another aspect of the invention is directed towards a profiler for inspecting microstructures, comprising an elongated stylus arm having a stylus tip for interacting with a specimen by an interaction force, said arm having a pivot, means for causing relative motion between the tip and the specimen so that the tip rotates about the pivot and measures the specimen; and stylus displacement measurement means including an interferometer or optical lever sensor to provide a position signal to indicate position of the stylus tip relative to the specimen when the tip and the specimen move relative to one another.

One more aspect of the invention is directed to a method for controlling stylus force in a profiler, the method comprising the steps of: providing an elongated stylus arm having a stylus tip for interacting with a specimen by an interaction force, said arm having a pivot, and causing relative motion between the tip and the specimen so that the interaction force causes the tip to rotate about the pivot to measure the specimen; and providing a position signal to indicate position information of the stylus tip relative to the specimen when the tip and the specimen move relative to one another by means of an interferometer or optical lever sensor.

Yet another aspect of the invention is directed towards a profiling apparatus for inspecting microstructures, comprising at least two profilers; and means for causing relative motion between the specimen and the at least two profilers.

One more aspect of the invention is directed towards a displacement sensing apparatus, comprising means for supplying two orthogonally polarized beams and means for introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams. The apparatus further comprises means for directing said two orthogonally polarized beams towards a sample after the first phase difference has been introduced so that the beams are modified (e.g. reflected) by the sample and means for monitoring a second phase difference between the two modified beams. Yet another aspect of the invention is directed towards a displacement sensing method, comprising the steps of supplying two orthogonally polarized beams and introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams. The method further comprises directing the two orthogonally polarized beams towards a sample after said first phase difference has been introduced so that the beams are modified by the sample and monitoring a second phase difference between the two modified beams. BRIEF DESCRIPTION OF THF πRAWTNOS

Fig. 1A is a side perspective view of a sensor assembly employing a magnetic means for causing a stylus tip to apply a desired force to a sample taken from the parent application useful for illustrating the invention.

Fig. IB is a cross-sectional view of a portion of the sensor assembly of Fig. 1A.

Fig. 1C is an end perspective view showing details of the magnetic stylus force biasing means of the sensor assembly of Fig. 1A.

Fig. ID is a block diagram of the electronics for a stylus force adjustment useful for illustrating the invention. Fig. 2 is a top view of a micromachined stylus arm, pivot and supporting frame that illustrate a small stylus assembly that can be operated at a low interaction force to illustrate an aspect of the invention. Fig. 3 is a perspective view of a differential interferometer and a portion of the stylus arm and pivot of Fig. 2, illustrating how a differential interferometer may be used to measure the displacement of a stylus arm of a profiler to illustrate an embodiment of the invention.

Fig. 4A is a schematic diagram of a portion of a differential interferometer and a portion of a stylus arm of a profiler of the type in Figs. 1A-1C to illustrate another aspect of the invention. Fig. 4B is a perspective view of a calcite beam displacer to illustrate another embodiment of an interferometer that may be used for detecting the position and displacement of a stylus arm in a profiler.

Fig. 5 is a schematic diagram of an optical lever sensor and a portion of the stylus arm assembly of Fig.

2 to illustrate how an optical lever sensor may be used to detect the position and displacement of a stylus arm of a profiler to illustrate another aspect of the invention.

Fig. 6 is a schematic view of a differential interference contrast (DIC) displacement interferometer sensor sensing a displacement of the stylus arm of a profiler to illustrate the preferred embodiment of the invention.

Fig. 7 is a top view of an array of notch flexure profiler stylus arms to illustrate an embodiment of the invention.

Fig. 8 is a side view of the stylus arms of Fig. 7.

Fig. 9 is a schematic diagram of a profiler assembly employing the array of stylus arms of a profiler in Figs. 7 and 8 for scanning and measuring a surface .

Fig. 10 is a schematic diagram of a portion of an interferometer and a portion of one of the stylus arms of Figs. 7 and 8 to illustrate the use of an interferometer to measure the position and displacement of the stylus arm to illustrate an embodiment of the invention.

Fig. 11 is a block diagram of a profiler system for moving and calibrating the array of notch flexure profiler stylus arms of Figs. 7 and 8.

Fig. 12 is a top view of the stylus arms of Figs. 7 and 8 and a specimen to illustrate how the arms may be used to scan and measure the specimen.

Fig. 13 is a graphical plot of the profile of the surface of the specimen along three different paths of the specimen surface as measured by the three stylus arms of Fig. 12.

Fig. 14 is a block diagram of a feedback control system for controlling an interaction force exerted between the tip of a stylus of a profiler and a specimen when the tip is used to measure the specimen. Figs. 15A-15D are cross-sectional views of portions of a stylus arm with a notch to show notches of four different cross-sections for illustrating the invention.

Figs. 16A and 16B are respectively the bottom and side views of a stylus arm having a substantially rectangular shape in the bottom view to illustrate the invention.

Figs. 17A and 17B are respectively the bottom and side views of a stylus arm with a substantially triangular shape m the bottom view to illustrate a preferred embodiment of the invention.

For simplicity m description, identical components are identified by the same numerals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT The following description of a profiler m reference to Figs. 1A-1D is taken from the parent application.

With reference to Fig. 1A, a diamond stylus tip 11 having a radius of 0.01 mm or less is adhered to an end of a slender stainless steel wire 13 which is bent at a right angle. The wire radius is about 0.25 mm. The diamond tip is adhesively mounted to a squared-off end of the wire 13 , while the opposite end of the wire 13 is inserted into an elongated hollow aluminum arm 15 which has a length of approximately 2 cm and a wall inside radius of approximately 0.018 cm. The aluminum arm is sufficiently rigid that it will not bend when sensing step heights, yet sufficiently low mass that its moment of inertia can be kept low. The overall mass of the arm, wire and diamond tip should preferably not exceed approximately 0.05 grams. Arm 15 fits into a support block 19 and is operably connected to flexural pivot 21, which also fits into support block 19. In this manner, the aluminum arm 15 has a center of rotation about the flexural pivot 21. The flexural pivot 21 has enough torsion to lightly hold the stylus tip 11 downwardly against a surface to be measured, such as specimen or sample 10. The entire mass on the stylus side of the pivot should preferably not exceed 0.50 grams, including a lever 59 described below.

An electrical solenoidal coil 51 is comprised of wire coil 53 around a plastic bobbin 50. The wire used is preferably thousands of turns of fine copper wire. The coil 51 becomes magnetized on application of current by means of wires 55, seen in Fig. IB. The magnetized coil 51 attracts a ferromagnetic tip of an aluminum lever 59. The lever 59 has an end opposite the ferromagnetic tip which is affixed to the support block 19. The ferromagnetic tip is preferably a magnet that is made of a material that is very hard magnetically and has a very strong field for its size, such as a neodymium-iron-boron magnet. A magnet 57 is shown in a holder 52 attached to the end of lever 59 opposite support block 19 in Figs. 1A-1C. Lever 59 is preferably curved so that magnet 57 may be positioned directly above flexural pivot 21. By applying current to the wires 55 and magnetizing the coil 51, magnetic force is exerted on the lever 59 causing a force bias in the form of a pull toward or away from the center of coil 51. The lever 59 should be lightweight, yet stiff so that the lever will not bend on the application of magnetic force. The magnet 57 and magnetic coil 51 are part of the stylus force biasing means of the present invention. Variations in the force exerted as the magnet 57 moves may be minimized and the magnitude of the force maximized by placing the magnet 57 near the position of the peak magnetic field gradient, i.e., on the axis of the coil 51 and proximate to the plane of the end of the coil winding. The magnet 57 is spaced apart from the coil winding 51 to prevent it from traveling inside the center bore of the coil. At its closest position, magnet 57 is nearly touching the coil 51. The placement of magnet 57 allows for easy adjustment of the position of the magnet .

The use of a very powerful material for the magnet 57, such as a neodymium- iron-boron material, allows the magnet to be very small and light in weight and to still generate useful amounts of force. The magnet is 3 mm in diameter and 1.5 mm thick. The corresponding low current requirement minimizes the power dissipated in the coil, which minimizes the heat generated. This, in turn, minimizes the thermally- induced expansion and contraction of the materials comprising the sensor assembly. These thermally- induced size changes can cause undesirable drift in the measured profile of the sample or specimen.

The underside of a support body 71 has attached a transducer support 72 which acts as an elevational adjustment for a pair of spaced-apart parallel capacitor plates 35 and 37. The spacing between the plates is approximately 0.7 mm, with an air gap between the plates. A small spacer, not shown, separates plate 35 from plate 37 and a screw fastens the two plates to transducer support 72. The area extent of the plates should be large enough to shield the vane 41 from outside air, so that the vane experiences resistance to motion due to compression of air momentarily trapped between the closely spaced plates. A pair of electrical leads 39 of Fig. IB is connected to the parallel plates, one lead to each plate. Between the parallel plates, a low mass electrically conductive vane 41 is spaced, forming a capacitor with respect to each of the parallel plates 35 and 37. The range of motion of the vane, indicated by arrows A in Fig. IB, is plus or minus 0.16 mm. Moreover, vane 41, being connected to the support block 19 and flexural pivot 21, damps pivoting motion as the vane attempts to compress air between the parallel plates. This damping motion of the vane serves to reduce vibration and shock which may be transmitted into arm 15. Vane 41 is connected to a paddle 43 which is the rearward extension of support block 19, opposite stylus arm 15, serving to counterbalance the arm. The total mass of the vane, paddle and pivot member on the vane side of the pivot should preferably not exceed about 0.6 g. Movement of the vane between plates 35 and 37 results in change of capacitance indicative of stylus tip motion. Such a motion transducer is taught in U.S. Patent No. 5,309,755 to Wheeler.

The illustrated configuration of the support body 71, L- shaped bracket 73, and transducer support 72 is intended only as an example of a support for the sensor stylus assembly of the invention of the parent application. Additionally, the stylus displacement measurement means or motion transducer described and positioned relative to the stylus tip is preferred, but may be substituted by an equivalent means for indicating the stylus tip motion, as explained below.

In operation, the stylus tip 11 scans a surface to be measured, such as a patterned semiconductor wafer. Scanning may be achieved either by moving the stylus arm frame with respect to a fixed wafer position or alternatively moving the wafer, on an X-Y positioning wafer stage such as fine and/or coarse stages with the position of the stylus fixed, or a combination of the two motions. In the latter instance, the stylus arm may be moved linearly in the X direction while the wafer is advanced in the Y direction after each lengthwise X direction scan. The stylus tip 11 is maintained in contact with or in close proximity to the surface of the wafer at a steady level of force by an appropriate bias applied through the coil 51 into the lever 59. The bias is preferably great enough to maintain contact, but not great enough to damage the surface being measured. Deflections of the tip 11 are caused by topological variances in the surface being measured and these are translated rearwardly through the flexural pivot 21 to the vane 41. Vane 41 resists undesirable large amplitude motion due to vibration because of the air displacement between the parallel plates 35 and 37. However, as the air is compressed and displaced, the vane 41 moves slightly causing a signal in electrical leads 39 reflecting a change in an electrical bridge circuit (not shown) connected to these wires. At the end of a scan, the tip 11 is raised to protect it from damage in the event that a wafer is changed.

The invention of the parent application signifies an improvement over the prior art because it allows for a dynamic change in the force coil current as the stylus moves vertically, thereby eliminating the stylus force variability of previous devices. The instrument may be calibrated by supplying the drive current to move the non-engaged stylus (that is, the stylus not in contact with specimen) to regularly spaced positions to create a table of position versus current settings. That table provides the data for a polynomial curve fit approximation. A digital signal processor 84 of Fig. ID uses the curve fit to dynamically change the force setting as the position measurements are taken, with a specimen in place. A positive, constant force is generated by adding a steady current offset to the fit polynomial, as a direct fit would result in zero force. Fig. ID provides an illustrative block diagram of the above stylus force adjustment electronics. The electrical signals produced by motion transducer 81, i.e., vane 41 in conjunction with parallel plates 35 and 37, are selected and stored within a signal conditioning circuit 82 for specified vertical positions, creating data points, while the stylus tip 11 is not in engagement with the specimen 10. Since the stylus tip is supported by a flexure, i.e., a torsion spring, the data points are directly proportional to force levels because of the spring law, F=kx. The signals are then converted to a digital format by converter 83 and a digital signal processor 84 generates a polynomial curve for the data points. The curve is then adjusted by processor 84 to represent the force desired upon stylus tip 11 during profiling. The adjusted curve provides modulation instructions; i.e., feedback signals, which are converted to an analog format by converter 85 and signal the circuit 86 driving the coil 51 to modulate current 87 within the coil for constant stylus force.

The above description of the sensor assembly 60 is taken from the parent application. While the above-described sensor assembly 60 is advantageous for many applications, with the continual reduction in size of semiconductor devices and increasing demand for nanometer scale vertical measurements and submicron lateral measurements, it may be necessary to employ a very sharp stylus for the measurements at low interaction force. Even though the stylus arm illustrated above in reference to Figs. 1A-1C is of relatively light weight, it may be desirable to employ a yet smaller stylus assembly, such as the one illustrated in Fig. 2. The entire sensor assembly 100 may be manufactured starting with a planar piece of silicon or silicon on insulator (SOI) . By means of conventional techniques used in the semiconductor industry, a wafer of silicon or SOI may be etched to form an arm 102 having a wider section 102' and a narrow section 102". At the end of the thinner section 102" is attached a tip 104 (not shown in Fig. 2) preferably made of diamond. Alternatively, where tip 104 is made of the same material as section 102", the tip may form an integral part of the section. Integral with the arm 102 is a support piece 106 which may be used to support a force coil (not shown) . The support 106 together with the arm 102 are connected to the remainder frame portion 110 of the wafer or plate by means of flexure hinges 108. The force coil may comprise a layer of electrically conductive material deposited or implanted onto the surface of the support 106. Preferably, the layer of material is in the shape of a spiral . A magnet (not shown) may then be attached to the frame portion 110 in close proximity to the force coil. In this manner, when a current is passed through the force coil, electromagnetic interactions between the force coil and the magnet will apply a force to support 106. Since support 106 is integral with arm 102 and both are attached to the portion 110 through hinges 108, the force applied to support 106 will also be applied to the arm. In other words, the magnet and the force coil serve the same functions as magnet 57 and solenoid coil 51 of U.S. Patent No. 5,309,755. The article entitled "Scanning Force Microscope Springs Optomized for Opticalizing Beam Detection and With Tips Made by Controlled Fracture," by Gustafsson et al . , J^". Appl . Phys . 76(1), July 1, 1994, pp. 172-181, described in reference to Fig. 6 a torsion spring design made from silicon or silicon nitride where the design is somewhat similar to the configuration in Fig. 2. This type of design enables a profilometer having low mass and rapid response to be constructed.

Instead of using a capacitance gauge as in Figs. 1A-1C, the position and therefore the displacement of tip 104 may also be detected and measured using a differential interferometer as shown in Fig. 3. A Nomarski-type differential interferometer is disclosed by T.C. Bristow in Surface Topography, 1 (1988) 85-89. The differential interferometer 120 is of the Nomarski- type described in the Bristow article referenced above. By means of interferometer 120, the position and therefore the displacement of stylus arm 102 over time may be detected and measured. Preferably interferometer

120 is rigidly connected (not shown) to the outside frame portion 110 supporting arm 102. The Nomarski-type differential interferometer employing a Wollaston prism may also be used to measure the position and therefore the displacement of the stylus arm in the profiler 60 of Figs. 1A-1C as shown in

Fig. 4A. Thus, as shown in Fig. 4A, a portion 130 of a differential interferometer including a Wollaston prism

132 is used to detect the position and therefore the displacement of stylus arm 15 as the stylus tip 11 moves up and down to follow the contour of a surface by rotation at a flexure pivot 21. The interferometer, of which 130 is a portion, measures the displacement of arm

15.

Fig. 4B is a perspective view of a calcite beam displacer which may be used in place of a Wollaston prism and objective lens in the configurations of Figs. 3 and 4A for measuring the displacement of the stylus arm. A calcite beam displacer is shown in the configuration of Fig. 6 described below. As shown in Fig. 4B, the calcite prism separates an incoming light beam into two orthogonally polarized beams, and can recombine the reflections of the two beams into one beam.

Fig. 5 is a schematic diagram of an optical lever sensor which may also be used to detect and measure the position and displacement of the stylus arm 102. The optical lever sensor 140 in Fig. 5 may be of the type described by Meyer et al . in "Novel Optical Approach to Atomic Force Microscopy," Appl . Phys . Lett . , Vol. 53, No. 12, Sept. 19, 1988, pp. 1045-1047. The sensor 140 includes a laser 142 supplying light to the arm 102 which reflects the beam, a mirror 144 reflecting the light from laser 142 that has been reflected from arm 102, towards a position sensitive detector 146. However, different from and as an improvement over that in the Meyer et al . article, mirror 144 may be rotated in a controlled manner by an actuator/transducer to null the signal at detector 146 so as to increase dynamic range and accuracy of detection, as described below.

When the stylus arm 102 is made of silicon, its surface may be machined to become a mirror surface which reflects radiation such as light. Alternatively, a mirror (not shown) may be connected to arm 102, and reflections from this mirror may be used in conjunction with the optical lever system 140 (or the interferometer described below) for sensing the position of tip 104. The optical lever system 140 may comprise a mirror 144 with actuator/transducer m either one or two orthogonal degrees of freedom, a split or quadrant photodetector 146, controller 150 and central processing unit (CPU) 152, as shown in Fig. 5. The laser beam from laser 142 is reflected specularly from arm 102 towards mirror 144. When the tip 104 and arm 102 are at their reference positions initially, the position of mirror 144 is such that it steers the reflected beam from arm 102 towards the center of photodetector 146 so that detector 146 will provide equal voltages from the split or quadrant detector portions. This is commonly referred to as a null condition. As the stylus tip profiles a surface, the resulting vertical motions of the stylus arm 102 alters the angle of incidence of laser light from source 142 on the arm. In the absence of feedback, the reflected beam from arm 102 and mirror 144 will wander away from the center of photodetector 146 and the output of the detector will wander away from null condition. The voltages at the output of detector 146 are supplied to controller 150 which derives a drive voltage from the detector output to the actuator/transducer of the mirror 144, causing mirror 144 to rotate so that the reflector beam is again centered at detector 146 to return the detector output to the null condition. The drive voltage from controller 150 or the output of a transducer attached to mirror 144 then provides an indication of the movement of arm 102 and the position of tip 104. The drive voltage is provided to CPU 152 which derives the position of tip 104 from the drive voltage.

By locating mirror 144 closer to the photodetector than the stylus arm, the mirror orientation must be altered significantly more than the change in angle of the stylus arm to keep the output of the detector in a null condition; this amplification allows the use of standard, commonly available, actuators and transducers. The feedback loop comprising controller 150 and actuator/transducer of the mirror 144 may be used to perform the initial alignment of the laser with the stylus arm and photodetector.

While preferably a position/sensitive detector comprising a split or quadrant photodetector may be used and preferably the optical lever sensor 140 operates in the null condition, it will be understood that this is not required and other detectors may be used and optical lever sensors not operating in null condition may also be used and are within the scope of the invention.

Even though the sensor or stylus assembly 100 in Figs. 2, 3 and 5 are of a different construction compared to the stylus arm 15 of Figs. 1A-1C, the force control and calibration of the stylus assembly may be performed in the same manner as that of stylus arm 15 in reference to Fig. ID. In other words, stylus assembly 100 may be calibrated by applying different currents to the force coil on support 106, thereby causing the stylus 104 to be displaced by different amounts. The current and therefore the force applied and the corresponding displacement of the tip 104 are then recorded as data points according to which a data processor may generate a polynomial curve for the data points. The curve may then be adjusted by the processor to represent the desired force to be applied by stylus tip 104 against the surface of the specimen during profiling. Modulation instructions are then derived from the adjusted curve for controlling the current and the force coil so that tip 104 applies a substantially constant force to the specimen surface as the tip 104 scans the surface in the contact mode.

Fig. 6 is a schematic diagram of a differential interference contrast (DIC) displacement sensor for sensing the displacement of a stylus arm of a construction illustrated in Fig. 7. While in Fig. 6, the DIC sensor is used to sense the displacement of a stylus arm, it will be understood that the DIC sensor may be used for sensing any displacement and any such application is within the scope of the invention. In the DIC sensor, a variable first phase difference is introduced between two substantially co-axial, orthogonally polarized light beams. The light beams are sheared by a specific distance, and then directed towards the surface to be measured so that the beams are reflected by the surface. A second phase difference between the reflections of the two beams is then monitored when the first phase difference is varied (or not varied as explained below) to measure the displacement of the surface. In order to generate the two orthogonally polarized beams, a light source 162 supplies circularly polarized light towards a linear polarizer 164 which passes a linearly polarized beam towards a quarter-wave plate 166. The fast and slow axes of quarter-wave plate 166 are oriented substantially at 45° to the fast and slow axes of a calcite or Wallaston prism 168. The quarter-wave plate 166 causes the linearly polarized beam from polarizer 164 to be converted to two orthogonally polarized beams. Each of these beams is separated by prism 168 into two separate beams of substantially equal intensity that are incident upon the stylus arm 172. The output beam from the quarter-wave plate 166 passes through an amplitude beam splitter 170 and is reflected by mirror 171 towards prism 168.

The reflections of the two beams 174, 176 from arm 172 are combined by prism 168 into a single beam, reflected by mirror 171 and by beam splitter 170 towards a polarization beam splitter 182 which is oriented at 45 degrees to the two polarizations contained within the incident beam. The beam splitter therefore mixes the two incident polarizations where the mixed beams interfere with one another to provide two output beams that modulate the intensities detected at detectors PD1, PD2. One of the two output beams travels along path 184 towards detector PD1 , while the other output beam passes along path 186 towards detector PD2. The two output beams containing mixed components emanating from the beam splitter are substantially 180 degrees out of phase irrespective of the orientation of the linear polarizer 164 relative to the quarter-wave plate 166 and prism 168. The detectors PD1, PD2 detect the intensities of the two beams along paths 184, 186 and supply signals proportional thereto to a differential amplifier 188 where the common DC terms in the intensities of the two mixed beams are cancelled. Thus, the output of differential amplifier 184 is proportional to COS (Δβ-δ+θ) ; where Δβ is an arbitrary constant phase associated with the optical path of the system; δ is the phase term proportional to the path length difference between the two measurement beams 174, 176 caused by a rotation of the arm 172 about pivot 194; and θ is a phase difference proportional to the angular displacement or orientation of the linear polarizer.

Since the fast and slow axes of the quarter-wave plate are oriented substantially at 45° to the corresponding axes of prism 168, the intensities of beams 174, 176 will be substantially equal regardless of the angular orientation of the linear polarizer. Also, if the linear polarizer 166 is rotated by an angle, this will introduce a phase difference equal to twice such angle of rotation of the linear polarizer between the two beams 174, 176. Differential amplifier 184 supplies the output given above to control software and electronics 190 which generate the control signal to motor/encoder 192 for rotating the polarizer 164 in order to control the phase difference between the two beams separated by beam splitter 182. If the initial conditions are such that there is no phase difference between the two beams detected by the detectors PD1, PD2 , as stylus arm 172 rotates, this introduces a phase difference between the two beams 174 and 176 so that the output of amplifier 188 is no longer 0. Such output signal is then supplied to control software and electronics 190 which derives a control signal to motor/encoder 192 for controlling the rotation of polarizer 164 so that the phase difference introduced by the rotation of the polarizer tends to cancel out the phase difference caused by the rotation of arm 172. As a result, the output of amplifier 188 is again 0. This is referred to as the null condition of the sensor 160. When operated in the above described null configuration, the DIC sensor 160 can accommodate a greater range of measurement. Motor/encoder 192 attached to the polarizer 164 monitors the instantaneous orientation of the polarizer. Changes in the orientation of the polarizer can be related to displacements of the stylus 172 as it moves across the surface .

Instead of operating sensor 160 in the null configuration, it is also possible to operate the sensor in open loop. In such event, no feedback signal is applied to control the rotation of the linear polarizer

164 which remains substantially stationary. The phase difference introduced by the quarter-wave plate 166 therefore also remains substantially constant. When operated in this mode, DIC sensor 160 is capable of measuring subangstrom displacements of the stylus 172 over a range of about 1 micrometer. The measurement range corresponds to a height change between the beams

174, 176 of approximately 1/4 of the wavelength of light employed in the optical path of sensor 160. The stylus arm may be several times longer than the beam shear; the range of the sensor 160 may be scaled proportionally.

Thus if the length of the arm from tip to pivot (notch) is L, the beam shear is Δ, and the wavelength of the laser 162 is λ, then the stylus range is given approximately by Lλ/4Δ.

In another open loop mode of operation, no feedback signal is applied to control the rotation of the linear polarizer 164, but the linear polarizer is continually rotated, and the output of differential amplifier 188 proportional to the quantity COS (Δβ-δ+θ) described above is monitored to detect a particular relative phase between beams 184 and 186. For example, if the initial conditions are set so that the argument in the expression above for the output of amplifier 188 is equal to π/2, then control software and electronics 190 monitors the condition where the output of amplifier 188 is 0. When this happens, control software and electronics 190 would cause motor/encoder 192 to record the orientation of the linear polarizer 164. The phase difference introduced by the linear polarizer relative to the initial settings will then yield a value for θ. From this value of θ, the path length difference between the beams 174, 176 (equal to the distance rotated by arm 172) can be calculated as a quantity proportional to θ. While in the preferred embodiment, the two orthogonally polarized beams applied to prism 168 are formed by applying circularly polarized light to a linear polarizer and quarter-wave plate, other means for generating two orthogonally polarized beams may be used and are within the scope of the invention. For example, liquid crystal variable retarders or Soleil-Babinet compensators may replace the motorized linear polarizer 164 for generating a controlled phase different between two orthogonally polarized beams. While the stylus assembly 100 of Fig. 2 is advantageous since it is small and of lightweight, the assembly can be further simplified. Fig. 7 is a top view of an array of notch flexure profiler stylus arms including three stylus arms to illustrate an embodiment of the invention. Fig. 8 is a side view of the stylus arms of Fig. 7. Thus, as shown in Figs. 7 and 8, each of the stylus arms 202 includes an elongated plate or beam with a notch 204 on one of its planar surfaces as best shown in Fig. 8. Preferably the relative cross- section or dimensions of arm 202 are such that at least one dimension of the beam at the notch 204 is not more than 1/3 of a dimension of another portion of the arm away from the notch. Thus, where the notch is on one of the planar surfaces of the arm as shown in Figs . 7 and 8 , the arm at the notch has a thickness of not more than 1/3 of the overall or average thickness of the plate- shaped arm. The overall thickness of the arm is simply the thickness of the arm t as shown in Fig. 8, where the thickness of the arm at the notch is disregarded, while the average thickness of the arm will be slightly less than t, where the thickness of the notch is also taken into account in the averaging process .

The notch 204 serves as a flexural pivot for arm 202. In other words, instead of using a rigid arm 15 without a notch mounted on a flexure spring pivot 21 shown in Fig. 1A, the combined structure (15, 21) may be replaced by arm 202 with a notch 204 that functions in the same manner. In the same manner as stylus arm 15, as the tip 104 moves over a specimen, interaction force between the tip and the specimen causes the tip to rotate about the notch, and a force biasing means applies a force to the arm and tip in response to the displacement (i.e. rotation) of the tip so as to control the interaction force between the tip and specimen. If a constant interaction force is desired, the biasing means would apply a force to the arm so as to substantially cancel the variation in the interaction force caused by the variation in the amount of bending of the arm about the notch, while leaving the tip to follow the surface of the specimen at constant force. Each of the stylus arms 202 is such that when the stylus tip 104 is in contact with the specimen during profiling, the arm bends only at the notch 204 while the remainder portions of the arm remain substantially rigid. In this manner, the stylus arm 202 operates and functions in essentially the same manner as stylus arm 102 of Figs. 2, 3 and 6, as well as that of stylus arm 15 of Figs. 1A-1C and 4. The notch 204 may be located at any point of each of the arms 202 if interaction forces between the tip 104 and the surface measured cause each of the arms to bend substantially only at the notch. Preferably the notch is located at an intermediate position between an end of each of the arms carrying the tip and the other end connected to a support . The displacement of arm 202 may be measured by a Nomarski-type differential interferometer a portion 130 of which is shown in Fig. 10. In reference to Fig. 9, the array 200 of stylus arms may be mounted onto a fine motion stage 210 which is in turn connected to Z elevator 212 for moving the array up or down. The sample 10 is supported by a coarse motion stage 214 for moving the sample in a plane substantially parallel to the surface of the sample 10 to be profiled, that is, in the XY plane, where the direction normal to the surface of the sample 10 is along the Z direction or axis. Thus, the fine motion stage 210 would move the entire array 200 relative to the sample in the XY plane for profiling the sample. A number of differential interferometers may be used, each for monitoring and measuring the position and displacement of a corresponding stylus arm 202. In other words, each of a plurality of interferometers (only the portion 130 of which is shown in Fig. 10) may be employed to measure the position and displacement of a corresponding stylus arm 202. Displacements of the stylus arms can also be measured using optical levers. In this case, a light beam would be reflected off of each of the arms. The reflected beams may then impinge on separate detectors or onto a single imaging detector (e.g. charged coupled devices) . The bemas can, but need not be, steered using rotatable mirror (s) and feedback in the manner described above .

Fig. 11 is a block diagram of a profiler system employing the array 200 of Figs. 7 and 8 for profiling a specimen. As in the case of stylus arms 15 or 102, the flexure characteristics of arm 202 may be calibrated by a process similar to that illustrated in Fig. ID in order to compensate and cancel out the change in stylus force due to the flexure of the stylus arm at the notch 204. Thus, digital data processor 250 causes a drive current from a power supply (not shown) to be applied to a force coil 252. A magnetic film 256 is deposited or otherwise attached to arm 202. The magnetic force exerted by the force coil 252 on film 256 would cause arm 202 to bend at the notch 204, and the amount of displacement of the arm 202 would be sensed by interferometer 120 as the Z displacement signal. This signal is applied to processor 250 which correlates the current applied to the force coil with the Z displacement signal to provide a data point in plot 260 which is stored in memory 262. Plot 260 is made by obtaining multiple data points. The processor 250 then adjusts the plot or curve in 260 to allow for a substantially constant or other predetermined interaction force to be applied between tip 104 and the specimen surface. Then as before, the Z displacement signal from interferometer 120, while tip 104 is measuring a surface, is used to control the amount of force applied to the tip. In the same manner as described above for stylus arms 15, 102, the variation in the force applied by the arm 202 due to the bending of the arm at notch 204 is taken into account by making use of the adjusted plot or curve and the corresponding drive current would then be applied by processor 250 to force coil 252 to alter the current applied to the coil so as to counterbalance and cancel out the change in force applied to the tip due to the flexure of the arm at the notch. In this manner, tip 204 is caused to apply a substantially constant (or a predetermined variable) force against the specimen surface. Alternatively, where a non-constant interaction force is desired, the plot 260 can be adjusted accordingly. Alternatively, processor 250 may control the stylus arm so as to scan a surface in contact, intermittent contact or non-contact modes. In Fig. 11, the magnetic film 256 may be a thin superparamagnetic material deposited directly onto the arm. Or, a small magnet might be glued to the arm. Alternatively, small magnetic particles might be "painted" onto the arm. The binder could be dried in the presence of a magnetic field in order to control the orientation of the magnetic particles.

Fig. 12 is a top view of the stylus array 200 used for scanning and profiling a specimen 10. Since each of the three stylus arms is in contact with a different portion of the specimen surface, and the position and displacement of each of the stylus arms is measured independently of the other stylus arms, the three interferometers corresponding to the three stylus arms would provide three different Z displacement signals which are illustrated in Fig. 13. Thus, the tips 104 of the three stylus arms in Fig. 12 form a row 270 and the tips are scanned in a scan direction which is transverse to row 270 so that the tips are used to profile along scan paths 272, 274, 276 covering an area of the specimen surface.

Fig. 14 is a block diagram illustrating a control system for maintaining a desired interaction force between the stylus tip and the specimen. While a magnetic force applied between a force coil and a permanent magnet described above may be used for applying a biasing force on the stylus arm, other means for applying a biasing force may also be used. Thus, in reference to Fig. 14, stylus arm 302 has a notch 304 therein, where notch 304 has a substantially rectangular cross-section which may be advantageous for some applications, while the triangular cross-section notch

204 may be advantageous for the same or other applications. Other cross-sectional notches may also be used, such as a curved cross-section in the shape of a

Gaussian or bell shaped cross-section. Thus in reference to Fig. 14, a piezoelectric or magnetostrictive layer 306 may be employed which is in contact with arm 302 immediately above or in the vicinity of notch 304. Alternatively, the film (layer) may be deposited on the bottom of the notch flexure or on both the top and bottom surfaces of the flexure. Actuator 310 then applies a voltage (for piezoelectric layer 306) or magnetic field (in the case of a magnetostrictive layer 306) to layer 306. Layer 306 would expand or contract in response to the voltage or magnetic field, thereby causing arm 302 and tip 104 to rotate about notch 304 in order to control an interaction force between tip 104 and specimen 10. The displacement of tip 104 is sensed by displacement sensor 320 which may be any one of the above described sensors above, such as capacitive, optical lever, interferometer or DIC sensor. Arm 302 may be calibrated in the same manner as that described above for arm 202 and the force calibration data is stored in block 360. Control electronics and software 350 then matches the displacement of tip 104 from displacement sensor 320 against the calibration data in 360 to generate a force control signal to actuator 310 which in turn controls the interaction force between tip 104 and specimen 10. Figs. 15A-15D are schematic cross-sectional views of portions of stylus arms and their supports to illustrate possible shapes of notches in the arm, where such notches perform essentially the same function as notch 204 in Figs. 7, 8, 10-12 and notch 304 of Fig. 14. A notch that has a rounded surface such as 202c shown in Fig. 15C will reduce the stress points at the notch whereas the more angular type of notches such as 202a, 202b, 202d shown in Figs. 15A, 15B and 15D are easier to make. While notches are shown only on one of the surfaces of the arm in Figs. 15A-15D, it will be understood that notches may be formed on more than one surface, such as on both the top and bottom surfaces of the arm 202.

Figs. 16A and 16B are respectively the bottom and side views of a stylus arm to illustrate yet another embodiment of the invention. As shown in Figs. 16A and

16B, arm 402 has a tip 104 that is close to one end 402a and a notch 404 in an intermediate position between end

402a and the other end 402b connected to a support (not shown) . As shown in Fig. 16A, arm 402 is substantially rectangular in shape. If the surface scanned by tip 104 on arm 402 is such as to cause torsional forces on the arm, it may cause the arm to twist. Furthermore, the corners at end 402a may come into contact with some surfaces . Figs. 17A and 17B are respectively the bottom and side views of arm 502 which is substantially triangular in shape. Arm 502 has a tip 104 mounted at an apex

502a, and notch 504, where the base 502b of the triangle opposite to the apex is connected to a support 510. Preferably, arm 502 has a portion of the arm removed in the center, leaving a space 512 to reduce the weight of the arm.

While the invention has been described above by reference to various embodiments, it will be understood that changes and modifications may be made without departing from the scope of the invention, which is to be defined only by the appended claims and their equivalents. Thus, the interaction force referred to above may include contact related forces (e. g. van de Waal forces) as well as electromagnetic or magnetic forces .

Claims

WHAT TS CLAIMED IS:

1. A profiler for inspecting microstructures, comprising: an elongated stylus arm having a notch therein, said notch defining a flexure pivot, said arm having a stylus tip for interacting with a specimen by an interaction force; means for causing relative motion between the tip and the specimen so that the tip follows and measures a surface of the specimen, wherein the interaction force between the tip and the surface causes the tip to be rotated about the notch; and stylus displacement measuring means to provide a position signal to indicate position of the stylus tip when the tip moves over the specimen.

2. The profiler of claim 1, wherein the interaction force causes the arm to bend substantially only at said notch when the tip is in contact with or in close proximity to the specimen.

3. The profiler of claim 1, said arm at the notch having a dimension not more than a third of a dimension of another portion of the arm away from the notch.

4. The profiler of claim 3, said arm at the notch having a dimension not more than about 1 mm.

5. The profiler of claim 1, said arm having the shape of an elongated plate having an overall or average thickness, said arm at the notch having a thickness not more than a third of the overall or average thickness of the plate.

6. The profiler of claim 1, said arm being substantially rigid except at the notch when the tip is in contact with or in close proximity to the specimen.

7. The profiler of claim 1, further comprising means for supporting the arm, said stylus displacement measuring means being rigidly connected to said supporting means.

8. The profiler of claim 1, said causing means causing the tip to be in constant or intermittent contact or not in contact with the specimen.

9. The profiler of claim 1, said stylus displacement measuring means including an interferometer or optical lever sensor.

10. The profiler of claim 9, said optical lever sensor including a reflective element, a light source and a position sensitive detector.

11. The profiler of claim 10, further comprising means for rotating and monitoring the rotation of the reflective element .

12. The profiler of claim 11, said rotating means rotating the reflective element to null an output of the position sensitive detector.

13. The profiler of claim 12, wherein the reflective element is placed so that, in order to null the output of the position sensitive detector, the reflective element is to be rotated by a larger angle than the arm.

14. The profiler of claim 9, said interferometer including : means for supplying two orthogonally polarized beams ; means for introducing a first phase difference between the two orthogonally polarized beams; means for directing said two orthogonally polarized beams towards a surface after said first phase difference has been introduced so that the beams are modified by the surface; and means for monitoring a second phase difference between the two reflected beams.

15. The profiler of claim 14, said introducing means introducing a variable or substantially constant phase difference between the two orthogonally polarized beams .

16. The profiler of claim 14, wherein said introducing means varies the first phase difference introduced between the two orthogonally polarized beams, said monitoring means monitoring the second phase difference between the two modified beams when the first phase difference is varied to measure the displacement of the surface.

17. The profiler of claim 16, wherein said introducing means varies the first phase difference so that the second phase difference is substantially constant .

18. The profiler of claim 1, further comprising: a stylus force biasing means connected to the stylus arm to control a force between the tip and the surface, so that the stylus tip remains in contact with or in close proximity to the specimen; and feedback means in response to the position signal from the stylus displacement measuring means for controlling the stylus force biasing means to apply a desired level of force to the specimen.

19. The profiler of claim 18, wherein said feedback means includes a device storing a plurality of reference values, wherein said feedback means controls the stylus force biasing means in response to the stored reference values.

20. The profiler of claim 19, wherein said reference values indicate reference positions of the tip .

21. The profiler of claim 18, wherein said feedback means comprises a digital circuit .

22. The profiler of claim 18, said stylus force biasing means further comprising: a magnetic element or a piezoelectric or magnetostrictive element connected to the arm in the vicinity of the notch; and means for applying a voltage, a magnetic or electromagnetic field to the element.

23. The profiler of claim 22, said magnetic element being a permanent magnet or a conductive coil.

24. A method for controlling stylus force in a profiler, the method comprising the steps of: providing an elongated stylus arm having a notch therein, and a stylus tip mounted on the arm for interacting with a specimen with an interaction force; causing relative motion between the tip and the specimen so that the tip rotates about the notch in response to the interacting force; and measuring the specimen.

25. The method of claim 24, further comprising: providing a position signal to indicate position information of the stylus tip when the tip moves over the specimen; and modulating a force applied by the tip to the specimen as a function of the position information in order to apply a desired level of force to the specimen.

26. The method of claim 25, further comprising storing a plurality of positions of the tip, wherein said modulating step includes modulating the force as a function of the stored reference positions.

27. The method of claim 26, further comprising: applying a force of different magnitudes to bend the non-engaged stylus tip about the notch so that the tip is at a plurality of specified vertical positions, and generating a signal corresponding to each of said vertical positions, wherein said storing step stores said signal or signals derived therefrom and the magnitudes of the force applied.

28. The method of claim 25, said modulating step including digital signal processing.

29. The method of claim 25, said stylus arm having or connected to a light reflective surface, said position signal providing step employing an optical lever sensor including a reflective element, a light source and a position sensitive detector, said providing step including rotating the reflective element.

30. The method of claim 29, said rotating step rotating the reflective element to null an output of the position sensitive detector.

31. The method of claim 25, said position signal providing step including: supplying two orthogonally polarized beams; introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams; directing said two orthogonally polarized beams towards a surface of the stylus arm after said first phase difference has been introduced so that the beams are reflected by the surface; and monitoring a second phase difference between the two modified beams.

32. The method of claim 31, wherein the introducing step varies the first phase difference so that the second phase difference is substantially constant .

33. The method of claim 24, said causing step causing the tip to be in constant or intermittent contact or not in contact with the specimen.

34. The method of claim 24, said measuring step measures a surface of the specimen by means of the tip.

35. A profiler for inspecting microstructures, comprising: an elongated stylus arm having a stylus tip for interaction with a specimen by an interaction force, said arm having a pivot ; means for causing relative motion between the tip and the specimen so that the tip rotates about the pivot in response to the interaction force and measures a surface of the specimen; and stylus displacement measuring means including an interferometer or optical lever sensor to provide a position signal to indicate position of the stylus tip when the tip and the specimen move relative to one another .

36. The profiler of claim 35, further comprising: a stylus force biasing means connected to the stylus arm for urging the stylus tip into contact with or in close proximity to the specimen; and feedback means in response to the position signal from the stylus displacement measuring means for controlling the stylus force biasing means to apply a desired level of force to the specimen.

37. The profiler of claim 35, said arm being connected to a pivot so that the arm rotates about the pivot when the tip moves over the specimen.

38. The profiler of claim 35, said arm having a notch therein at an intermediate position of the arm, said notch defining a flexure pivot, so that the arm bends substantially only at the notch when the tip is in contact with or close proximity to the specimen.

39. The profiler of claim 35, said interferometer being a differential interferometer.

40. The profiler of claim 35, said stylus arm having or connected to a light reflective surface, said optical lever sensor including a reflective element, a light source and a position sensitive detector.

41. The profiler of claim 40, further comprising means for rotating the reflective element.

42. The profiler of claim 41, said rotating means rotating the reflective element to null an output of the position sensitive detector.

43. The profiler of claim 42, wherein the reflective element is placed so that, in order to null the output of the position sensitive detector, the reflective element is to be rotated by a larger angle than the arm.

44. The profiler of claim 35, said interferometer including : means for supplying two orthogonally polarized beams ; means for introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams; means for directing said two orthogonally polarized beams towards a surface of the stylus arm after said first phase difference has been introduced so that the beams are modified by the surface; and means for monitoring a second phase difference between the two reflected beams.

45. The profiler of claim 44, wherein the introducing means varies the first phase difference so that the second phase difference is substantially constant .

46. The profiler of claim 44, said monitoring means monitoring the second phase difference between the two modified beams when the first phase difference is held substantially constant .

47. A method for controlling stylus force in a profiler, the method comprising the steps of: providing an elongated stylus arm having a stylus tip for interacting with a specimen by an interaction force, said arm having a pivot; causing relative motion between the tip and the specimen so that the tip rotates about the pivot in response to the interaction force to measure the specimen; and providing a position signal to indicate position information of the stylus tip when the tip and the specimen move relative to one another by means of an interferometer or optical lever sensor.

48. The method of claim 47, further comprising modulating the interaction force as a function of the position information in order to apply a desired level of force to the specimen.

49. The method of claim 47, said stylus arm having a light reflective surface, said position signal providing step employing an optical lever sensor including a reflective element, a light source and a position sensitive detector, said position signal providing step including rotating the reflective element .

50. The method of claim 49, said rotating step rotating the reflective element to null an output of the position sensitive detector.

51. The method of claim 47, said position signal providing step including: supplying two orthogonally polarized beams; introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams; directing said two orthogonally polarized beams towards a surface of the stylus arm after said first phase difference has been introduced so that the beams are modified by the surface; and monitoring a second phase difference between the two reflected beams.

52. The method of claim 51, wherein the introducing step varies the first phase difference so that the second phase difference is substantially constant .

53. A profiling apparatus for inspecting microstructures, comprising: at least two profilers; and means for causing relative motion between the specimen and the at least two profilers .

54. The apparatus of claim 53, said causing means causing the at least two profilers to scan along at least two paths to increase throughput of the apparatus .

55. The apparatus of claim 54, said at least two profilers having tips arranged in a row, said causing means causing the array to move over the specimen in a direction transverse to the row to substantially simultaneously inspect an area of the specimen.

56. The apparatus of claim 53, each of said profilers comprising: an elongated stylus arm having a notch therein at an intermediate position of the arm, said notch defining a flexure pivot, and a stylus tip mounted thereon for contact with a specimen; and stylus displacement measuring means to provide a position signal to indicate position of the stylus tip when relative motion is caused between the tip and the specimen.

57. The apparatus of claim 56, further comprising: a stylus force biasing means connected to the stylus arm for urging the stylus tip into contact with the specimen; and feedback means in response to the position signal from the stylus displacement measuring means for controlling the stylus force biasing means to apply a desired level of force to the specimen.

58. The apparatus of claim 56, the stylus arms in said profilers being substantially coplanar.

59. The apparatus of claim 56, the stylus arms in said profilers comprising a silicon material or compound or a diamond material .

60. The apparatus of claim 56, said causing means comprising: a fine stage for moving the array in a direction substantially parallel to a surface of the specimen to be inspected; and one or more stages for moving the specimen in a direction substantially parallel as well as normal to said surface of the specimen.

61. The apparatus of claim 56, said stylus displacement measuring means including an interferometer or optical lever sensor.

62. A measurement apparatus, comprising: means for supplying two orthogonally polarized beams ; means for introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams; means for directing said two orthogonally polarized beams towards a surface after said first phase difference has been introduced so that the beams are modified by the surface; and means for monitoring a second phase difference between the two modified beams.

63. The apparatus of claim 62, wherein the introducing means varies the first phase difference so that the second phase difference is substantially constant .

64. The profiler of claim 62, said monitoring means monitoring the second phase difference between the two modified beams when the first phase difference is held substantially constant.

65. The apparatus of claim 62, said supplying means including: means for generating linearly polarized light; and a quarter-wave plate.

66. The apparatus of claim 65, said generating means including a source of circularly polarized light and a linear polarizer.

67. The apparatus of claim 66, said supplying means further including a calcite or Wallaston prism with its fast and slow axes substantially at 45 degrees to those of the quarter-wave plate.

68. The apparatus of claim 66, said introducing means including means for rotating the linear polarizer.

69. The apparatus of claim 68, further comprising means for controlling the rotation of the linear polarizer in response to the second phase difference to null the second phase difference.

70. A apparatus of claim 62, wherein said introducing means includes a liquid crystal variable retarder and/or a Soleil-Babinet compensator.

71. A displacement sensing method, comprising the steps of : supplying two orthogonally polarized beams; introducing a first variable or substantially constant phase difference between the two orthogonally polarized beams; directing said two orthogonally polarized beams towards a surface after said first phase difference has been introduced so that the beams are modified by the surface; and monitoring a second phase difference between two modified beams.

72. The method of claim 71, wherein the introducing step varies the first phase difference so that the second phase difference is substantially constant .

73. The method of claim 72, said introducing step including varying the first phase difference introduced by the introducing step in response to the second phase difference to null the second phase difference.

74. The method of claim 72, said supplying step including supplying circularly polarized light to a linear polarizer and rotating the linear polarizer.

75. The method of claim 72, wherein said introducing step increases or decreases continually the phase difference introduced, said monitoring step detecting the first phase difference when the second phase difference is substantially a predetermined value.

76. The method of claim 71, said monitoring step monitoring the second phase difference between the two modified beams when the first phase difference is held substantially constant .