Methods and devices for correcting errors in atomic force microscopy

The present application is related to U.S. provisional patent application Ser. No. 61/385,618 filed on Sep. 23, 2010, entitled “METHOD AND APPARATUS FOR ATOMIC FORCE MICROSCOPY” from which priority is claimed under 35 U.S.C. §119(e) and which application is incorporated by reference herein in its entirety.

The present application relates to atomic force microscopy (AFM). AFM may be used to measure and characterize surface features of semiconductor devices, magnetic recording devices, and microelectromechanical system (MEMS) devices, among others.

AFM systems can use tube scanners to scan surfaces. Tube scanners can include electrodes composed of piezoelectric materials, which cause linearity and hysteresis errors. The linearity and hysteresis errors can vary when applying different voltages to the piezoelectric materials, thereby causing inaccuracies in the measurement and characterization of surface features.

In certain embodiments, a probe scans a surface to produce a first scan. The first scan is used to estimate a vertical offset for scanning the surface to produce a second scan. In certain embodiments, an AFM device engages a probe to a surface using a piezo voltage. The probe scans the surface to produce a first scan. The first scan is used to estimate a vertical offset such that the probe uses the piezo voltage to engage the surface for a second scan at the same separation distance between AFM tip and sample surface.

FIG. 1 depicts an exemplary tube scanner.

FIG. 2 provides an exemplary operation of an AFM device in accordance with various embodiments of the present invention.

FIG. 3 provides an AFM device routine illustrative of steps carried out in accordance with various embodiments.

FIG. 4 provides an AFM device routine illustrative of steps carried out in accordance with various embodiments.

FIG. 1 shows parts of an AFM system, including a tube scanner 100 that includes a first electrode 102 , a second electrode 104 , a third electrode 106 , and a probe 108 , which is located on a distal end of the tube scanner 100 . The probe has a probe tip 110 .

The first electrode 102 and second electrode 104 control the tube scanner's horizontal motion while the third electrode 106 controls the tube scanner's vertical motion. Each electrode 102 , 104 , and 106 can be made of piezoelectric materials. When a voltage is applied to the first electrode 102 or the second electrode 104 , the tube scanner 100 bends, thereby causing horizontal displacement. When a voltage is applied to the third electrode 106 , the tube scanner 100 elongates, thereby causing vertical displacement. Altering the applied voltage alters an electrode's displacement, for example, increasing the applied voltage increases displacement.

For each applied voltage, piezoelectric materials naturally have associated linearity and hysteresis errors. As a result, tube scanner linearity and hysteresis errors differ among different elongation states. These differences in errors can cause inaccuracies when measuring and characterizing surface features and when using scan correction methods like image subtraction.

FIG. 2 shows parts of an AFM systems, including a tube scanner 200 including a first and second electrode 202 and 204 (for horizontal displacement), a third electrode 206 (for vertical displacement), and a probe 208 . The probe 208 is shown engaged with a sample surface 210 of a workpiece 212 at a first scan location (tube scanner outlined in dotted lines) and at a second scan location (tube scanner outlined in solid lines). In use, as outlined in FIG. 3 , the tube scanner 200 and the sample surface 210 can be moved in a horizontal or vertical direction by a step motor or other suitable devices to position the tube scanner 200 over a first scan location (step 300 ). The tube scanner 200 engages the probe 208 with the surface 210 by applying a voltage to the third electrode 206 , thereby elongating the tube scanner 200 (step 302 ). Once engaged, the probe 208 scans the surface 210 and produces a first scan (step 304 ). The first scan produces an image of the surface 210 and can contain information like a surface tilt φ (step 306 ).

Then, the tube scanner 200 can be offset in a horizontal direction (e.g., ΔX) to a second location for a second scan (step 308 ). Using the horizontal offset and the surface tilt φ from the first scan, the tube scanner 200 is offset in a vertical direction (e.g., ΔZ) by using a step motor or other suitable method but not using the third electrode 206 so that the tube scanner 200 is substantially the same distance away from the surface 210 as during the first scan (steps 310 and 312 ). Alternatively, the sample surface 210 , by itself or in combination with the tube scanner 200 , may be offset in the horizontal and vertical directions. In one example, the vertical offset ΔZ is calculated using the following equation:

Δ Z=ΔX ×tan(φ)  (Equation 1)

After vertically offsetting the tube scanner 200 or sample surface 210 , the third electrode 206 is elongated by applying substantially the same amount of voltage applied to the third electrode 206 in the first scan, thereby engaging the probe 208 with the surface 210 (step 314 ). This is because the same separation distance between AFM tip and sample surface was maintained by adjusting Z position using step motor or other suitable method at the second location. Applying substantially the same amount of voltage permits the probe 208 to perform a second scan, at a different vertical height or at a different elevation along the surface 210 , with substantially the same tube scanner linearity and hysteresis errors (step 316 ). This, in turn, reduces the effect of having different linearity and hysteresis errors in the first and second scans because the hysteresis behavior is substantially the same for both scans. The second scan produces an image of the surface 210 .

In some larger size scan, AFM images contain artificial tube scanner bow error, but these errors can be mitigated. After the first and second scans have been produced, an image subtraction method is performed to obtain an accurate image of the topography of the surface 210 (step 318 ). Image subtraction is generally performed to eliminate effects like bowing effects, which are artificial errors caused by run out variation during scans. A corrected image can be created by subtracting the first scan from the second scan.

FIG. 4 outlines an alternative image subtraction method 400 , which removes a second scan thereby avoiding the influence of linearity and hysteresis errors. Using the first scan, a fitted mean profile of the first scan is created (step 402 ). Using a flat and undamaged region of a first scan image, a fitted mean profile is created by a polynomial fit method (step 404 ). A virtual reference scan image is generated by replicating the polynomial fit to the same size of the first image or extrapolating the polynomial fit to cover the length scale of the first image. Finally, a corrected image can be created by subtracting the virtual reference scan image from the first scan (step 406 ). The virtual reference scan image method can be used for samples with limited surface area and therefore limited scanning area. Alternatively, the virtual reference scan can be used as a reference scan for additional scans, thereby reducing the number of reference scans needed, which improves throughput.

It is to be understood that even though numerous characteristics and advantages of various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of various embodiments of the invention, this detailed description is illustrative only, and changes may be made in detail, especially in matters of structure and arrangements of steps within the principles of the present invention to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.