FIELD OF THE INVENTION This invention relates generally to the scanning probe microscopes and integrated optical waveguide devices and methods and apparatus for detecting the displacement of the cantilever and cantilever arrays used in scanning probe microscopes with high sensitivity.

Such cantilevers and cantilever arrays are used in surface characterization, lithography systems and data storage systems.

BACKGROUND The advent of scanning probe microscopes introduced a very powerful tool to investigate electrically conducting and non-conducting materials, to pattern, modify and repair surfaces at nanometric scale, by acting as lithographic tools and to sense physical, biological and chemical processes such as topography, friction, magnetism and electrical charge. In all of these applications, scanning probe microscopes (SPMs) such as scanning tunneling microscope (STM), atomic force microscope (AFM), magnetic force microscope (MFM), lateral force microscope (LFM) and scanning near field optical microscope (SNOM), require the measurement of the cantilever displacement with high sensitivity. A good example is AFM, based upon the principle of sensing the forces between a tip and a surface. These forces induce displacement of the tip mounted on the cantilever. There is a need to determine the displacement of the cantilever with high sensitivity to work out the attractive and repulsive forces between surface and the tip. There are many methods to determine the tip displacement.

Most widely used technique is the optical lever technique, described in US Pat. No 5,144, 833.

In this technique, an external laser light is reflected from the cantilever surface and detected with a position sensitive detector. Although this method is very simple and cheap, it has low sensitivity and requires alignment during the surface scan and it is not very suitable for cantilever arrays. A highly sensitive technique is the interferometer which uses a cleaved end of a fiber and the back side of the cantilever as a Fabry-Perot interferometer (see, Rugar et al., Rev. Sci. Instrum. 59 (11) 1988, pp. 2337-2340). Using this technique, it is possible to achieve 0.01 A resolution, however, it requires positioning the fiber accurately on the cantilever and keeping the alignment during the surface scan, which limits the scan area. This method is not suitable for cantilever arrays. Another technique is the piezoresistive detection (see; Albrecht

et al. , US Patent 5,345, 815). In this method, a piezoresistive element integrated on the cantilever is used to sense the generated stress on the surface due to the bending of the cantilever via the change of the resistance of the element due to stress. This method is very compact and it does not require any alignment, but it has less sensitivity than the other methods. A further method is to use two sets of interdigitated fingers, one reference set being attached to the substrate from which the cantilever extends and the other movable set being attached to the tip of the cantilever. [U. S. Patent No: 5908981]. The interdigitated fingers form an optical phase grating. The deflection of the cantilever is measured by directing a light beam onto the optical phase grating and measuring the diffracted light in reflection. This method provides the highest sensitivity up-to-date, however, it requires alignment of laser beam and also the photodetector. Finally, a scanning probe microscope utilizing an optical element in a waveguide for dividing the center part of the laser beam perpendicular to the waveguide has been proposed [U. S. Patent No: 5231286]. This approach employs a Fresnel lens as well as a beam splitter on an optical waveguide to detect cantilever displacement. However, the sensitivity of this approach is not very good. Despite above mentioned methods, there still is a great need for integrated and highly sensitive sensors for displacement detection. The present invention provides a highly sensitive integrated optical displacement sensing probes for scanning probe microscopes. Integrated sensors are very attractive because of their compactness, no need of alignment, applicability to cantilever arrays and potential for mass production.

SPMs are very effective tools to analyze and modify surfaces. Next generation data storage technology and lithography systems require atomic resolution and SPMs are good candidates for this purpose. Parallel operation of SPMs dramatically reduce the time it takes to scan large surfaces and increase the throughput of the system. Previously, parallel AFM has been demonstrated by several authors. Minne et. al. [S. C. Minne, S. R. Manalis and C. F.

Quate, Appl. Phys. Lett. 67 (26) (1995) 3918] have built two cantilevers that use piezoresistive sensors and piezoelectric actuators that operate in parallel to obtain AFM images. The piezoresistive sensors are suitable for parallel operation because they do not require any alignment during operation and output is linear over a wide range. However, piezoresistive sensors are Johnson noise limited and have poor vertical resolution and less displacement sensitivity. Thermomechanical reading and writing by heating a silicon cantilever intended for data storage applications has also been studied [W. King et. al. J. Microelectromech. Syst., vol. 11 (6) (2002) 765, and H. J. Mamin Appl. Phys. Lett. 69 (3) (1996) 433]. Long thermal time

constants on the order of 0.4 ms have later been reduced down to ~1 pLS by better designs [B. W. Chui, et. al. J. of Microelectromech. Syst. Vol. 7 (1998) 69.] 5x5 cantilever arrays as well as 32x32 square arrays have been used [M. Lutwyche, et. al. Sensors and Actuators Vol.

A73 (1999) 89. and P. Vettiger, et. al. ISM Journal of Research and Development Vol. 44 (2000) 323] and features as small as 2 um has been measured. While this approach gives satisfactory results for writing on polymer surfaces, temperature rise as high as 350 °C may perturb surface properties of many materials under study via physical and chemical processes induced by the temperature rise. The interdigitated fingers forming an optical diffraction grating on a cantilever to measure cantilever displacement has been applied to one and two dimensional arrays with very high displacement sensitivity [U. S. Patent 5908981] However, this approach requires alignment of both the laser and the photodetector, a cumbersome task at best. The present invention provides very suitable cantilever array probes for parallel operation without any or little alignment.

SUMMARY OF THE INVENTION The present invention provides an integrated device comprising a cantilever with a tip and an optical waveguide device. Embodiments of this invention fall into two groups: the first group uses optical waveguide devices and make use of the stress generated during the displacement of the cantilever to detect the cantilever displacement; in the second group, integrated optical grating couplers are used to couple light in or out of optical waveguide devices integrated on the cantilevers and deflection of the entering or exiting light beams are used to measure the displacement of the cantilevers. The first group of embodiments of this invention has been made in view of the fact that the stress along the cantilever changes the local refractive index of the materials that make up the integrated optical device. The changes in the refractive index modify the optical transmission characteristics of the waveguide devices. Monitoring the power of the transmitted light, it is possible to determine the cantilever displacement with high sensitivity. The first group of embodiments of this invention provides for optical waveguide devices with varying sensitivities. These optical waveguide devices may include anyone of the following: ring resonators, race-track resonators and grating loaded optical waveguide Michelson interferometers. With this invention, it is possible to detect cantilever displacement without any alignment. These integrated optical devises are commonly used for sensor applications such as pressure, chemical and biological, temperature and strain sensors.

The second group of embodiments of this invention also provides a cantilever with an optical waveguide integrated with a grating coupler to scatter light in or out of the waveguide.

In one embodiment of this group of the invention, the grating on the optical waveguide acts as an optical coupler and it coherently scatters the light out of the waveguide to be detected by an external photodetector. It is possible to control the angle of scattering by appropriate choice of optical waveguide parameters and the period of the grating. In the present embodiment, the direction of the scattered light is altered with the displacement of the cantilever. The scattered light is detected with a position sensitive detector connected to a differential amplifier where the displacement of the cantilever changes the output of the amplifier as in the optical lever technique. Compared with the optical lever technique, this method provides higher directionality in scattered light which increases the sensitivity. With this invention, it is possible to detect the scattered light with minimum alignment during the surface scan. Under appropriate design conditions it is possible to scatter light out of the grating coupler in multiple directions. Coupling light to more than one direction allows to simultaneously measure the scattered light with more detectors which provides more sensitive measurements.

In an alternative embodiment of the second group, the optical waveguide grating coupler is used to couple light into the waveguide from an external coherent light source such as a laser and the light that travels in the waveguide is detected by a photodetector placed at the end of the waveguide which may be integrated or coupled by fiber optics. In this mode, a photodetector, placed at the output of the waveguide, can detect the variation of the optical power due to the cantilever displacement. The coupling efficiency of the grating depends on the angle between the waveguide and laser light. Small variations in the angle can be detected.

The second group of embodiments of this invention also provide for optical waveguide devices with varying sensitivities and applications. These optical waveguide devices may include either of the following: optical waveguide integrated grating input coupler or optical waveguide integrated grating output coupler. These integrated optical devises are commonly used for sensor applications such as pressure, chemical and biological, temperature and strain sensors. In accordance with another aspect of this invention, the cantilever in a SPM is deflected by an integral actuator which uses electrostatic forces, piezoelectricity or thermal heating to actively control displacement of the cantilever. All embodiments of this invention may use V-groove geometries, to couple fibers in and out of the optical waveguides integrated on the displacement sensors. Finally, in a dynamic version of the embodiments, differential

amplification, AC signals and lock-in detection may be used to improve the signal to noise performance of the sensors mentioned herein.

All the embodiments of this invention can be applied to various kinds of scanning probe microscopes such as AFM, LFM, MFM, and SNOM.

The throughput problem of SPMs can be alleviated by increasing the scan rate and the number of cantilevers that operate in parallel in the form of one and two dimensional arrays of cantilevers with integrated displacement sensors as described above. Low sensitivity and tedious alignment is a common problem for all prior arts. The present invention provides suitable probes for one and two dimensional cantilever arrays for parallel operation without any or minimum alignment.

The main objective of the current invention is to design a highly sensitive integrated optical waveguide sensor for scanning probe microscopes to measure cantilever displacement.

It is still another objective of the present invention to provide displacement sensors compatible with cantilever arrays.

It is still another objective of the present invention to provide displacement sensors compatible with cantilever arrays to study physical, chemical and biological processes.

It is still another objective of the present invention to provide displacement sensors for lithography systems and data storage technology.

It is another objective of the present invention to provide displacement sensors which do not require any alignment or little alignment in the case of the waveguide grating coupler, of laser light during surface scanning.

It is still yet another objective of the present invention to provide one and two dimensional arrays of displacement sensors to increase the throughput.

Additional objects and advantages of the invention will be set forth in the description which follows and in part will be obvious from the description or may be learned by practice of the invention.

The advantage of the present invention over the prior art is that the present invention provides for simple and compact integrated optical waveguide displacement sensors that measure the cantilever displacement without any or little alignment. The present invention can also be used for independent measurement of force applied on the cantilever. Other advantage of the present invention is that it is suitable for cantilever arrays which find application in lithography systems and data storage technology.

BRIEF DESCRIPTION OF THE FIGURES The objectives and advantages of the present invention will be better understood reading the following detailed description in conjunction with the drawings, in which; FIGS. 1A and 1B illustrate top and side views of the displacement sensor where the cantilever is integrated with an optical waveguide ring resonator, respectively.

FIG. 2 illustrates top view of the displacement sensor where the cantilever is integrated with an optical waveguide racetrack resonator.

FIG. 3A and 3B illustrate top view of the displacement sensor where the cantilever is integrated with an optical waveguide grating loaded Michelson interferometer and free space representation of the same, respectively.

FIG. 4A, 4B illustrate side and top views of the displacement sensor where the cantilever is integrated with an optical waveguide grating coupler, respectively.

FIG. 5 illustrates view of the displacement sensor where the cantilever is integrated with an optical waveguide grating input coupler.

FIG. 6A and 6B illustrate the coupling mechanism of the optical waveguide grating coupler and graph of coupling efficiency versus incidence angle, respectively.

FIG. 7A and 7B illustrate the view of displacement sensor where the cantilever is integrated with an optical waveguide grating output coupler and representation of diffraction mechanism, respectively.

FIG. 8 illustrates view of the displacement sensor where a U-shaped cantilever is integrated with an optical waveguide grating output couplers and integrated light source (laser).

FIG. 9 illustrates top view of one dimensional array of displacement sensors where cantilevers are integrated with optical waveguide ring resonators.

FIG. 10 illustrates top view of one dimensional displacement sensors where array of cantilevers are integrated with optical waveguide grating loaded Michelson interferometers.

FIG. 11A and 11B illustrates one dimensional array of displacement sensors where cantilevers are integrated with optical waveguide grating couplers in the out-coupling mode and in-coupling mode, respectively.

FIG. 12 illustrates view of two dimensional array of displacement sensors where where cantilevers are integrated with optical waveguide grating couplers.

FIG. 13 illustrates view of a displacement sensor where a cantilever is integrated with a piezoelectric actuator and a grating coupler.

FIG. 14A and 14B illustrates a displacement sensor where a cantilever is integrated with thermal actuator, top and bottom view, respectively.

FIG. 15A-15H illustrates the fabrication steps of displacement sensors where cantilevers are integrated with optical waveguide grating couplers.

FIG. 16A-16F illustrates the fabrication steps of displacement sensors where cantilevers are integrated with optical waveguide coupled ring resonators.

DETAILED DESCRIPTION OF THE INVENTION The present invention provides highly sensitive integrated optical displacement sensors for scanning probe microscopes. The present invention has a large application area with variations and modifications of the following descriptions upon the same principle. Present invention consists of two groups of embodiments. In the first group of embodiments, the sensing principle is based on monitoring the transmission of coherent light in optical waveguide devices integrated on the cantilever. The transmission characteristics of the devices are altered by the variation of local refractive index of the materials that make up the devices.

Thus, as the cantilever displaces, stress occurs and reaches to its maximum value at the supporting point of the cantilever. This stress changes the refractive index of the integrated optical device due to photo-elastic effect and can be written as n = n0 + #Ci#i where n0 is refractive index of waveguide material, Ci are the elements of the photo-elastic tensor and ai are the elements of stress vector. In the second group of embodiments, a waveguide grating coupler is used to scatter coherent light in or out of the optical waveguide to detect the displacement of the cantilever tip.

Ring Resonator Coupled optical Waveguide Integrated Displacement Sensors Fig. lA shows the schematic drawing of the displacement sensor (100) with cantilever (101), with the integrated optical waveguide ring resonator (102), and the optical waveguide (103) coupled to the ring resonator (102). Optical waveguide (103) is placed on the substrate (104) side of the supporting point of the cantilever (101). Ring resonator (102) is placed on the cantilever (101) at the supporting point of the cantilever (101) in close proximity with the

optical waveguide (103). Ring resonators (102) with radii as small as 5 urn can be fabricated in material systems with large index differences such as silicon and silicon dioxide. Large index differences prevent radiative losses in the ring resonators (102). Other materials may also be used. Due to wavelength selective nature, the ring resonators are frequently used in integrated optical devices as wavelength selective filters and add and drop multiplexer. Optical transmission of an optical waveguide (103) coupled to a ring resonator (102) drops down to zero periodically at specific (resonance) wavelengths. One can derive the transmission of such a ring resonator coupled optical waveguide as, where L is the circumference of the ring, k is the coupling coefficient between the ring and waveguide and a is the round trip loss and n is the effective index of the ring resonator (see, K.

Okamoto, Fundamentals of Optical Waveguides, Chp. 4, p. 162, Academic Press, 2000, San Diego). The sensitive parameter of the ring resonator system is the refractive index of the materials making up the system. Refractive indices of materials can change under stress. Any change in the refractive index shifts the resonance wavelengths. A narrow band coherent light source (laser) (105) tuned to one of the resonance wavelengths of the unstressed displacement sensor, is used at the input of the straight optical waveguide (103). As the resonance wavelength changes with stress due to displacement of the cantilever the change in the transmitted intensity at the output of the optical waveguide (103) will be detected by the photodetector (106). This allows for detection of cantilever (101) and attached tip (107) displacement. Both the photodetector (106) and the laser (105) may be integrated to the optical waveguide (103) or can be coupled by fiber optics. The sensitivity of the sensor depends on optical transmission characteristics of the straight optical waveguide (103) coupled ring resonator (102) system and the photo-elastic coefficient of the materials used. A cross sectional schematic of the cantilever in Fig. lA along AA'is given in Fig. 1B. Typical materials used for the fabrication of cantilevers are silicon and gallium arsenide, both of which have large photo-elastic constants. Furthermore, in order to increase the sensitivity, the total length of the ring can be increased. Fig. 2 represents another embodiment of the ring resonator where the length of the ring is extended to form a race-track shaped ring resonator (202). In this embodiment, the stress, which is maximum at the supporting point of the cantilever (200), decreases linearly along the cantilever (200), towards the free end of the cantilever (200).

Laying the long side of the race track resonator (202) along the cantilever (200) increases the length over which the light traveling in this portion of the race track resonator (202) experiences refractive index changes due to stress on the cantilever (200) surface leading to higher sensitivity. Similar to the ring resonator, a narrow band coherent light source (laser) (204) tuned to one of the resonance wavelengths, is used at the input of the optical waveguide (201). As the resonance wavelength changes with stress due to displacement of the cantilever the change in the transmitted intensity at the output of the optical waveguide (201) will be detected by the photodetector (205) and the tip (203) displacement will be determined.

Grating Loaded Optical Waveguide Michelson Interferometer Integrated Displacement Sensors Fig. 3A depicts schematically a displacement sensor (300) with a cantilever (301) integrated with a grating loaded optical waveguide Michelson interferometer. Coherent light from a narrow band light source such as a semiconductor laser (302) is injected to the input arm (303) of a directional coupler (304). Directional coupler (304) is designed to provide 50% power splitting between the upper (305) and lower (306) arms at the output side. The upper output arm waveguide (305) is loaded with a waveguide grating (307) before it is terminated short of the support point of the cantilever and the lower output arm waveguide (306) extending to the free end of the cantilever (301) is loaded with a waveguide grating (308) towards the free end of the cantilever (301). Gratings (307) and (308) act as wavelength selective mirrors. Under operating conditions where no stress due to cantilever displacement is experienced, light reflected from the grating (308) at the free end of the cantilever (301), will recombine with the light reflecting from the grating (307) at the directional coupler (304) and light will be directed to the lower input waveguide arm (309) to be detected by a photodetector (310) which may also be integrated to the waveguide arm (309). Any stress on the cantilever surface due to the displacement of the cantilever will change the optical path and hence result in a phase difference, AC, along the lower arm (306) leading to decreased optical signal at the output waveguide (309) to be detected by photodetector (310).

Fig. 3B illustrates the free space analogy of the interferometer. Directional coupler (304) serves the purpose of a beam splitter 311 and Bragg gratings (307), and (308) serve as mirrors (312), and (313). Light input from a coherent light source (314) is detected at the output by a photodetector (315). The output of the interferometer depends on the phase difference between the two reflected beams which can be written as

where Io is the input light intensity, and A is the phase difference and can be written as Ll and L2 is the length of the two arms and n is the refractive index of the materials that make up the waveguide. When the cantilever displaces, stress induced refractive index change produces a phase shift and the output intensity at the reflection wavelength will be changed.

Maximum sensitivity can be achieved when the two arms of the interferometer are in quadrature where the total phase shift is 2 This condition can be obtained by adjusting the L1 and L2 so that #L = m#/8n where m is an integer number. Comparison of reflected optical power with and without stress on the cantilever (301) will result in the measurement of cantilever (301) displacement.

Input and Output Optical Waveguide Grating Coupler Integrated Displacement Sensors Grating couplers provide a very attractive and efficient method for the excitation of waveguide modes and to couple light in and out of optical waveguides. Due to these properties, they are extensively used in the optoelectronic and integrated optical devices. A displacement sensor (400) with a cantilever (401) with an integrated optical waveguide grating coupler (402) is shown in the Fig. 4A. The waveguide grating (402) can be created thorough different methods such as electron beam lithography, or holographic (interferometric) lithography. Waveguide grating coupler (402) is built on the straight optical waveguide 403 that extends from the substrate (404) to the free end of the cantilever (401) where the tip (405) is located. Fig. 4B illustrates the AA'cross section of this device. In the first embodiment of this device as shown in Fig. 5, an incident coherent light (500) from a laser source (not shown) strikes the grating (501) on the optical waveguide to couple to the optical waveguide (502) under phase matching conditions and propagate in the optical waveguide (502) to be detected by a photodetector (503). The phase matching condition can be written as,

where ksour and ksi, t are the wave vectors of the incident light in ambient and propagating light in the waveguide, respectively. q is an integer, A is the period of the grating. This condition can be easily seen in Fig. 6A where it can be written that; where/ is the propagation constant of the waveguide modes and na is the refractive index of air. The coupling efficiency of the waveguide grating coupler (600) depends on many parameters such as grating height, h, length, L, incidence angle, 0, etc. The detailed analysis of the waveguide grating coupler (600) can be found in the article written by Harris et al., Applied Optics 11 (10) 1972 pp. 2234-2241. The most critical parameter of the coupling efficiency is the incidence angle, O, Present embodiment of this invention provides a method to determine the cantilever displacement using this critical parameter. When the coherent light (601) from a laser (not shown) strikes on the waveguide grating (600) at the critical angle, light will couple to the optical waveguide (602) and is detected at the end of the optical waveguide) (602) with a photodetector (603) which may be integrated or externally coupled with fiber optics. As the cantilever (604) displaces, the incidence angle changes and the coupling efficiency will decrease and the detected power will decrease. The sensitivity of this method depends mainly on the angular dependence of the coupling efficiency. Fig. 6B shows the graph of coupling efficiency versus incidence angle. With suitable design AO can be made in the order of 0. 01 degrees. In order to increase sensitivity, AS has to be kept as small as possible. Increasing the length of grating (600) makes AS small and this provides higher sensitivity. In contrast with the optical lever method, the advantage of this invention is that it requires only the initial alignment of the incidence angle with collimated light source such as laser, which is not difficult. In the second embodiment of this invention, waveguide grating coupler can also be used to out-couple coherent light already in the optical waveguide injected by a laser. Fig 7A shows the waveguide grating (700) used as an output coupler. The waveguide grating coherently scatters some of the light out of optical waveguide (701) injected to it by a coherent light source such as a semiconductor laser, (702) placed on the substrate side (703) of the cantilever (704) which may be integrated or coupled with fiber optics. As illustrated in Fig. 7B, in this embodiment, the incident wave is the propagating wave in the optical waveguide which has a propagation constant of/3 and the remaining formalism is same as in the previous embodiment. The scattered light (705) is detected with a

position sensitive photodetector (706). Displacement of the cantilever (704) and hence of the tip (707) is detected by the change in the out-coupling angle, 6. This method has advantages over the optical lever method as it does not require an alignment of the laser light on the cantilever (704) and provides more directional light which increase the sensitivity. Another advantage of this invention is that, under certain conditions grating (700) can scatter the light into multiple directions, which allows the measurement of the scattered light with more than one detector, further increasing sensitivity.

Fig. 8 illustrates yet another embodiment of this invention where two optical waveguide gratings (800) are fabricated on the arms of a U-shaped cantilever (801). The waveguide gratings (800) are fed with coherent light sources such as semiconductor lasers (802) which may be integrated or coupled with fiber optics to the optical waveguides (803) and the operational principles are the same as the that of the waveguide grating output coupler as shown in Fig. 7A and 7B. The light out coupled from the waveguide gratings are detected by position sensitive detectors (804), which may be a two dimensional array of photodetectors.

In addition to detecting the cantilever (801) displacement, such an arrangement as shown in Fig. 8, also allows the detection of lateral forces due to friction acting on the cantilever (801) tip (805) during the operation of the cantilever (801). The displacement and torsion of the cantilever (801) may be measured by differential amplification of the signals obtained from the photodetectors (804). In another embodiment of this device, the U-shaped cantilever (801) may be operated under the same principles as the cantilever (504) as illustrated in Fig. 5. In this embodiment, the grating couplers (800) couple light from external coherent light sources such as laser to the optical waveguides (803) to be detected by photodetectors placed at the position of the lasers (802). The displacement and torsion of the cantilever (801) may be measured by differential amplification of the signals obtained from the photodetectors placed at the position of the lasers (802).

Cantilever Arrays with Integrated Optical Displacement Sensors The above mentioned embodiments of this invention, integrating a cantilever with optical waveguide devices may be implemented in one and two dimensional arrays.

Fig. 9 illustrates a one dimensional array of cantilevers (900) each same as the cantilever (100) in Fig. 1. In this embodiment, light from coherent light sources such as lasers (901) either integrated or coupled by fiber optics to optical waveguides (902) coupled to ring resonators (903) are used as displacement sensors. Light propagating in the optical

waveguides (902) will be detected by photodetectors (904) which may be integrated and separately addressable, which provides for easy readout. The integrated ring resonators provide very high displacement sensitivity without any alignment. Any displacement of any one of the cantilevers can be detected individually. By using optical waveguide interconnects, it may be possible to reduce the number of light sources and photodetectors. Likewise, using optical directional couplers, it is always possible to monitor the light from the laser (901) sources to minimize the noise from laser (901) fluctuations.

Fig. 10 illustrates a one dimensional array of displacement sensors with cantilevers (1000) each same as the cantilever (300) in Fig. 3A. The operating principles of displacement sensors with cantilevers (1000) are also the same as the cantilever (300). In this embodiment, light from coherent light sources such as lasers (1001) either integrated or coupled by fiber optics to input waveguides (1002) provide the coherent light for the operation of the grating loaded Michelson interferometers (1003). Reflected light will be detected by photodetectors (1004) which may be integrated and separately addressable which provides for easy readout.

One dimensional array of displacement sensors (1000) where cantilevers are integrated with grating loaded optical waveguide Michelson interferometers (1003) provides high sensitive displacement detection for each cantilever separately without any alignment. Any displacement of any one of the cantilevers can be detected individually monitoring the power on the photodetectors (1004) for each cantilever, separately.

Fig. 11A illustrates a one dimensional array of displacement sensors with cantilevers (1100) which are similar to the cantilever (704) in Fig. 7A and operate in the same manner as described earlier. Coherent light input provided by semiconductor lasers (1101) which may be integrated or fiber coupled to the optical waveguides (1102), is injected to the optical waveguides (1102) arriving at the waveguide grating couplers (1103) and will be scattered by the waveguide grating coupler (1103) to be detected by position sensitive photodetectors (1104) external to the displacement sensor array (1100). Such position sensitive photodetectors (1104) may be two dimensional continuous array of photodetectors which will further ease the alignment process. In fact, since what is needed is the detection of shifts in the positions of the scattered beams, a two dimensional detector with large number pixels, should ease the alignment process, immensely.

Fig. 11B illustrates a one dimensional array of displacement sensors with cantilevers (1105) each same as the cantilever (504) in Fig. 5. In this embodiment, waveguide gratings (1106) are used to couple light from an external coherent light source such as a laser (not

shown) to the optical waveguides (1107) on the cantilevers (1105). Light propagating in the optical waveguides (1107) will be detected by photodetectors (1108) which may be integrated and separately addressable which provides for easy readout. A single beam of collimated coherent light (1109) can be used to illuminate the cantilevers, (1105) which makes alignment of the input light beam (1109) considerably easier. Any displacement of any one of the cantilevers can be detected individually.

The scanning capability of SPMs can be further improved by using two dimensional array of cantilevers. Fig. 12 illustrates the two dimensional application of optical waveguide grating couplers integrated on a cantilever as displacement sensors, as an example. The displacement sensors of the kind shown in Figs 1, 2,3A, 5,7A and 8 may be used in such a two dimensional array. In all cases, the displacement sensor array has N rows of sensors of M rows of sensors in each row forming an NxM matrix of sensors. The number of sensors in a two dimensional array may be limited by available micromachining technology. As an example Fig. 12 illustrates a two dimensional array of sensors (1200) using a integrated optical waveguide grating couplers (1201) operating either in the in-coupling mode or in the out-coupling mode. In the case of operating the two dimensional array (1200) in the out- coupling mode, the coherent light may be provided by a semiconductor laser (1202) which may be integrated or may even be supplied by a single laser to all the devices connected by optical waveguide interconnects. The out-coupled light can be detected by a two dimensional array of photodetectors (not shown). The same configuration may be used for the in-coupling mode of operation for optical waveguide grating couplers, in which case lasers (1202) will have to be replaced by photodetectors and coherent light may be provided external sources such as lasers (not shown).

In cases where ring resonator coupled optical waveguides, race-track resonator coupled optical waveguides or grating loaded optical waveguide Michelson interferometers are integrated on the cantilevers of the two dimensional arrays, the required coherent light may be provided by a single source such as a laser for each row that may be integrated. Suitable design of optical interconnects feeding the input of the optical waveguide devices on the cantilevers of the sensor array may even reduce the number of coherent light sources to a single (laser) source which may be integrated or fiber coupled to the input optical waveguides.

The transmitted light through the optical waveguides may be detected by individual photodetectors integrated on the substrate side of the sensors, all of which may be individually addressable. In addition to the high sensitivity, in the case of optical waveguides coupled to

ring resonators or optical waveguides coupled to race-track resonators, or grating loaded Michelson interferometers integrated on the cantilevers of the array, a significant further advantage of this invention is that there is no need for any alignment of either the input light to the optical waveguides or the transmitted light detected by the photodetectors during the operation of the array.

Integrated Actuators for Cantilevers with Integrated Optical Displacement Sensors A further requirement for a sensor or sensor array may be controlling the tip sample surface distance, individually in the case of arrays. Individual tip movement can be controlled by active arrays using integrated actuators. In the prior arts, different types of integrated actuators are described. The integrated actuator which is most often used is described in US Patent 5 883 705 includes a piezoelectric element in thicker section of the cantilever near its fixed end. The piezoelectric element is used to control the tip-sample surface separation. Such an actuator (1300) for a sensor with an optical waveguide grating coupler integrated on a cantilever same as the sensor (704) in Fig. 7A is shown in Fig. 13. The same approach can be extended to all other embodiments of this invention and the arrays thereof. The resonant frequency of the piezoelectric element is higher than the cantilever and this provides higher scan rates. Actuators using materials such as ZnO films or lead zirconate titanate (PZT) devices may be used in all embodiments of this invention to externally control the cantilever displacement [U. S. Pat. No. 5883705] Second most commonly used method to actuate cantilevers is to use an integrated thermal bimorph actuator (see, Bullen et al., Proceeding of SPIE, Smart electronics, MEMS and Nanotechnology SPIE's 9th annual international symposium on Smart Structures and Material, and Sulchelk et al. Appl. Phys. Lett. 75 (11) 1999 pp. 1637-1639). In this method, cantilevers are coated with a metal heater such as gold and aluminum, and the cantilever is heated using ohmic heating and cantilever bends along its length due to the different thermal expansion coefficient of the cantilever and the metal heater. Such an actuator (1400) is shown in Fig. 14A (bottom side) and 14B (top side) for an optical waveguide grating coupler integrated on a cantilever. This design results in a compact actuator and high probe density.

The common problem for both actuation methods in prior art is the electrical coupling between the actuator and the integrated sensor. When a piezoelectric actuator is used with a piezoresistive sensor on a cantilever, electrical coupling between the piezoelectric actuator and piezoresistive sensor limits the vertical resolution. In the thermal bimorph method, heat used

for actuation causes a temperature gradient on the sensor and this causes the variation of the resistance of the sensor and giving way to a large variation of the output signal. This invention provides optical waveguide devices integrated on cantilevers as displacement sensors that are immune to electrically active environments. The piezoelectric and thermal bimorph actuators are incorporated herein by reference. Finally, electrostatic forces may be used to control the deflection of cantilevers. Provided sample surface is conductive (or an underlayer) deflection of the cantilever may be controlled by coating the cantilevers with a metal layer [U. S. Pat. No.

5908981].

Fiber Coupling to Integrated Optical Displacement Sensors In all embodiments of this invention, where the coherent light is required at the input of the optical waveguides, light sources such as lasers may be integrated (semiconductor lasers) or coupled by fiber optics. In the case of coupling with fiber optics, V-grooves etched on the cantilever substrates may be used to couple light in the optical waveguides. The same may be true where light is detected at the output of the optical waveguides by photodetectors which are either integrated or coupled by fiber optics. In the case of fiber coupling, fibers may be mechanically positioned in V-grooves for easy alignment to optical waveguides on the cantilever to couple light out of optical waveguides of the optical waveguide devices, mentioned herein.

Fabrication of Integrated Optical Waveguide Displacement Sensors Fabrication process of cantilevers and cantilever arrays integrated with optical waveguide devices described herein are available in the literature and separately well understood. Here, we summarize possible fabrication processes, parts of which will be referred to available patents and publications, the disclosure of which will be incorporated herein, by reference, as though recited in full. The fabrication process of cantilevers integrated with ring resonator coupled optical waveguide integrated displacement sensors and other embodiments of this invention will be addressed separately below.

The fabrication process steps of input and output optical waveguide grating coupler integrated displacement sensors are summarized in Fig. 15. It begins with a silicon (Si) wafer (450, um thick) (1500) on which a layer of SiO2 (oxide) (0. 5 urn thick) (1501) is conventionally grown which is then conventionally coated with a layer of photoresist (not shown) and a small area masked and exposed by conventional lithography. The exposed area on the oxide layer is first etched with BHF exposing the Si wafer (1500). Using the oxide

(1501) as a mask and anisotropic etch, a pyramidal pit (1502) in Si wafer (1500) is etched and exposed. [U. S. Patent No: 5908 981] as shown in Fig 15A. The oxide layer (1501) is stripped with BHF and a layer of Si3N4 (nitride) (1503) is coated (5 um thick) on the Si wafer (1500) filling and covering the pit (1502), Fig. 15B. Illustrated in Fig. 15C, is an optical waveguide (1504) fabricated on the Si3N4 layer (1503) by covering the surface with a layer of photoresist, masking and exposing and finally etching Si3N4 using the photoresist as a mask.

A grating (1505) is then fabricated on the optical waveguide (1504) by first generating a grating pattern of desired period either by electron beam (e-beam) exposure of a e-beam resist or by interference lithography, which is then used as a mask to etch the grating (1505) onto the optical waveguide (1504), Fig 15D. Fig. 15E illustrates the definition of the cantilever (1506).

This is achieved by first covering the surface with a photoresist and conventional lithography to define the cantilever (1506) area. The cantilever width and length are typically 50 u, m and 500 urn, respectively. Fig. 15E illustrates the result of etching the nitride layer (15039 using photoresist as a mask and removing the photoresist. Attaching the cantilever (1506) to a holder requires the growth of a second oxide layer (1507) on the substrate side (1508) of the cantilever (1506) as shown in Fig. 15F. The substrate side of the cantilever is covered with an oxide layer (1507) using conventional lithography and growth techniques to which glass (1509) is bonded, as illustrated in Fig. 15G. Fig. l5H illustrates the final form of the cantilever (1506) with integrated optical waveguide grating (1505) coupler after the silicon wafer (1500) is etched away using conventional techniques, thereby freeing the cantilever (1506) and exposing the cantilever tip (1510). The cantilever (1506) with integrated optical waveguide grating (1505) coupler may be used to couple light in and out of the waveguide and as such this process covers the fabrication of both embodiments of the cantilever (1506) with integrated optical waveguide grating (1505) coupler (Fig. 5 and Fig. 7A). The process described herein may also be applied to the fabrication of optical waveguide grating loaded Michelson interferometer integrated displacement sensors (Fig. 3A). Other materials, such as multilayered GaAs/GaAlAs crystals may also be used to fabricate these devices. Such a material system is proposed to fabricate cantilevers integrated with ring resonator or race-track resonator coupled optical waveguide integrated displacement sensors. Fig. 16A-16F summarizes steps in the process of fabricating a cantilever integrated with ring resonator coupled optical waveguide integrated displacement sensors. The process begins with a wafer 1600 where thin layers of GaAs (typically 5 u. m thick) (1601) and GaAlAs (typically 5 urn or more) (1602) are grown epitaxially on a GaAs substrate (450 am thick) 1603 using what by

now are conventional techniques. The aluminum mole fraction of the GaAlAs layer (1602) is chosen to provide strong confinement of light in the optical waveguide to be made in the GaAs layer (1601) (typically 30%). The thickness of the GaAlAs layer (1602) is chosen to be thick enough to prevent radiative leakage of the light from the GaAs optical waveguide and to provide for the body of the cantilever that will make up the displacement sensor (typically 5 urn or more). Both sides of the wafer (1600) are coated with a layer of silicon dioxide. As illustrated in Fig. 16B silicon oxide (1604) (1, um thick) is masked by a photoresist (1605) where the tip is to be formed by conventional lithographic techniques. The bottom oxide layer (1606) is also masked with a layer of photoresist and part of the oxide layer is etched. [Atalar et. al. in U. S. Pat. No. 5908981]. The tip (1607) of the cantilever is formed by etching partially through the GaAs layer (1601), using photoresist layer (1605) and the remaining portion of the of the oxide layer (1604) as a mask by a suitable etch. The thickness of the remaining GaAs layer outside the tip area is chosen so as to obtain a single mode optical waveguide in the following steps. The tip (1607) may be sharpened by repeatedly using anisotropic chemical etches. The result is a sharp tip (1607) made of GaAs on the top GaAs layer (1601) as illustrated in Fig. 16C. The tip is then protected by coating it with a layer of dielectric such as an oxide, a nitride or a polymer. As shown in Fig. 16D, the ring resonator (1608) coupled optical waveguide (1609) structure is fabricated by masking it with a thick layer of photoresist and etching the exposed areas either by a dry etch such as in a chlorine plasma or a wet chemical etch. To prevent light leakage from the waveguide, single mode GaAs optical waveguides (1609) must be etched all the way down to the bottom cladding layer of GaAlAs (1602). The definition of the cantilever (1610) area is illustrated in Fig. 16E. As the cantilever (1610) is relatively thick, attention must be paid to use a mask that will withstand the etch process. This may be a metal layer sandwiched between two photoresist layers where the mask is transferred from the top photoresist to the metal layer and then to the photoresist layer below it. Resulting metal mask is used to etch the bottom cladding layer of GaAlAs layer (1602) to define the cantilever (1610) area. The metal mask is removed by lifting off the bottom photoresist layer using a chemical solvent. The exposed reverse side of the wafer (1600), is etched from the backside with a chemical etch using the oxide layer (1606) as a mask to remove the bottom GaAs layer (1603) to release the cantilever (1610) as shown in Fig 16F. Same process is applicable to race-track resonators without any change.

The foregoing discussion is illustrative and limiting. The invention is applicable to all other technologies that use cantilevers or micro-cantilevers to detect, repair or modify a variety of chemical biological and physical properties and events, such as: to determine static and dynamic properties of thin films, to measure stress, fatigue and other mechanical properties of materials, to measure temperature, pressure and humidity, to sense presence of various chemicals, to study phase transitions heat capacity and specific resistivity, to detect radiation, to mark and scribe materials, to measure absorption spectrum of materials, to measure magnetic properties of materials, to investigate electron-spin and nuclear magnetic resonance phenomena. The present invention may be used to measure cantilever displacement to achieve each and every one of above mentioned examples. The present invention is therefore intended to cover all such embodiments and those skilled in the art will appreciate that various modifications, additions and substitutions are possible without departing from the scope and spirit as disclosed in the accompanying claims.