Atomically Traceable Nanostructure Fabrication

Performing metrology on the nanostructures described above requires the ability to bridge the tip positioning during HDL and pattern location using other tools such as AFM and SEM. In contrast to other well-developed patterning tools with high-resolution position encoding such as e-beam lithography, the HDL performed here was performed with an STM without well controlled coarse positioning, so extra position identification protocols were used, as shown in Figure 3. First, a long-focal-length microscope is positioned outside the UHV system approximately 20 cm from the tip-sample junction. The sample is patterned with a square grid of 10 μm wide lines, 1 μm deep, on a pitch of 500 μm to facilitate identification of the tip location on the surface.

Figure 5. Pattern location images of sample. (A) Optical image of STM tip (left) and its reflection (right) in the Si(100) surface on an area of the sample with 500 μm pitch line pattern. The lines are 1 μm deep and 10 μm wide prior to UHV processing. Guide lines are included to show the line directions. (B) Close-up, de-skewed optical image of the tip (lower left) and its reflection (upper right). The centerpoint location between the tip and its reflection is identified within the 500 μm x 500 μm fiducial square. C: Closeup of patterning location with a 50 μm spot included for scale. D: 5 μm x 5 μm AFM image of an entire patterned area after ALD. E: 1 μm x 1 μm SEM image of one of the locator patterns after RIE. Please click here to view a larger version of this figure.

The first step to locating nanostructures is identifying the tip location on the surface,^32,33 which is accomplished in this case using a long working distance microscope. Figure 5A shows an optical image of the tip when engaged with the sample, with dotted lines added to guide the reader for the directions of the fiducial grid. To locate the tip/sample junction the optical image is unskewed to make a square grid, as shown in Figure 5B, although there are errors in high temperature processing of the samples due to significant surface atom migration. This reduces the depth and visibility of the fiducial grid as imaged here, increasing the uncertainty in the tip position.³² While it has been shown previously that high temperature sample processing will induce significant atomic scale surface reconstruction, the grid pitch used here is great enough to have little effect on the surface reconstruction in the middle of the squared defined by the grid.³⁴ However, near the edges of the patterned areas, step bunching does occur with asymmetry that depends upon the direction of current flow during sample preparation. ³⁴ Since the optical imaging is performed at an oblique angle relative to the surface, small changes in height on one side of a trench relative to the other will induce additional uncertainty in pattern location—especially when compared to plan-view imaging as in AFM or normal SEM. After the tip engages the sample, the 10 μm focal spot size of the microscope coupled with the ~20 μm post-processing fiducial linewidth results in an approximate uncertainty in pattern position identification of ±27 μm. This defines the search window for using various techniques for pattern identification.

To facilitate the location of the smaller 10–100 nm features, additional large serps are added near the nanoscale patterns, as shown in Figure 5B. These 800 nm x 800 nm serps are written using FE mode HDL with vertical lines and gaps of 15 nm each. By aligning the AFM fast scan direction to be perpendicular to the serp lines (i.e., horizontal scanning), these patterns tend to show a high contrast in the AFM phase image due to the high spatial frequency of the topography, further facilitating pattern location. Once these patterns are found, it becomes much easier to find the smaller nanoscale patterns which are placed with approximately 100 nm accuracy relative to the large patterns.

For this nanostructure fabrication process, the sample undergoes atmospheric exposure between each major process step once HDL has been performed, as shown schematically in Figure 1. Given this, it must be assured that the sample does not degrade at any point in the handling. As shown above, there is a finite amount of background deposition during ALD, which is assumed to seed on background defect sites.³¹ Thus, improper handling such as extended atmosphere exposure can increase the number of background defects and reduce the apparent ALD selectivity. An additional surface degradation mechanism can occur during venting of the sample from the UHV load-lock to atmospheric conditions.²⁹ To alleviate this problem, a spring-loaded sapphire chip which was mounted onto a linear actuator in UHV makes contact to the 125 μm thick sample mounting foil which contacts the sample to prevent surface degradation. Once the sample is at atmospheric conditions, the rate of dangling bond accumulation remains low (i.e., <0.1%/hr) for at least several hours, so as long as the sample is inserted into a stable environment such as ultra-pure Ar within less than 1 hr, the additional background deposition due to surface damage should remain low. At this point, it should be noted that the sample should not be stored in a vacuum environment, as this requires an additional vent/pump-down cycle, adding to the possibility of surface damage. This time between HDL and ALD is the point at which the sample is most sensitive since the etch mask has not yet been applied. After ALD, the sample still needs protection, but only to prevent additional mask growth due to silicon dioxide formation, a comparatively slow process.

In the patterns shown in Figure 4, the HDL removed >80% of the background H within the center of the patterns, with a spatial roll-off in the efficiency of the depassivation as the edge of the line is reached.²⁴ Given the limits of very limited ALD on the background and incubation free growth on fully depassivated patterns (Figure 3), the edges of FE mode patterns where there is a transition from fully effective HDL and no HDL, show a transition of effectiveness of ALD mask growth. Below 70% H removal during HDL is where this transition starts to occur, indicating an approximate region of ~2 nm on each side of an FE mode line where partial mask deposition occurs.³⁵ Additionally, ALD growth occurs in a “mushroom” manner,³⁶ further broadening the mask relative to the HDL patterns so that a mask of 2.8 nm broadens any mask features by that amount. To summarize, the ALD linewidth can be expressed as W_m = W_sat + f(δH) + M where W_m is the total width, W_sat is the width of the line where the HDL has saturated to remove >70% of the surface H, f(δH) is the additional width due to the growth at each point due to the density of H remaining on the surface, and M is the additional linewidth due to the mushrooming of growth. δH depends upon the spatial distance from the saturated edge of the HDL pattern, so f(δH) becomes f(r) since there is spatial dependence of the HDL. Of these terms, W_sat plays the primary role in the overall linewidth, and the other terms determine the degree of roll-off of the line edges.

With the ultimate nanostructure fabrication, the ALD mask alone does not determine the total feature size. Instead, the pattern size depends upon the degree of erosion of the substrate under the mask. The total etched linewidth is expressed as W_t = W_m – W_e = W_sat + f(r) + M – W_e, where W_e indicates an erosion linewidth, or pattern size reduction due to the etching process. This depends upon, among other things, the thickness and quality of the etch mask as described above for W_m. For a case where the linewidth simply requires the removal of mask before etching occurs, the W_e term is zero, however it is observed that there is a modification to the feature size after etching relative to the mask shape, suggesting that more complicated dynamics are at play.

Of the elements determining linewidth limitations, W_sat can be reduced to a minimum width of ~4 nm before the growth stops appearing the same as bulk ALD.³⁵ Of the other elements the mushroom growth effect, M (and as a consequence W_m), can only be reduced if the total film thickness is reduced, correlating with the total nanostructure height after etching. The line broadening effect due to the spatial dependence of the density of H remaining on the surface, f(δH), can be reduced to nearly zero by using multi-mode HDL which produces HDL line edges with negligible line edge roll-off.²⁴ To demonstrate the effect of this reduction in f(δH), Figure 6 shows a pattern array of squares produced using multi-mode HDL. The array includes patterns with HDL linewidths of 7 nm, 14 nm, and 21 nm from top to bottom, and inner HDL opening sizes of 7 nm, 14 nm, and 21 nm from left to right. While there is a slight misalignment of the multi-mode HDL in the bottom row, along the top row the registration is precise to <1 nm. After RIE, the lines remain primarily intact to widths of 5 nm with two small defects, and the openings between lines are resolvable for all of the patterns with the 7 nm holes barely resolvable using this metrology tool.

Figure 6. Linewidth and holewidth test. (A) STM of HDL of boxes written using multi-mode HDL. The linewidth of rows is 21 nm, 14 nm, and 7 nm from the bottom to the top, respectively, and the holewidth of the columns is 7 nm, 14 nm, and 21 nm from left to right, respectively. (B) SEM of the same patterns after ALD and RIE. Please click here to view a larger version of this figure.

The ultimate limits of this process depend upon selectivity of the ALD process, the quality of the HDL, the resistance of the mask to etching, and the desired feature shapes themselves. Methods to improve selectivity based on chemistry and background defect mitigation has already been addressed above. It has been shown previously that leaving H defects in the patterned areas reduces the quality of mask growth, and thus the resistance to etching.³⁵ Also, lack of careful control over the patterned line edges results in a mask “roll-off”, or excessive thinning of the mask along the edges of patterns which acts as a proximity effect preventing close placement of patterns. Fortunately, the selectivity of the etch process depends upon the mask thickness, so for spurious deposition on the background or defects along the edges of patterns the net effect is small. Furthermore, for structures shorter than 20 nm, thinner mask layers will likely be possible. Since the ALD growth occurs in a mushroom manner, thinner masks due to shorter structures will result in even better lateral control and smaller features than those demonstrated here. While the ultimate feature size reductions are not known for this process, certainly some down-scaling is likely.

While the SEM metrology leaves uncertainty regarding feature size and positioning, the first metrology step described in the top of Figure 1 gives atomic precision with regard to the HDL pattern as written. Since the Si(100)-H surface consists of a very regular lattice, and since the STM can be operated in a non-destructive imaging mode, the HDL patterns can be imaged without inducing further surface damage or further patterning, in contrast to other techniques such as e-beam lithography. With the atomic scale imaging of the invariant Si(100) lattice, the STM metrology eliminates the largest part of the positioning uncertainty related to the AFM and SEM metrology steps. In Figure 6B the box array appears skewed, for example. With the high resolution metrology of the STM giving atomic precision of the feature positions within the array, the apparent skew can be confirmed to be due to SEM imaging artifacts. Also, with highly precisely known spacings between the array features, an additional calibration uncertainty with respect to linewidths in the SEM images is eliminated.

This manuscript describes a nanofabrication method which utilizes the atomic precision of scanning tunneling microscope-based hydrogen depassivation lithography (HDL). HDL produces chemically reactive patterns on a Si(100)-H surface where atomic layer deposition of titania produces a localized etch mask with a lateral dimension demonstrated down to below 10 nm. Reactive ion etching then transfers the HDL patterns into the substrate, making 17 nm tall patterns with high precision lateral control. In order to achieve these results, samples must be protected during venting and transfer between instruments. With careful control of sample handling, nanostructures with traceability to the atomic lattice can be fabricated with atomic position precision and ~1 nm size precision.