Hydrogen lithography

To begin automated HL, the location of every hydrogen atom in a select area needs to be determined for accurate STM tip registration during fabrication. Slight errors in the tip position can result in incorrect atoms being removed. Fast, fully autonomous lithography also requires the location of each atom to be known, such that the surface does not need to be reimaged after each removal event to determine the next site. The periodicity of the hydrogen-passivated Si(100)-2×1 surface (Fig. 1a) permits the location of every hydrogen atom to be determined from a single STM image (while accounting for nonlinearities in the scanner) through the use of Fourier analysis31 (Fig. 1b–f). Such an analysis is relatively immune to the presence of small surface defects and DBs due to their spatially localized nature in images compared to the extended periodicity of the surface itself. Figure 1 illustrates the basic features of this process. In practice, we use images between 10 × 10 and 40 × 40 nm2 to determine the location of the hydrogen atoms on a given sample terrace. Once the surface has been characterized, the desired pattern is mapped onto the lattice. Next, the tip is brought over each lattice site of the pattern and 20 ms voltage pulses between 1.8 and 3.0 V are applied at a fixed tip-height (V = 1.4 V, I = 50 pA) until the successful removal of hydrogen has been detected. Figure 2a, b, d shows the process of building DB structures (Fig. 2e) with HL while using HR to correct errors (Fig. 2c). Unlike conventional HL16,32, the STM feedback control is disabled during the voltage pulses. This allows jumps in the tunneling current to be used as an indicator of success15, which can be detected faster and more accurately than jumps in tip-height through the feedback circuitry (see Methods).

Fig. 1 Surface geometry of hydrogen-passivated Si(100)-2×1. a (V = 1.4 V, I = 50 pA, T = 4.5 K, 2 × 3 nm2) Scanning tunneling microscope (STM) image of hydrogen atoms bonded to the Si(100)-2×1 surface. The distance between identical sites along and across a dimer row are shown in yellow (solid). The surface geometry allows for the creation of single atom bits at ultra-high densities. The area of one bit (including spaces between atoms), 0.590 nm2, is outlined in yellow (dashed), and is defined as a binary zero. The hydrogen atom denoted with a dot (green) is removed with an STM tip to create a dangling bond, changing the bit to a binary one. b 2D-Fourier transform (power spectrum) of a, where the three dominant spatial frequencies have been isolated. c–e Filtered STM images of a, for each dominant frequency (peak) shown in b, c bottom peak (blue), d top peak (yellow), e middle peak (red). f The resulting image from the sum of frequencies shown in c–e, reconstructing the essential features of the STM image shown in a, allowing for the position of each surface hydrogen atom to be determined. Scale bar, 1 nm Full size image

Fig. 2 Fabrication and correction on hydrogen-terminated Si(100)-2×1 via hydrogen lithography and hydrogen repassivation. a (V = 1.4 V, I = 50 pA, T = 4.5 K, 10 × 10 nm2) Scanning tunneling microscope image of the fabrication area used to determine the location of each hydrogen atom through Fourier image analysis. b (V = −1.7 V, I = 50 pA, T = 4.5 K, 10 × 10 nm2) Fabrication of the characters “150” using hydrogen lithography (HL), one atom has been removed in error above the characters and one atom is incorrectly placed inside the “5”. The created dangling bonds (DBs) appear as bright protrusions. c, d Using the hydrogen repassivation technique, the errors were subsequently erased, then the correct hydrogen atoms were removed with HL to create an error-free structure. e (V = −1.7 V, I = 50 pA, T = 4.5 K, 11 × 21 nm2) The completed structure of 54 DBs, depicting the characters “150” and a maple leaf. Scale bar, 5 nm Full size image

Using this procedure, the probability of detrimental uncontrolled apex changes is low. By beginning removal attempts at 1.8 V (see Methods), higher voltages, which are more likely to change or damage the tip, are only reached when necessary. Conservatively, on the order of 10 DBs can be created consecutively without some type of minor modification to the tip. However, we have found that HL efficiency is not particularly sensitive to minor changes of the tip, so the actual number of DBs that can be created without altering removal efficiency during fabrication is often larger. Should the tip change so much that it is no longer suitable for HL purposes, an automated tip forming routine can be called to recondition the tip through controlled contact with the surface33. This routine takes advantage of a machine learning algorithm, and STM image data for training sets, to automatically identify the quality of the probe by imaging a DB, initiating reconditioning when necessary33.

An important consideration inherent in all scanned probe lithography is the existence of thermal drift and creep, both of which can also cause uncertainty in the position of the tip, leading to errors. At 4.5 K, these factors can be well controlled by allowing the STM to stabilize over a period of several hours. However, at warmer temperatures or in situations where allowing the STM to stabilize is not an option, a more active solution is required. To address these factors, we implemented periodic image realignments into the HL workflow. Before initiating the HL procedure, an area near the lithography location (around 10 × 10 nm2) is imaged as a reference. After a set time, lithography is paused, and this area is reimaged to determine how much the tip has been offset from its intended position due to creep and drift. The remaining sites in the pattern are shifted appropriately to compensate and lithography resumes. The effectiveness of this realignment can be increased by reducing the interval between reference checks, permitting an optimization between speed and accuracy depending on a given application. We found that without realignment the lithographic accuracy during HL using a non-stabilized STM was near 35% for a particular structure. Under the same conditions using moderate active realignment it was over 85%, which is within a suitable range to then correct the remaining errors using HR.

Hydrogen repassivation

To correct errors, we use the STM tip to address individual lattice sites with atomic precision to repassivate select DBs (Supplementary Figure 1). Ab initio calculations have revealed that certain silicon terminated tips with a hydrogen atom bonded to the apex (functionalized) are capable of repassivating DBs34. Through controlled contact of the STM tip with the surface during tip-conditioning, silicon atoms may become affixed to the tip to form the necessary structure for HR5,14,34. When the tip becomes functionalized with a hydrogen atom, there is a change in STM imaging resolution5,14. With a functionalized tip, the first step of HR is to position it over a DB at a sample voltage of 1.4 V and current of 50 pA. The feedback control is then disabled, and the sample voltage is changed to a value between 100 mV and 1.0 V. While recording the tunneling current, the STM tip is brought 500–800 pm towards the sample, then is retracted to its original position. The voltage is reset to the original value of 1.4 V and the feedback control is restored. This entire process, once a user has selected a site, has been automated, taking approximately 1 s. It can be initiated at the press of a button and repeated until a successful repassivation signature is observed. Work is underway to integrate this new HR process within the HL workflow to enable fully autonomous fabrication and correction. Errors will be automatically detected via image recognition, and subsequently corrected using the HR technique (see Methods).

A necessary element for the implementation of fully automated HR is the existence of a reproducible, easily distinguished, signature indicative of successful repassivation, much like the jump in tunneling current used during HL. We usually observe a sudden increase in the tunneling current while approaching the surface when a DB is successfully repassivated that is not present as the tip retracts (Fig. 3a). The measured current is related to the overlap of the imaging orbital of the tip, and orbitals of features on the surface16,35. We associate this signature (type-I) with the removal of a hydrogen at the tip apex and a return to pre-hydrogen-functionalized imaging resolution. The increase in current is possibly due to the new apex orbital having a larger spatial extent, creating a greater overlap between the tip and sample surface compared to that between a DB and a hydrogen-functionalized tip (held at the same tip–sample separation).

Fig. 3 Tunneling current signatures of hydrogen repassivation and the number of successful events with tip approach distance. a (V = 0.4 V, T = 4.5 K) The recorded tunneling current as the scanning tunneling microscope (STM) tip (set over a dangling bond (DB) at 1.4 V and 50 pA) is brought towards the surface (blue) and as the STM tip is retracted (red) during hydrogen repassivation (HR). A sudden increase as the tip approaches the surface can be observed in the tunneling current (blue), indicating a successful HR event. This signature (type-I) is typically associated with a change in STM imaging resolution, and a removal of hydrogen from the tip apex. b (V = 0.2 V) A second distinct signature (type-II) has also been observed during HR, with a sudden decrease in tunneling current during the approach towards the surface (blue). This type-II signature is not typically associated with a change in STM imaging resolution. c The tip approach distance where either signature occurred was recorded for 119 unique HR events. Nearly 90% of events occur before 550 pm. The inset depicts the STM tip approaching a DB on the surface Full size image

Following the same HR procedure with a tip that is not functionalized, we have found it is also possible to repassivate DBs with no associated change in imaging resolution. Since STM imaging resolution is particularly sensitive to changes in the tip apex orbital14,16,35, we assume the imaging orbital is unchanged and that the hydrogen atom comes from the off-apex region. In addition to functionalization, there are several mechanisms through which atomic hydrogen may be available on the (off-apex) surface of a tip for HR. The surface of silicon terminated tips can become passivated with hydrogen34, allowing atomic hydrogen to physisorb on top14 (see Supplementary Note 1, Supplementary Figure 2). If the tip is not silicon terminated, it has been shown that hydrogen is also capable of physisorbing on the surface of some metallic tips29,30,36. Since the precise structure of the tip is unknown, the available hydrogen on the tip is likely due to a combination of these mechanisms. We have found that the STM tip can be loaded with hydrogen atoms through the creation of several DBs (far from the current structure) using HL14. We observed that an average of three DBs (to a maximum of five) can be repassivated successively, without the need to load the tip with additional hydrogen (Fig. 4a). During HR, when the DB is repassivated with an off-apex hydrogen we see a second signature in the tunneling current (type-II), a sudden decrease (Fig. 3b) (also observed during HR at 77 K, Supplementary Figure 3). The sudden drop in current is due to a reduction in overlap between the tip and sample. DBs appear as bright protrusions on the sample surface (Supplementary Figure 1b) compared to the surrounding hydrogen (orbital more spatially extended towards tip). There is a decrease in the size of the surface orbital after the DB has been repassivated, which reduces overlap/current, as the tip orbital remains unaltered.

Fig. 4 8-bit and 192-bit atomic rewritable memories. a (V = −1.75 V, I = 50 pA, T = 4.5 K, 2.4 × 10 nm2) Scanning tunneling microscope (STM) images of a rewritable 8-bit memory constructed from dangling bonds (DBs). The DBs appear as bright protrusions in the images, the black defect is an area of missing silicon. The letters of the English alphabet were converted to their ASCII binary forms (e.g., A = 01000001) and written to the same area using hydrogen lithography and hydrogen repassivation techniques. An average of three bits could be repassivated sequentially between the letters shown. Scale bar, 3 nm. b (V = −1.8 V, I = 50 pA, T = 4.5 K, 21.5 × 10.7 nm2) An STM image of an expanded 192-bit memory, storing 24 simplified notes (converted into binary) of the popular Mario videogame theme song. The notes consist of 62 DBs, forming the largest error-free DB structure to date. Both memories have a bit density of 1.70 bits per nm2 and can be read directly from the STM or from a stored image. Scale bar, 4 nm Full size image

Unlike the type-I process, which theoretically relies on a particular tip state to enable the transfer of hydrogen34, the type-II process appears to be much less restrictive. We have been able to observe type-II HR events even after impressing the tip approximately 1 nm into the sample surface. Both processes have been observed and reproduced on a number of physically different tungsten tips, during the fabrication of numerous different structures. The structures in Figs. 2 and 4 were created using different tips for example. We recorded the location of type-I and type-II signatures in the tunneling current for 119 successful HR events collected using seven different tips (see Supplementary Figure 4 for additional recordings). Figure 3c shows the distribution of distances the tip traveled towards the surface for HR to occur. The majority of events (around 90%) occur before moving 550 pm. Closer tip approaches have an increased tendency to change the tip structure. This value provides an ideal parameter for fully automated HR, optimizing the probability of repassivation while mitigating that of harmful apex changes.

During our observations, we noted that when a tip is hydrogen-functionalized, as indicated by a change in STM imaging resolution, it is still possible to transfer an off-apex hydrogen to the surface (type-II signature) without altering the apex itself (leaving the tip functionalized). That is, with a hydrogen-functionalized tip, it is not guaranteed to first remove the apex atom and observe a type-I HR signature, as an off-apex hydrogen may be more mobile and easily transferred to the surface first, causing a type-II HR signature. We have also never observed sequential type-I signatures without deliberately re-functionalizing the tip in between HR attempts, suggesting the tip is unable to functionalize spontaneously with off-apex hydrogen. This observation is consistent with experimental imaging data, where spontaneous changes in image resolution with a tip suitable for HL/HR are rare.

With the ability to know when a tip is hydrogen-functionalized, and when the apex atom has been removed, it may now be possible to correlate dI/dV spectroscopy curves over hydrogen-terminated silicon with the specific tip states necessary for hydrogen transfer to occur34 (type-I). If such a correlation is found, dI/dV spectroscopy would provide a new, more rapidly acquirable metric to determine if the tip is suitable for HR. These curves could then be used as training data in the automated tip forming routine to always ensure the tip is in the required state, without the need to take an entire STM image33.

Atomic-scale memories

We used HL combined with HR to create two different working atomic-scale memories (Fig. 4). We defined a bit with four lattice sites, giving a one atom buffer between neighboring DBs (Fig. 1a). Due to the ideal geometry of the hydrogen-passivated Si(100)-2×1 surface, this arrangement yields an extremely high bit density of 1.70 bits per nm2. Conservatively, estimating a 20% reduction in storage density to integrate practical control architecture, this system still provides the highest density solid-state storage medium available. We used the 8-bit memory to sequentially encode the ASCII binary representation of each letter of the English alphabet, overwriting the previous letter each time (Fig. 4a). Writing each letter took between 10 and 120 s, depending on how many DBs needed to be created and repassivated. Random changes in the tip apex occurred infrequently, altering HL and HR efficiencies (remedied through controlled contact of the STM tip with the surface to condition the apex structure). The HR stage is currently the slowest step, limited by the number of available hydrogen atoms on the surface of the tip. Moving away from the structure to reload the tip after repassivating several DBs introduced a significant delay. This may only be a factor for structures with a small number of DBs, like those we have presented. With structures requiring more DBs, there will be a continued source of hydrogen to the tip as each new DB is created (equivalent to the reloading procedure). With enhanced automation to incorporate periodic intervals for HR/error correction during HL, the need to travel away from the structure to reload the tip can be reduced or altogether eliminated. Further improvements to HR speeds may be possible through tip materials like platinum, which is able to hold at least 1000 atoms of hydrogen on its surface29.

We created a larger 192-bit memory with the same bit density, encoding the iconic notes (simplified) of the Mario videogame theme song (Fig. 4b). This 62 DB arrangement took 250 s to create, and represents a leap forward in HL capability. We can read the 192-bit memory directly from the STM as the tip scans over the structure, or later from recorded images (see Supplementary Movie 1). The room-temperature stability of DB structures has already been well established in prior works4,17,18,26,28. DBs on the surface of silicon are known to face barriers to diffusion in excess of 1.4 eV in either direction12,13. Taking an exponential prefactor of A = 2·1014 s−1 and the lowest energy barrier of 1.46 eV13 (intradimer diffusion), a DB is expected to experience only one hop in over 500 years due to thermally activated processes at 300 K. Such stability and density make DB-based memories a unique candidate for data archival and long-term storage.

Held at 4.5 K, we detected no unintentional changes in the memory at the end of 72 h of observation (Supplementary Figure 5). In this environment, we have worked with samples that have shown no significant surface degradation after half a year. The ultra-high vacuum (UHV) requirements may also eventually be relaxed as isolated DBs can be protected against spontaneous reactions through the appropriate choice of doping level of the silicon substrate. Highly doped n-type silicon results in negatively charged DBs, which have a barrier to reaction with closed shell species37. There is also only a subset of entities that are known to readily react with DBs27,37,38,39. Molecular hydrogen, which is commonly present in vacuum environments, requires two directly adjacent DBs along a dimer row for adsorption40, reducing the likelihood of spontaneous repassivation of isolated DBs at greater separations (as in the memory) due to ambient gas. The inability to eliminate all naturally occurring DBs during sample preparation in an environment of 1·10−6 Torr of hydrogen gas, or with intentional chemical dosing further supports this notion27.