Highly oriented pyrolytic graphite Albrecht [1989], Bernhardt [1990],

Foster [1988], Li [1992], Miller and Hocken [1990] (and this thesis), Mizutani [1990], Penner [1991], Rabe [1990], Schneir [1988a], Shedd [1990],

Terashima [1990], Yau [1991a,b].

Holes as small as 2 nm and as large as 200 nm were produced by pulsing. There was a tip-dependent amplitude threshold necessary for

174 formation. Reports of this threshold varied, but the magnitudes were always greater than 2.5 V with some tip-sample configurations not producing any bits for amplitudes as high as 10 V. Near the threshold, the formation of mounds instead of holes was often observed. Mounds were typical with negative pulses applied to the sample. This thesis found mounds to be irreproducible by showing the necessity of increasing the pulse amplitude to continue mound formation. This thesis also shows these mounds to diffuse within minutes indicating that they would not be durable enough for a memory. The mounds diffused almost immediately when the bias was switched from negative to positive for a couple of seconds and then back to negative. Although longer times were necessary, holes were also observed to

disappear. Other reports claim stability of features. One possible explanation for mound stability could be in the material of the feature. If one feature is produced from tip material, while another is the result of contamination in the ambient, their surface diffusivities would also be different. This would be particularly true if one was charged and the other was not charged.

Using positive voltages, the predetermined reproducible formation of a mound or hole was not possible. The hole sizes were not found to be

reproducible but were often observed to increase in size with the number of bits produced. Given the same pulse parameters, this thesis shows holes varying in size from 1-20 nm. This irreproducibility is more than sufficient to make a memory inoperable. Although one tip successfully produced 496 holes out of 498 tries, not all tips produced bits.

Bits were made with pulse durations from 10 s to 10 ns. Bigger holes were not always associated with longer pulses. For pulse durations greater than 100 )s, the time to write 106 bits limits its capacity to be used as a high density memory.

UHV, and UHV vented to benzene or pure oxygen or 100 L water inhibited hole formation. In UHV, contamination deposits of 50 nm mounds were observed for 0.2 s pulses. A sample that had been stored in a container with water prior to alteration produced 10 nm bits during only 80% of the trials showing limited reproducibility. At a vacuum of 0.04 torr, increased pulse amplitudes were also required for successive alterations, illustrating

irreproducibility at this pressure. The changing daily ambient also played a role. Some experiments suggest that H2O in the ambient is necessary for hole

production1.

Under DI H2O, domes as small as 7 Å were produced having durability

>1 hr under normal imaging conditions. The use of water leaves something to be desired since the medium would have to be replenished due to

evaporation. The re-ionization of the water over time would produce an

unacceptable changing of parameters necessary for alteration. Coating of the tip except for the last few nm is also necessary due to higher leakage

currents.

In a 10-4 torr TMA (trimethylaluminum) ambient, pulsing produced features dependent on pulse duration. For 0.2 s pulses, Angstrom scale features were produced. The feature size is sufficient to produce an advanced memory, but the 0.2 s time required to write on the surface

176 makes it impractical. The reproducibility of features was also found to be unreliable in nature so that there is no assurance of alteration. In a 10-2 torr TMA ambient, an 18 nS × 440 nm (Al resonance) light pulse was used in conjunction with a 0.8-3 V bias. The result was deposits as small as 1 nm for a 1.1 V bias, excellent for a nanomemory bit size. By changing the light wavelength to 430 nm, deposition was rare. The effect of the light on reproducibility was not mentioned. The deposition showed linear I-V

measurements while the HOPG I-V was nonlinear. This would be excellent for an ac reading scheme.

Alterations in organic fluids, dimethyl phthalate, and decane were clouded by the fact that similar features were produced in air with similar pulse parameters. One experiment showed, however, that once a tip

produced a hole it was likely to form more holes indicating a tip dependence on alterability.

Under a drop of di(-ethylhexyl) phthalate, voltage pulses wrote features as small as 1-2 nm which could be erased or partially erased. The structures were not found to deteriorate during imaging. This provides an excellent write-read scheme. The 100 ns pulse required is also favorable for a memory. The reproducibility question was not addressed fully. The effect of subsequent voltage pulses was not consistent. Although this scheme seems promising, the reproducibilty may inhibit it from producing a viable memory. Experiments showing non-durability of bits on HOPG suggest using another substrate. There will also be a problem of maintaining the ambient over a long period of time.

Pulsing produced 1-2 nm adsorbate features in a monolayer

octylcyanobiphenyl film on HOPG. The duration of the adsorbate features varied from < 1 s to several minutes for others, not durable enough for memory. Holes, 10 nm in width were formed with 2 s duration pulses. This writing time would be to long for a memory.

Also reported was the inability to alter the surface under 1 atm of helium and 10 mbarr of dry toluene.

In a AgF solution, 50 )s pulses produced 30 nm mounds on 10 nm holes. The holes were produced during the first 5 )s of the pulse. Several pulses were necessary to initialize the tip, demonstrating that the tip had to be conditioned just right for alteration. The feature size may be acceptable for a first generation memory. The features demonstrated durability by not

changing shape on the order of hours and being unaffected by nearby depositions. The 90 % success at reproducing the structures was good, but for viability would have to be increased to enable memory segments to

accurately store information (possibly by read-rewrite scheme). It may be that an initial hole is necessary for formation. The main problem is maintaining the stability of the AgF solution over long time periods. The process needs to be characterized for the same tip over days and weeks.