Integrated Tip Strain Sensor for Use in Combination With a Single Axis Atomic Force Microscope

This patent application is related to U.S. patent application Ser. No. 07/568,451, filed Aug. 16, 1990, entitled "Method of Producing Ultrafine Silicon Tips for the AFM/STM Profilometry", U.S. patent application Ser. No. 07/608,043, filed Oct. 31, 1990, entitled "Nanometer Scale Probe for an Atomic Force Microprobe, and Method for Making Same", U.S. patent application Ser. No. 07/619,378, entitled "Microprobe-Based CD Measurement Tool", and U.S. patent application Ser. No. 07/767,300, filed Sep. 27, 1991, entitled "Combined Scanning Force Microscope and Optical Metrology Tool", and which are all assigned to the same assignee as this application, and the disclosure of all of them is incorporated herein by reference.

1. Field of the Invention

The present invention generally relates to metrology of linewidths, trenches and overlays on a submicron range using strain sensors and, more particularly, to probe assembly used in combination with an Atomic Force Microscope (AFM) for making measurements in three dimensions. The AFM is used to make measurements in the z direction and then the probe assembly is used in the x-y plane perpendicular to the z direction to profile a sample.

2. Description of the Prior Art

Quality control in the manufacture of integrated circuits (ICs) requires a technology for measuring surface geometries of the IC wafer. One such technology employs a small thin piezoelectric member affixed to a larger mounting member to which there is attached a probe tip. The probe tip is made to contact the surface being mapped thereby causing a flexing of the mounting member and a corresponding flexing of the piezoelectric member, producing an electrical signal. An example of this technology is shown in U.S. Pat. No. 4,888,550 to Reid which discloses a multiprobe tip assembly used for quality control testing of an integrated circuit wafer. The Reid device uses a strain sensing piezoelectric element at the base supporting the entire tip structure and, because of this construction, is limited to sensing only in the vertical direction.

In practice, the contact tip technology as known in the prior art has been useful in measuring micron scale or larger structures. Furthermore, the sensitivities of these structures has been relatively low, making them unsuitable for measurements of submicron scale structures such as those of the newer gigabit chips.

Scanning Tunneling Microscopy (STM) technology introduced a new concept in measurements using a probe type approach to map the surfaces of integrated circuit structures. The basic STM is disclosed in U.S. Pat. No. 4,343,993. Briefly described, a sharply pointed, electrically conductive tip is placed at a distance on the order of one nanometer from the conductive surface of a sample to be investigated, with an appropriate electrical potential applied across the gap between the tip and surface. As the electron clouds of the atoms at the apex of the tip and the surface touch, a flow of electrons will result giving rise to a tunneling current which happens to be extremely sensitive to changes in gap width. To render these changes as close as possible to zero, a feedback control system controls the distance of the tip from the surface, using deviations of the tunneling current from an initial value as a control signal. This control signal is also employed to generate a plot of the topography of the surface being investigated.

Atomic Force Microscopy (AFM) is a variation of the STM technology. In one design, a sensor consisting of a spring-like cantilever which is rigidly mounted at one end and carries at its free end a dielectric tip, profiles the surface of an object. The force between the object's surface and the tip deflects the cantilever, and this deflection can be accurately measured. A spatial resolution of 3 nm has been achieved.

Another version of the AFM includes optical detection instead of STM detection. In this version, a tungsten tip at the end of a wire is mounted on a piezoelectric transducer. The transducer vibrates the tip at the resonance frequency of the wire which acts as a cantilever, and a laser heterodyne interferometer accurately measures the amplitude of the a.c. vibration. The gradient of the force between the tip and the sample modifies the compliance of the lever, hence inducing a change in vibration amplitude due to the shift of the lever resonance. Knowing the lever characteristics, one can measure the vibration amplitude as a function of the tip-to-sample spacing in order to deduce the gradient of the force and, thus, the force itself.

A most critical component of the AFM is the spring-like cantilever. As a maximum deflection for a given force is needed, a cantilever is required which is as soft as possible. At the same time, a stiff cantilever with a high natural frequency is necessary in order to minimize the sensitivity to vibrational noise from the building. For meeting both requirements, dimensions of the cantilever beam are necessary that can only be obtained by microfabrication techniques. Another most critical component of the AFM is the tip itself. Tip "crashes" are the most likely failure cause of AFM tips, and such failures are catastrophic, requiring the replacement of the cantilever beam and tip subassembly. This is perhaps the greatest limitation to the use of AFM.

While STM and AFM both have the potential capability for profiling on an atomic diameter scale, the difficulty with both is the measurement of trenches in two dimensions. Both require sophisticated multi-connected interferometers to detect the two-dimensional tip motion and, for AFM to be useful as a reliable and accurate metrology tool, the sensor tip should be highly reproducible in the fabrication process and the tip shape should be known with high accuracy. Moreover, for profiling and critical dimension (CD) measurement of silicon (Si) memory trenches with submicron trenches with depths of 1 .mu.m or more, nanometer-scale tip shapes with high aspect ratio will be required.

It is therefore an object of the present invention to provide a measurement technology which is capable of meeting the advanced metrology requirements imposed by the dimensions of gigabit chips.

It is another, more specific object of the invention to provide a strain sensor device in combination with an AFM that is capable of three-dimensional measurements of submicron geometries with high sensitivity.

It is a further object of the invention to provide an AFM measurement system which protects the tip of the cantilever from catastrophic "crashes".

According to the invention, there is provided a cantilever beam carrying an integrated tip stem on which is deposited a piezoelectric film strain sensor. A high-resolution direct electron beam (e-beam) deposition process is used to grow a sharp tip onto a silicon (Si) cantilever structure. The direct e-beam deposition process permits the controllable fabrication of high-aspect ratio, nanometer-scale structures. A piezoelectric jacket with four electrical contacts superimposed is deposited on the tip stem. The piezoelectric sensors function in a plane perpendicular to that of the probe in STM; that is, any tip contact with the linewidth surface will cause tip deflection with a corresponding proportional electrical signal output. This tip strain sensor, coupled to a standard single axis AFM tip, allows for three-dimensional metrology with a much simpler approach while avoiding catastrophic tip "crashes". The potential for a tip "crash" is detected by the electrical signal output from the strain sensor exceeding a predetermined threshold. When the threshold is exceeded, the feedback control system stops the movement of the tip and then backs the tip away from a surface feature with which it has come into contact. Two-dimensional edge detection of the sidewalls is used to calculate the absolute value of the linewidth or overlay, independent of the AFM principle. The technique works on any linewidth surface material, whether conductive, non-conductive or semiconductive.

The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:

FIG. 1 is an isometric view showing a scanning probe tip attachment for a one-dimensional AFM system combined with a two-dimensional integrated tip strain sensor element according to the invention;

FIG. 2 is a cross-sectional view of the tip element shown in FIG. 1 showing the piezoelectric film divided into four quadrants and provided with overlying electrical contacts;

FIG. 3 is a graph showing a characteristic voltage output of the strain sensor elements as a function of tip lateral displacement in the X-axis and/or the Y-axis;

FIG. 4 is a block diagram showing the AFM system combined with the integrated tip strain sensor element; and

FIG. 5 is an enlarged view, partially in cross-section, showing the integrated tip strain sensor element within a trench and illustrating the operation of the system.

Referring now to the drawings, and more particularly to FIG. 1, there is shown the scanning probe tip attachment 10 for a one-dimensional AFM system (not shown). Starting with a standard cantilever beam 11 with an integrated tip stem 12, fabricated by micromachining techniques, the extended tip stem 15 and tip structure 16 is deposited by established e-beam techniques described, for example, by K. L. Lee and M. Hatzakis in "Direct electron-beam patterning for nanolithography", J. Vac. Sci Technology, November/December 1989, pp. 1941-1946. A piezoelectric film 17 is deposited on the tip stem 12 and divided into four quadrants by e-beam etching to define four piezoelectric sensors 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4. The preferred implementation of this invention is to use lead zirconate titanate (PZT) material as the piezoelectric film 17 to construct the strain sensor elements 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4, shown in more detail in FIG. 2. PZT material can be deposited with a variety of different processes known in the art, including rf-magnetron sputtering and chemical methods. Metal film is patterned to provide electrical conductors 18.sub.1, 18.sub.2, 18.sub.3, and 18.sub.4 which make electrical contact with respective strain sensor elements 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4 and extend up the tip stem 12 and along the under surface of the cantilever beam 11, and from there electrical connections 19.sub.1, 19.sub.2, 19.sub.3, and 19.sub.4 are made to the measuring circuitry (not shown). The common base electrode for the strain sensor elements 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4 can be either the tip stem 12 itself or a conductive coating, preferably Pt, or a conductive oxide. In either case, an insulative coating is provided to separate the conductive stem or coating from the electrical conductors 18.sub.1, 18.sub.2, 18.sub.3, and 18.sub.4. Additionally, a conductive path is provided as the common or return path to the measuring circuitry. The conductive coating, if used, and the corresponding common or return path are not shown in the drawings in order to simplify the illustration, but those skilled in the art will understand that the connection is implied.

The implementation of the strain sensor elements 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4 can be accomplished using different geometrical configurations, the configuration shown in FIGS. 1 and 2 being preferred. One specific reduction to practice is described below:

1. The stem 12 is first prepared by depositing an insulative layer of oxide, if the stem itself is the conductive return path for the sensors, or creating a conductive return path by depositing layers of a conductive coating (e.g., Pt or a conductive oxide) and an insulative layer.

2. A continuous PZT coating 17 is applied to the tip stem 15 and etched to form the four quadrant structure shown.

3. The deposited material is heat treated in an oxygen containing environment to crystalline into the proper PZT phase and to oxidize the exposed cantilever structure 11 and tip stem 12, achieving an electrical insulating coating on the same.

4. A continuous metal film is applied onto PZT coating 17 and at least the bottom surface of the cantilever beam 11 as an overcoat.

5. The metal film is patterned to yield the four quadrant contacts on the PZT coating 17 and the electrical conductors 18.sub.1, 18.sub.2, 18.sub.3, and 18.sub.4 which extend up the tip stem 12 and along the bottom length of the cantilever beam 11 to provide the output signal on lines 19.sub.1, 19.sub.2, 19.sub.3, and 19.sub.4 from the strain sensor contacts.

The patterning of the metal film into quadrant contacts and conducting lines is most conveniently accomplished by laser ablation, ion milling or e-beam processing.

As shown in FIG. 1, the tip 16 extends into a trench 21 formed in the surface of a chip 20 which is to be profiled. The trench 21 has side walls 22 and 23 and a bottom 24. In operation, the system starts up as an AFM probe vibrating the tip 16 in the z direction and moving the tip from the surface to the bottom 24 of the trench 21. Vibration of the cantilever beam 11 is produced by a bimorphous crystal 26, and trench depth is defined by the AFM viewing the vibration of the end of the cantilever beam 11 via lens 27. At this point, tip vibration is terminated, the z dimension of the trench having been established. The tip is backed off from the surface (e.g., 20 to 80% of depth). The side walls are located by moving the tip in the x-y directions until strain is detected by the piezoelectric sensors due to tip deflection. FIG. 3 shows the voltage output for one of the piezoelectric sensors as a function of deflection of the tip 16. By a simple geometric calculation based on the relative amplitudes of the signals from each of the four sensors 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4, the x and y coordinates are defined, and the tip 16 is moved to the opposite sidewall. When detected, the x and y coordinates of the opposite sidewall are also defined. Then, again using a simple geometric calculation, the linewidth or overlay is calculated.

FIG. 4 shows in block diagram form the overall system including the AFM. The chip 20 is mounted on a stage 30 for movement in the x-y plane. The AFM itself includes, in addition to the lens 27, a laser heterodyne system 31, the output of which is the z feedback signal to an amplifier 32 which provides an a.c. driving signal from generator 33 to the bimorphous crystal 26. The z feedback signal is also supplied to a microprocessor controller 34 which controls the application of the driving signal to the crystal 26, this being terminated when the z dimension has been determined. After the z dimension has been determined, the tip 16 is withdrawn to a predetermined distance above the bottom of the trench and moved relative to the trench in the x-y plane. This is accomplished by the controller 34 moving the stage 30. A feedback loop defined by the output signals from the sensors 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4 to the controller 34 is used to control relative tip motion. The three dimensions determined in this fashion are output by the controller 34 to a suitable output device 35 which displays, prints and/or stores the data.

A most valuable and important part of the invention is its ability to prevent "crashes" of the tip. "Crash" prevention is accomplished by the microprocessor controller 34 including a programmed routine which compares the absolute value of the voltage output from the sensors 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4 with a predetermined threshold. As generally indicated in FIG. 3, the absolute value of voltage is approximately a linear function of tip deflection, and the threshold is selected to correspond to a tip deflection well within elastic limit of the tip 15. The routine simply compares each of the outputs of the sensors 17.sub.1, 17.sub.2, 17.sub.3, and 17.sub.4 with the predetermined threshold. An alternative, although more complex routine, performs a simple trigonometric calculation on the two positive and two negative voltages from the sensors to resolve the direction of the tip deflection. Such a routine is part of the profiling computation performed by controller. The resolved absolute value of voltage amplitude is then compared to the threshold. When the threshold is exceeded, the controller 34 stops the motion of the tip 15 relative to the sample being profiled and then backs the tip away from the surface feature with which the tip has come into contact.

The preferred measurement regimen is illustrated in FIG. 5. After the z dimension is determined, the tip 16 is first withdrawn by 20% of the trench depth, and the x and y coordinates are determined for point A on side wall 22 and then point D on side wall 23. Then the tip 16 is withdrawn by 80% of the trench depth, and the x and y coordinates are determined for point B on side wall 22 and then point C on side wall 23. The calculated width of the trench 21 is based on the root mean square of the width AD and the width BC.

While the invention has been described in terms of a single preferred embodiment, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.