Abstract
A method of batch-fabricating an array of probe devices for a surface analysis instrument, such as an atomic force microscope (AFM), includes providing a wafer, and photolithographically forming a base and a cantilever for each probe. The cantilever includes a built-in angle, θ, relative to the base, and the base is substantially parallel to a sample holder when the probe device is mounted in a probe holder of the surface analysis instrument.
Claims
1. A method of batch-fabricating an array of probe devices for a surface analysis instrument, the method including: providing a wafer; photolithographically forming a base and a cantilever for each probe device; and wherein the cantilever includes a built-in angle, θ, relative to the base.
2. The method of claim 1, wherein the base is substantially parallel to a sample holder when the probe is mounted in a probe holder of the surface analysis instrument.
3. The method of claim 1, wherein the providing step includes bonding an off-axis wafer (111) with a substrate wafer.
4. The method of claim 3, further comprising reducing the thickness of the off-axis wafer by at least one of grinding and polishing.
5. The method of claim 4, further comprising growing an oxide on the wafer and patterning the array.
6. The method of claim 5, further comprising etching the off-axis wafer to leave a portion of the off-axis wafer having an angled surface relative to the substrate wafer for each probe, and wherein the angle, θ, is 8°-15°.
7. The method of claim 6, wherein the etching step uses a KOH etch for a selected amount of time.
8. The method of claim 7, further comprising: depositing a silicon nitride layer on the wafer; photolithographically patterning the backside of the wafer; and etching the backside of the wafer with a KOH etch to release the backside, and stripping the silicon nitride layer.
9. The method of claim 8, further comprising depositing a second layer of silicon nitride on the wafer, and then patterning the cantilevers of the probe devices.
10. The method of claim 3, wherein the substrate is a silicon wafer with an oxidized surface.
11. A probe device for a surface analysis instrument including a probe holder, the probe device comprising: a base; a cantilever; wherein the probe device is mounted in the surface analysis instrument at an angle, β, between the base and a surface of the probe holder of the surface analysis instrument that is substantially parallel to a mounting surface of a sample holder of the surface analysis instrument, and wherein β is 0°.
12. The probe device of claim 11, further comprising: a tip; and wherein the cantilever extends from the base at an angle, θ, thereto, so that when introduced to a sample, the tip extends in a direction substantially orthogonal to a surface of the sample.
13. The probe device of claim 12, wherein the angle, θ, is 8°-15°.
14. The probe device of claim 13, wherein the angle, θ, is about 12°.
15. The probe device of claim 11, wherein the cantilever is made of silicon nitride (Si.sub.3N.sub.4).
16. The probe device of claim 12, wherein the tip is a diamond-like carbon (DLC) tip.
17. The probe device of claim 11, wherein the surface analysis instrument is an AFM.
18. An AFM probe device microfabricated by a process comprising the steps of: providing a wafer; photolithographically forming a base and a cantilever for each probe device; and wherein the cantilever includes a built-in angle, θ, relative to the base.
19. The probe device of claim 18, wherein the base is substantially parallel to a sample holder when the probe device is mounted in a probe holder of the AFM, and wherein the angle, θ, is 8°-15°.
20. The probe device of claim 18, wherein the providing step includes bonding an off-axis wafer (111) with a substrate wafer.
Description
BRIEF DESCRIPTION OF THE DRAWINGS
[0024] A preferred exemplary embodiment of the invention is illustrated in the accompanying drawings in which like reference numerals represent like parts throughout, and in which:
[0025] FIG. 1 is a schematic illustration of a Prior Art surface analysis instrument such as an atomic force microscope (AFM);
[0026] FIG. 2 is a schematic side elevational view of a Prior Art probe fabricated in conventional fashion mounted in a probe holder, the probe holder manufactured with an angle so as to allow the tip of the probe to extend orthogonally to the sample surface during AFM operation;
[0027] FIG. 3 is a schematic side elevational view of a probe mounted in an AFM head and fabricated according to a preferred embodiment with a cantilever extending from the base of the probe with a known angle to accommodate orthogonally positioning the probe tip relative to the sample;
[0028] FIG. 4 is a flow chart illustrating a method of fabricating a probe with an angled cantilever such as that shown in FIG. 3;
[0029] FIGS. 5A-5L are schematic side elevational views of an AFM probe being microfabricated according to the method of FIG. 4 so as to have a cantilever angled relative to the probe base, according to a preferred embodiment;
[0030] FIG. 6 is a schematic side elevational view of a probe fabricated according to the method shown in FIGS. 4 and 5A-5L; and
[0031] FIG. 7 is an image of a probe fabricated according to the method shown in FIGS. 4 and 5A-5L, according to a preferred embodiment.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0032] Referring again to FIG. 2, probe device 50 for use in a surface analysis instrument such as an atomic force microscope (AFM) and fabricated in conventional fashion must be mounted in the AFM head/probe holder 52 at an angle ß, between a bottom surface 69 of base 54 of probe device and a bottom surface 68 of the AFM/probe holder. In this way, the probe tip is substantially orthogonal to the plane of a sample holder 67 supporting a sample 64 having a surface 65 to be imaged. This is typically accomplished with an appropriately machined probe holder that can then be mounted in the AFM head. The drawbacks of this arrangement have been discussed previously, including XY positional variation of the probe that directly effects the tip's Z position when employing an angled mount.
[0033] Turning to FIG. 3, a probe device 70 including probe 71 having a cantilever 72 extending from a base 74 of device 70 formed from a substrate (e.g., photolithographically patterned on a silicon wafer) includes a free end 76 supporting a tip 78. Tip 78 of probe device 70 includes a distal end 80 (i.e., apex) that may have a hemispherical shape of varying radii, and a conical or pyramidal shaped base or body. Notably, unlike the cantilever of probe 56 of probe device 50 of the prior art (FIG. 2), which extends linearly from its interface with the base to its distal end supporting its tip 62, cantilever 72 extends from base 74 at a set angle, θ. The fabrication of probe device 70 with built-in angle, θ, is discussed further below. The operational difference is illustrated by considering FIGS. 2 and 3 together.
[0034] With the known probe 56 in FIG. 2, the AFM head or the probe holder 52 must be configured with an angle ß so that when probe device 50 is mounted in the head, the laser deflected off the backside of the cantilever is directed to the sensor of the optical deflection detection arrangement. In this design, tip 62 extends substantially orthogonally to the sample surface 65 as the two are brought in to contact. A spring-loaded clip 66 holds probe device 50 in a probe holder 52 so that this relationship between probe tip 62 and sample 64 is maintained. In the preferred embodiments, in contrast, the AFM head/probe holder 82 does not have to be configured with an angle to allow this orthogonal relationship between tip 78 and a sample 90 supported by a sample holder 92. In this case, the angle between a bottom surface 77 of probe holder 82 and a bottom surface 75 of base 74 of probe device 70 is zero (ß=0°), and the probe device 70 itself is designed with the requisite angle (θ) to accommodate the deflection detection apparatus. Notably, angle, θ, may need to be different for different probe designs and different samples to maintain the orthogonal tip-sample relationship and accommodate the optical deflection detection scheme. Incorporating the angle in the probe design lends flexibility to the instrument design. To wit, when the probe is fabricated with a built-in angle, a single AFM head/probe holder design 82 can be used across varying applications with different types of probes and samples to be imaged.
[0035] The microfabrication process for making probes with this built-in angle is set forth in the flow diagram of FIG. 4, and will be described together with an illustration of each step of the probe being fabricated in FIGS. 5A-5L. In FIG. 4, a method 100 of fabricating AFM probes having a built-in angle is shown in FIG. 4. Initially, in Step 102, an off-axis wafer 200 with a bare surface is, preferably, thermal fusion bonded to a substrate wafer 202 (e.g., standard <100> orientation silicon wafer having an oxidized surface), leaving an oxide layer 204 between the two (FIG. 5A). It is the crystalline structure of the off-axis wafer that ultimately produces the angle of the cantilever of the probe, as described further below. The off-axis wafer can be defined by a: <111> orientation processed 0 degrees towards the <100> crystal direction. The bonded wafers shown in FIG. 5A have a front side 201 and a back side 203. Next, method 100 includes reducing the thickness of the off-axis wafer 200 in Step 104 leaving a thickness, t, of wafer 200 (FIG. 5B), typically by grinding and/or polishing. This thickness, t, is in the range of about 20-40 microns, and preferably about 30 microns, and contributes to the length of the cantilever of the probe, as shown and described further below.
[0036] Method 100 then includes, in Step 106, growing an oxide layer as shown in FIG. 5C, leaving a layer 206 on the front side 201, and a thicker layer of oxide 208 on the backside 203 (thicker because of the oxidized surface of the standard wafer).
[0037] In Step 108, the front side oxide layer 206 is patterned using standard photolithography techniques including a masking film, thereby leaving portions of oxide 210 of oxide layer 206 as shown in FIG. 5D.
[0038] Turning to FIG. 5E, an etch of the off-axis wafer is performed in Step 110. The time and material of the etch is controlled to produce the desired lever angle. Preferably a KOH etch is employed. More particularly, the parameters of an anisotropic KOH etch are controlled to etch along the crystalline plane of the off-axis wafer to produce angle, θ. Typical ranges for angle, θ, are 8°-15°, and more preferably, 10-12° or 11-13°, with about 12° being preferred.
[0039] Next, in Step 112, silicon nitride Si.sub.3N.sub.4 is grown on the bonded wafer as a layer 212 (FIG. 5F), and then an appropriate mask is deposited and the nitride is patterned as shown. This backside lithography step, Step 114, will ultimately define each base of the field of probe devices being fabricated. The backside patterned nitride exposes wafer 202 at strategic locations 214 (FIG. 5F) relative to the frontside lithography that creates angle θ (FIG. 5E). In that regard, a backside etch (preferably, KOH) is performed in Step 116 (creating angled surfaces 216 of the probe device base, see FIG. 5G). The exposed surface locations 214 of the standard wafer are strategically placed so when the backside KOH step is performed the point the levers begin to angle is near the edge of the base (discussed and shown below).
[0040] The nitride is then stripped in Step 118 (FIG. 5H). The cantilever material is then grown, e.g., silicon nitride of a selected thickness (218 in FIG. 5I), in Step 120. Front side and back side surfaces include a silicon nitride layer as shown in FIG. 5I. Another photolithography step is used to pattern the field of levers on the front side of the bonded wafer in Step 122. As shown in FIG. 5J, the levers include a portion 220 of nitride 218 that extends along the surface of the substrate and a portion 222 that is angled and will be the operational portion of the cantilever. Next, a backside strip of the nitride is performed in Step 124 (FIG. 5K). Lastly, another KOH etch is performed in Step 126 to release the probes from the substrate and the oxide is stripped from the wafer. As a result, method 100 yields a plurality of probe assemblies 300 having cantilevers 302 extending from respective bases 304, as shown in FIG. 5L.
[0041] FIGS. 6 and 7 more specifically show the resultant product using method 100 of FIGS. 4 and 5. In the schematic illustration of FIG. 6, probe device 300 includes a cantilever 302 extending from a base 304. Lever 302 forms an angle θ to base 304 that again is preferably about 12°. At a distal end of lever a tip 306 is supported. Tip 306 may be glued (DLC tips) or formed using known techniques such as electron beam deposition (EBD SEM and can be oriented at any angle relative to the base. Overall, tight control of the location of the tip relative to the free end of the cantilever is realized with the preferred embodiments. An image of a resultant probe assembly is shown in FIG. 7. By forming the probe devices in this fashion, with cantilevers 302 having a built-in angle, θ, access to the laser beam reflected off the backside of the cantilever is provided without having to angle the AFM head or probe holder. Moreover, tip 306 will extend substantially orthogonally to the sample surface when the two are introduced. Without requiring that the AFM instrument provide the angle to accommodate the optical deflection detection apparatus, and achieve this substantially orthogonal relationship, AFM performance is improved. For example, the user benefits from better mass balance between mount and probe, leading to improved imaging in certain types of applications, such as in Peak Force Tapping (PFT) mode of AFM imaging.
[0042] Also, the user experience is very repeatable and consistent from probe to probe when batch fabricated as described herein. Moreover, any XY positional variation of the probe that may directly affect the tip's Z position with an angled mount is minimized. Transferring the angle from the mount to the built-in angle of the cantilever negates any Z effects due to XY probe positioning on the mount. An additional benefit of transferring the angle from the mount to the built-in angle cantilever is distributing most of the mass of the cantilever in a plane orthogonal to Z motion. In so doing, the dynamic stability of the scanning assembly can be improved, through the reduction of induced parasitic motions related to the tilting of the tip mass in the Z direction from an angled tip mount. In sum, application of the preferred probe design and fabrication techniques encompasses a broad spectrum of already existing AFMs, probes and their associated applications. Moreover, the tips on these angled cantilevers are typically diamond-like carbon, and therefore, the lifetime of the probe devices is enhanced.
[0043] Although the best mode contemplated by the inventors of carrying out the present invention is disclosed above, practice of the present invention is not limited thereto. It will be manifest that various additions, modifications and rearrangements of the features of the present invention may be made without deviating from the spirit and scope of the underlying inventive concept.
