A Fully Integrated, MEMS Based, Micro-Scale Printer for Cryogenic Thin Film Structures

Cryogenically produced thin film structures at the research scale facilitate many novels experiments. This paper discusses the construction of a fully integrated, MEMS based, printer, capable of printing micro and nano-scale features. In this millimeter-sized device, many features of a nanofab are incorporated to fabricate and characterize thin films in situ. The micro-scale printer comprises surface micromachined MEMS, digitally programmable source of atoms; a stencil lithograpghy tool that uses a MEMS nanopositioner for alignment; a substrate with on-chip leads for electrical characterization; a film thickness monitor; a substrate thermometer and heater. The device consists of three separate silicon die, flip-chip bonded to each other, forming a fully integrated, fabrication system of systems. This device creates micro and nano structures ranging from a few monolayers thick up to micron scale circuits. Applications range from searching for the Casimir Energy to the direct fabrication of quantum circuits, in situ, at cryogenic temperatures. [2022-0132]

rules at the local level that allow the fundamental elements to self-assemble. While capable of creating systems of great complexity (human beings, for example), the rules are highly complex and our understanding of how to do this is only slowly emerging. This is a high entropy solution to the problem.
The alternative approach, based on how we manufacture integrated circuits, is top-down. In this approach, we have deterministic control over the structures and devices and specify where and how material is placed. This approach endeavors to reduce the effects of entropy to zero, completely controlling where everything goes and minimizing the effects of randomness to as great an extent as possible. The current generation of 3D printers follows this top-down approach [5]. Manufacturing Science is being revolutionized by the development of 3D printing [6]. This technology allows for customized, high volume, low cost production of a wide range of objects and breaks the historical conundrum where one is forced to choose between custom/expensive or standardized/inexpensive.
Matter is composed of atoms and atomic-scale 3D printing is a universal and highly sought after goal by the researchers in this field [7], [8], [9]. Similarly, a micro-scale printer is a modest step in that direction. The micro-scale printer presented here can pattern deposits into various designs and thicknesses. These patterned films are quenched condensed and can be characterized with integrated electrical probes and on-chip heaters. In this paper, we describe the construction of such a micro-scale printer as illustrated in the conceptual rendering of Fig. 1. It comprises two basic technologies. The first, comparable to the conventional printer's ink source is a programmable atom source [10], [11], [12]. With it, we can evaporate many different kinds of atoms in amounts from attograms to nanograms. The second tool, comparable to the conventional printer's print head, is a dynamic lithography system that aligns the aperture with the substrate during evaporation [13], [14], [15], [16]. The dynamic lithography consists of a MEMS devices with separately controllable micromotors. These two technologies form the basis of the miniature printer allowing MEMS-based additive manufacturing at the micro and nano scale. This device provides a deterministic, top down control, building circuits and structures with a few monolayers at a time in situ at cryogenic temperatures [10]. This work builds upon the proposed idea presented in [12] and provides a novel and demonstrated solution. In this work, the writer die is introduced allowing dynamic stenciling, a larger capacity Physical Vapor Deposition (PVD) system is integrated to enable the dynamic writing of one or more materials, and the This printer is cable of the patterning of any structure or simple devices. Potentially, it can create metastable crystalline structures or molecules that do not naturally occur via normal kinetic pathways. Theoretically, this technology enables additive manufacturing to move beyond building existing items and begin exploring structures that chemistry cannot produce.

II. DESIGN
Actual images of the fully assembled printer are shown in Fig. 2. Visible are three separate silicon die that are flip-chip assembled to make the complete structure.
The bottom die is the programmable atom source [11]. With this source, one can thermally evaporate numerous metals in varying amounts, either concurrently or consecutively. The middle die is the lithography tool. It determines where the material is deposited. It consists of a MEMS plate with one or more apertures that is under the control of four electrostatic comb drives [17]. These motors move the plate in plane as the atoms pass through the backside of the die by way of a through wafer via. The position of the aperture(s) determines where the atoms land. The top chip is the target die. It consists of pre-positioned leads for in situ electrical measurements. The target die also contains a film thickness monitor, heater and thermometer for more refined metrology. Fig. 3a shows an actual printer, fully assembled with all three chips stacked together. As depicted, the die are shifted relative to each other to allow for access to the leads for each chip. In Fig. 3a, the source die is on the bottom, the writing die in the middle and the target die on top. The leads for the target die are  connected to pads on the writing die and those leads and the writing die leads are brought off chip via wire bonds on the middle die. The source die leads are also brought off chip directly to the wire bond pads seen on the lower chip. The schematic in Fig. 3c demonstrates how the electrical connections are brought off chip for the system and the internal setup. Fig. 3b is an optical micrograph of the assembled and packaged device. The package is a 20 lead, custom printed circuit board (PCB) with the height adjusted to allow for thermalization of experiments done at cryogenic temperatures. When the PCB is inserted into the cryostat the target die surface is in contact with the cold finger.
The result here is the construction a system of systems. This approach is feasible because the MEMS devices are fabricated with the MEMSCAP foundry services using the PolyMUMPs process for all the elements discussed here [18]. The foundry model ensures the process is stable from run to run, across an entire wafer. Therefore, all devices are consistent and perform reliably. This consistency provides a high degree of confidence that each assembled printer will perform as expected.
In what follows, each of the constituent chips are discussed separately: the source die, the writing die and the target die. Then the integration of the three die is reviewed and finally, experimental results are presented of the fully integrated system.

A. Atomic Source Die
The bottom chip of the printer shown in Fig. 1 is the atomic source die. The concept of its operation is shown in Fig. 4. The device is a micron scale PVD source. When heated, the atoms sublime or evaporate off the surface. Fig. 4a is a finite element model of the plate when tens of milliwatts of power are applied. The plate is fully suspended from the substrate and experiments are performed in a vacuum, so the only cooling mechanism is via blackbody radiation, thermal conduction through the springs, and the evaporant. This cooling process keeps the plate temperature quite uniform, to within ∼4 K [11]. The serpentine contacts to the plate are highly resistive due to their geometry and therefore heat when a current is applied. The springs' serpentine shape also minimizes mechanical stresses caused by thermal expansion of the plate (see Fig. 4a-c).
Metal atoms for deposition are prepositioned onto the source plates and boiled off due to its high temperature. In order to avoid forming a silicide with the metal atoms on the plate, the entire structure is coated using Atomic Layer Deposition (ALD) with 20 nm of Al 2 O 3 . This coating forms a physically strong, chemically unreactive layer for the deposition surface. Fig. 4c is an image of source plate (100 μm × 100 μm × 1.5 μm) used with this setup. The holes in the plate facilitate the release step of the MEMS device. Because of their small size, the thermal time constants of the atomic source plates are quite short, typically about a millisecond or so. This allows us to use Pulse Width Modulation (PWM) to control the rate at which atoms leave the surface of the structure. These sources have been extensively characterized and the details are described in Ref. 10-12, 19. In, Ag, Cu, Al, Pb, Fe, Sn and Au have been evaporated using these devices. Other materials are possible but have not been tested. By varying the area of the source and using the PWM technique, atoms can be emitted in amounts ranging from attograms to nanograms [11]. Due to a fast thermal time constant, a shutter is not needed. Fig. 4d shows an array of 30 sources used for the printer described here. The 5 × 6 array is divided into two groups so that two different metals can be evaporated separately or simultaneously. The sources in Fig. 4d are the dark grey squares near the center of the chip. The prominent white bars are on chip spacers which provide a standoff distance to protect the source plates from the writer die above. The wire bond pads are the white squares along the bottom edge of the die. As shown in Fig.1, this die is placed on the backside of the writing die, with the 30 source plates directed at the through wafer via.

B. Writer Die
The front face of the writer die is depicted in Fig. 5a. It consists of a square (150 μm × 150 μm × 1.5 μm) plate, connected via four pairs of tethers to four linear, electrostatic motors. One of the motors is seen in Fig. 5b and the writing plate is shown in Fig. 5c. As seen in Fig. 5a, the writing plate is located over a through wafer via [20], [21]. The through wafer via is marked by the darkened pattern behind the writing plate. The source die is positioned on the backside of the writing plate and the atoms from the source pass through the via and the aperture(s) on the writing plate to deposit on the target die in locations determined by the position of the plate.
The four bonding pads that surround the writing plate are used to make contact with the target die that is mounted facing this side of the writing plate. This orientation provides for the finest patterns. In this configuration, the writing plate is as close as possible to the substrate, reducing blurring effects.
Apertures are milled into the writing plate using a Focused Ion Beam (FIB). Such apertures can range from tens of nanometers to microns [22]. Nano-scale apertures are prone to clogging during deposition. Joule heating of the writer plate can prevent the aperture from clogging [23]. Fig. 5c shows an example of five apertures milled into the writing plate with the FIB and the depth of the through wafer via. The X axis displacement of the aperture plate as a function of applied voltage squared is seen in Fig. 6. The expected quadratic behavior is observed and the Y axis displacement is similar [24]. There are various writer designs that offer different capabilities. The writer in Fig. 5d allows the interfaces necessary to connect the mass sensor and temperature sensor. The large circular opening left of the writer plate permits the atomic flux to pass through the writer die to the mass sensor on the target die.

C. Target Die
The basic target die is shown in Fig. 7a. There are four gold leads which form a Van der Pauw geometry for resistance measurements [25]. The nearest corner to an adjacent lead is separated by 4.5 μm. The four gold contact pads at the corners of the cross make electrical contact with four similarly located pads on the writer die. These connections are then led off chip with wire bonds. Since deposited films are quite thin, special care must be taken to produce leads that taper such that atomically thin films can make contact. Fig. 7b displays the thinned end of one of the four gold leads. This lead is fabricated by using the native PolyMUMPs gold placed on the nitride layer that has been thinned by outlining the lead with a dimple layer plus the Poly1/Poly2 Via contour. This produces a gold film whose edges bleed out onto the substrate making it easier for electrical contact to be made with thin films and also produces a smoother surface for deposition.
A typical target die would have electrical leads on it as shown in Fig 7a. However, other devices and structures can be added for better characterization. In Fig. 7c, a target die is shown with both a thermometer and a film thickness monitor. The film thickness monitor is a MEMS trampoline resonator design shown in Fig. 7d [10]. This device has a resonant frequency of ∼80 kHz, a Q of ∼50,000 and a mass sensitivity with 10 s averaging of ∼2.5 fg, corresponding to ∼175 atoms/μm 2 . The device is designed so that the springs, seen in the print through of the structure surrounding the 4-sided diaphragm, are shielded from the atoms landing on the surface. The unshielded atoms then change the mass of the plate and not the spring constant of the structure. Thus, as atoms land on the structure, its resonant frequency is reduced by an amount proportional to the added mass [26].
The thermometer shown in Fig. 8a consists of a polysilicon layer directly deposited onto the nitride substrate layer of the PolyMUMPs process. It is a highly doped polysilicon layer that reduces the freeze out effect at cryogenic temperatures. Fig. 8b shows the resistance of this structure as a function of temperature. Using a conventional resistance bridge, we can resolve mK changes in temperature [10]. As depicted in Fig. 8b, the temperature sensor remains linear within the interested cryostat temperature range.

A. Integration of the Micro-Scale Printer
The three die described above, the atom source, writing plate and target die need to be processed, prepared and bonded to fully integrate the die set and create the micro-scale printer. This section describes the necessary steps to convert the die set as received from PolyMUMPs through the fully functioning assembly of the printer. Each die will be addressed in this process.

B. Writer Die Preparation
In order to create the void for the metal to evaporate through from the atomic sources, a backside etch (BSE) process is performed on the writer die. First, the non-device side of the die is thinned to ∼300 μm using a dicing saw. Next, a photoresist mask is patterned on the thinned side of the die and a Deep Reactive Ion Etch (DRIE) is performed where the native nitride layer of the PolyMUMPs process acts as a stopping layer. The die is then transferred to the FIB where a 95 μm hole is milled through the nitride layer at the bottom of the BSE under the writer plate. This produces an opening that allows access to the writer plate for the evaporant. Similarly, a 200 μm hole is milled through the nitride layer to allow access to the mass sensor. The aperture is then milled into the writer plate either from the top or bottom side of the die. Due to the depth of the BSE and difference between the angle of FIB and SEM images, it is easier to mill the aperture(s) directly from the top of the writer die [20], [21].
To release the moveable structures, the sacrificial oxide layer is removed with hydrofluoric acid. After releasing the writer die, it is placed in a critical point dryer to reduce stiction between the plate/tethers and the nitride layer. Since the target die has no direct ball bonding pads to the PCB, all electrical connections are made through the writer die. To generate this electrical and structural connection between these two die, a gold ball bump is employed. Ball bumps (∼ 90 μm diameter and ∼ 12 μm height) are created using a ball bonder on each of the gold pads as seen in Fig. 9a and their wire tails removed. Alternatively, Ag epoxy can be applied between the writer and target die bonding pads further reducing the distance between the writer plate and target.

C. Source Die Preparation
After releasing the source die, it is layered with 20 nm of Al 2 O 3 , as discuss previously. The source die is then loaded with the metal to be evaporated. This is done through PVD. Depending on the material being deposited either thermal or electron beam deposition is used. With the 5 × 6 source array up to 1.5 μm of material can be deposited on the source plates before the substrate and the plate begin to form a connection. For the information presented here, 1.0 μm of Pb was deposited onto the source die. During PVD onto the source die, the gold bonding pads are protected with a Ni shadow mask. This completes the source die preparation and the target does not require any preparation except for the sacrificial release.

D. Assembly of Micro-Scale Printer
The assembly of the printer requires the use of a Flip Chip (FC) bonder. Since the writer die has the through via it cannot be picked up by the FC bonder since the vacuum would destroy the nitride layer and writer plate. Therefore, the target die is picked up above the writer die and can be aligned with the split beam images to ∼1 μm. Fig. 9a shows such an alignment being conducted. Each die has alignment marks, which aid in the coarse positioning. For fine alignment, the aperture of the writer plate is repositioned with respect to the leads on the target die as in Fig. 9b. After alignment, the two die are bonded by applying 20 N at 200 • C for 5 minutes for Au ball bumps as shown in Fig. 9c. Alternatively, Ag epoxy FC bonding is conducted by applying 10 N at 150 • C for 10 minutes.
With a 5 × 6 array of atomic sources depositing over a distance of ∼300 μm there will be some blurring effect from the peripheral source plates. The 5 × 6 array dimensions are 675 μm×640 μm. To reduce the blurring effect, a spacer die is added to increase the standoff distance. Here, we use roughly the same dimensions as the writer die (2.5 mm × 2.5 mm × 0.310 mm). Fig. 9d shows the spacer used for our results. The spacer has a ∼500 μm × 500 μm void created by the same BSE process. This opening aligns with the source array and the through via of the writer die. By including the additional 310 μm spacer die the blurring effect is reduced by half. The FC bonding alignment is completed in the same manner as the target/writer die set for the remaining die. The two die are brought into contact with the spacer but no force or heat is applied. Since there is no electrical connection between die, a high viscosity cyanoacrylate glue is tacked to the sides of the die. This strongly adheres each die to the other but does not wick into the interior structure or interfere with the MEMS devices. Traces of the cyanoacrylate glue on the outside of the stack can be seen in Fig. 9c. The last die (source die) is then also FC aligned and attached. Again, the high viscosity glue is used instead of heating so that low melting point metals on the source plates are not affected. The assembled device is then electrically connected with ball bonds to the PCB and mounted in the cryostat.

IV. PERFORMANCE
When fully integrated, the system looks like the image shown in Fig. 2. Once all three die are aligned and bonded relative to each other, one has an integrated micro-scale printer. Below is the discussion of results from a series of different printers. Each experiment was designed to examine a different aspect of the printer. Because the printers are not reusable, each test below required a different printer.
Shown in Fig. 10a is the deposition of Pb at cryogenic temperatures as monitored by the mass sensor and the thermometer. A group of four source die loaded with 1 μm of Pb was evaporated by applying 5 V with 15 ms pulses every 2 seconds over 3 minute off and on cycles. The evaporation threshold for Pb is ∼ 4.5 V. In the beginning, the rate of evaporation is high as the sources are full of Pb. However, as the deposition progresses, the amount of material per pulse decreases until the sources are exhausted and 12 nm of Pb has been deposited. Note that the temperature increase during deposition is quite small, roughly 20 mK. This is a consequence of the nature of the source plates. The atoms leaving the atomic sources contain very little thermal energy. In a detailed study [12], we have shown that the atoms from the sources land on a substrate at cryogenic temperature with sufficiently little thermal energy that they do not form islanded structures as one normally finds at room temperature depositions. When the atoms land on a cryogenic substrate from these sources, they form disordered 2D films and do not ball-up. Thus, thin film nanoscale structures stay nanoscale. Fig. 10b is deposition of Pb data from a different printer using only half of the 5 × 6 array of source plates. Here, atomic sources were again loaded with ∼1 μm of Pb before assembling the printer. During this deposition, the pulse width was increased to 20 ms followed by a 2 second rest period. The pulse level was increased by 0.1 V every 30 seconds from 0 to 3.0 V. After 3.0 V the pulse level duration was increased to 1 minute for each 0.1 V incremental increase. Also, recorded in Fig. 10b is the cryostat sample temperature sensor and the printer temperature sensor located on the target die. There is excellent correlation between the two sensors. Both show a sharp increase in temperature with the printer temperature sensor being more responsive due to its proximity to the Fig. 10. Mass sensor and temperature sensor measurements. a) Measurements of the deposited mass and temperature, using sensor shown in Fig. 7d, during a deposition of Pb using sources shown in Fig. 4d. b) Cryostat temperature, temperature sensor resistance, bias voltage, and mass sensor frequency graphed as a function of time for the deposition of Pb from 15 atomic source plates. c) AFM recorded measurement of the Pb evaporated on target lead. Deposited Pb sample is highlighted on LHS with dashed outline. atomic sources. This increase in temperature, which peaks around the 1,500 seconds mark, is due to the evaporation of the Pb from the atomic source springs. The source plates resistance undergoes a dramatic increase from ∼14 to ∼170 . At this point, there is an expected corresponding decrease in temperature. The mass sensor also shows a comparable reaction to the sharp increase and decrease in temperature due to the change in Pb coverage of the atomic sources. When the pulse level is ∼ 4.6 V the Pb begins to evaporate, indicated by the decreasing frequency of the mass sensor. This evaporation of half the source plates results in ∼30 nm deposit of Pb.
For this printer to be useful, it must provide sufficient evaporated material on the target die. With the application of ∼1 μm of Pb to a 5 × 6 array of atomic source plates this is achieved. Using another printer and after fully depleting all atomic sources of Pb, AFM measurements were made on the Depicts alternative printer setups for increasing the deposition amount or patterning on other devices. a) Spacer die is attached over 400 μm × 400 μm source plate. b) Pb is loaded into the spacer die void and onto the source plate. c) Pre-bonded writer and target die is attached on top of the spacer die. d) Two die stack of source die and writer die shown for writing on other devices. target die. Fig. 10c shows that ∼70 nm of Pb (outlined in yellow) was deposited on the target die. Therefore, if loading two different metals on each half of the 5 × 6 atomic source array, ∼35 nm of material could be deposited simultaneously or consecutively. This amount of material is sufficient for many thin film research applications.
For dynamic stenciling, much more material is required for deposition to write structures. For this application a single more robust source plate [27] is substituted into the printer setup to increase the amount of material deposited. This source plate is fabricated using the MEMSCAP SOIMUMPs process [28]. Fig. 11a shows the 400 μm × 400 μm × 25 μm source plate with a spacer die attached using the same FC bonding process. The thickness of this source plate allows a large slice of a Pb pellet to be positioned on the plate within the void of the spacer die (Fig. 11b) using a micromanipulator. The alignment and assembly of the rest of the printer remains the same as seen in Fig. 11c. Using this deposition setup, applying 9 V to the source plate for 30 minutes, yielded ∼3.5 μm of Pb on the target die. With this printer setup, there is a wide range and amounts of deposited materials that can be achieved. These two options (5 × 6 array of 100 μm source plates or one 400 μm × 400 μm source plate) allow deposits ranging from a few monolayers to 3.5 μm of Pb.
The printer components are versatile and can be configured in many ways to accommodate the research goal. One unique option is using the printer without the target die attached. Here, the writer die can still be attached to either of the two source die and that die setup can then direct write onto other surfaces. Fig. 11d shows the setup, which allows customized application to fabricate nanoscale structures on fiber optics, metamaterials or other microscale devices. Fig. 12. Dynamic writing of patterns at ∼3 K using printer. The pattern of apertures in the writing plate is shown on the LHS of the image. The RHS shows two separate writing events. First, the lower structures were written, the source was turned off, the writing plate electrostatically repositioned and the upper set of figures were created. Printed in situ at cryogenic temperatures.
A moveable stencil is essential aspect of any printer. Fig. 12 shows the full functionality of the micro-scale printer. In Fig. 12 the LHS image shows the pattern of apertures on the writing plate. In the RHS the image displays the results of two depositions on the target die. First, the pattern in the LHS was positioned over the target die, the atomic sources were activated with 5 V and the lower pattern in the RHS was produced. The sources were turned off, the writing plate was electrostatically repositioned ∼1 μm by applying a 17.5 V bias to the Y axis electrostatic motor. The sources were reactivated and the upper pattern was created. These in situ depositions were conducted with Pb at temperatures of 3 K in Ultra High Vacuum (UHV). Each of the two evaporations yielded ∼10 nm of Pb and the observed blurring effect in both cases was negligible. The second deposited images did appear to narrow slightly by ∼50 nm perhaps due to the buildup of Pb around the aperture openings. The writer plate is estimated to have been ∼500 nm from the substrate. Here, Ag epoxy was used to adhere the writer die to the target die.
The target die leads were also tested as part of the printer. Using the nitride layer and the writing plate as a mask, Pb was deposited through a U-shaped aperture onto the four electrical leads (Fig. 13a). In this deposition the Ag epoxy bonded writer plate was ∼500 nm from the substrate and 1 μm was evaporated through the U-shaped aperture. This resulted in a film thickness of 66 nm of Pb on the target leads. Fig. 13a displays the deposited pattern and the blurring effect of 0.75 μm surrounding the deposited pattern.
In Fig. 13b another similar deposit was made in UHV and four point resistance measurements were recorded on the Pb sample between 5 K and 7 K. In this setup, 6 of the array source plates were partially evaporated having been preloaded with 1 μm of Pb. Fig. 13b shows resistance measurements of a superconducting sample with a transition temperature of 6.35 K. The superconducting transition temperature, resistance at 10 K and the mass sensor correlate to a deposit of ∼8.5 nm of Pb. This result, as expected, differs significantly from the bulk Pb superconducting transition temperature of 7.19 K due to the thin layer deposited [12].
The atomic sources can also be used as heaters. Exhausted source plates or unloaded plates can be used to heat the target die. In Fig. 13c the atomic sources, of the same printer presented in Fig 13b, are being used to heat the Pb sample on the target die leads. The first two peaks show the Pb transitioning from being superconducting while the cryostat temperature remains at 3.1 K. Unlike the cryostat temperature sensor, the calibrated target die surface mounted temperature sensor indicates superconducting transition temperatures. At ∼ 700 s in Fig. 13c, the temperature is being held on the shoulder of the superconducting transition by use of the heaters. Again, the proximity of the source die to the target when used as a heater allows very fine temperature changes. These fine adjustments can be made with as little as 10 mV increments and/or 1 ms adjustments to the pulse width. This fine tuning is a distinct advantage over the set point or heaters incorporated into typical cryostats.
Each aspect of the micro-scale printer has been evaluated. The printer using the 5 × 6 array of source plates is capable of depositing from a few monolayers to 70 nm of Pb. For dynamic writing, switching to the 400 μm × 400 μm source plate, the printer is capable of depositing 3.5 μm of Pb. The alignment between the writer aperture and the target leads is limited to 1 μm by the resolution of the FC bonder. The writer plate can be repositioned ±10 μm in both the X and Y axis. When applying 17.5 V to the electrostatic motor the redeposit was 1.06 μm ± 0.01 μm from the original deposit. Based on the current printer setup (writer plate thickness of 1.5 μm, distance between source plate and bottom of writer plate of 625 μm, smallest distance between substrate and top of writer plate of 0.5 μm, and effective source array width of 300 μm) the minimum blurring effect is 0.8 μm ± 0.1 μm. The temperature sensor resolves 1 mK temperature changes in close proximity of the deposit. The source plates, used as heaters, can accurately control superconducting transition with adjustments of either 10 mV or 1 ms of the pulse width. Lastly, there was good correlation between the mass senor and the AFM measurements of the deposited materials.
The micro-scale printer does possess some limitations. First, while the assembled device is inexpensive, it is not reusable. Preparation of the die set is time consuming but manageable. Separating the die set after bonding damages the MEMS devices and scatters debris on the die making them unusable. During the BSE process, the nitride layer can be damaged causing cracks and elevating the writer plate. Stiction between the writer plate and elevated nitride layer can impede movement of the plate. The current blurring effect for nano sized apertures is significant and limits achievable patterns. While capable of printing other metals [11], only Pb has been used in these results. As referenced earlier narrowing or closing of small apertures can limit the size of pattern. Au ball bumps increase the distance between the writer plate and the substrate. If blurring is a concern to the deposited pattern, Ag epoxy is an alternative to the Au ball bumps. The thermal response time of the 400 μm × 400 μm plate, due to the larger dimensions of the plate and increased mass of the evaporant, is on the order of seconds compared to that of the 5 × 6 array source plates of just a few milliseconds. Since PWM is not used with the larger source plate when depositing, an increase in the sample temperature by 3.4 K has been observed rather than the 150 mK increase using the 5 × 6 array with PWM.
The micro-scale printer can be further optimized to resolve some of the limitations highlighted above. One of the first steps would be to use the printer to characterize the deposit of multiple metals either simultaneously or sequentially in layers. Another enhancement would be to reduce the blurring effect. By pre-thinning the writer plate with the FIB to 200 nm, doubling the thickness of the spacer, and limiting the source plate effective width to 300 μm, the blurring effect could be reduced to 30 nm. Another writer configuration [29] (not shown) provides 5 degrees of freedoms. Incorporating this writer into the printer with its z axis writer plate adjustment would help further reduce blurring from source plates on the exterior of the array. Doubling the area of the writer die would allow the incorporation of the 5 degrees of freedom writer design, a shutter for use with the 400 μm × 400 μm source, and a circuit to heat the writer plate to reduce clogging of the aperture.

V. CONCLUSION
A couple of technologies allow this approach to be possible: the sensitivity of MEMS technologies that permit a writer plate to be positioned with nanometer accuracy and the capability of FIBs to mill silicon structures with nanometer scale apertures. The basic idea behind this device is that one can print with a stream of atoms by engaging one or multiple source plates while patterning the emission with the dynamic writer plate. There are many advantages to this approach: 1) One can deposit small amounts of atoms; (a) the thermal response time of the micrometer-sized sources allows for rapid start/stop action, (b) a responsive shutter can be incorporated to limit the number of atoms, and (c) the tens of nanometers sized aperture minimizes the number of atoms passing through. 2) Many control features and options for fast processing and unique structures are available; (a) arrays of apertures on the writer plate can fabricate many devices in parallel and (b) specifically sized and shaped apertures allows for the creation of novel structures with the dynamic patterning. 3) Multiple materials are capable of being deposited in situ; (a) conducting and nonconducting materials could be deposited to fabricate new structures and junctions and (b) because of these conducting and insulating materials, NEMS as well as electrical circuits are possible. 4) Due to the small footprint of the printer, depositions can take place in specialized environments, such as a cryostat [10]. Consequently, the integrated printer can address four important application areas. These include combinatorial materials science, direct printing of molecules, printing optical metamaterials with small numbers of monolayers and in situ printing of superconducting Quantum circuits and Casimir energy experiments [19], [30], [31], [32], [33].
In this paper, we described how we built a fully integrated, micro-scale printer. By combining a source die, a writer die and a target die, one can construct a system of systems for doing micro and nanoscale research on novel circuits and materials.