Laser-induced forward transfer digital printing of silicon thin films and devices

Abstract

Laser-induced forward transfer (LIFT) digitally prints silicon thin films and devices using engineered thin donors with a silicon thickness to optical absorption depth ratio L Si / d Si < 1, where d Si is the optical absorption depth of silicon at the transfer laser wavelength (~532 nm). Silicon droplets ejected through a transparent donor solidify on a receiver as voxels; tessellating the voxels with controlled overlap (~65%) forms dense films about 0.5–2 μm thick. The films are laser-sintered with a defocused beam to reduce surface roughness (e.g., ~450 nm to ~150 nm) and to increase free-carrier mobility (up to ~100 cm² / V·s) with minimal change in carrier concentration. Donor doping (n-type or p-type) sets carrier density across semiconducting and semi-metallic regimes. Printed structures include p-n diodes, n-p-n bipolar transistors, and interconnects on glass or polymer substrates, enabling integrated circuit elements.

Description

LASER-INDUCED FORWARD TRANSFER DIGITAL PRINTING OF SILICON THIN FILMS AND DEVICESRELATED APPLICATIONS

[0001] This application claims the priority benefit of U.S. Provisional Application 63 / 760,263, filed 19 February 2025.TECHNICAL FIELD

[0002] The present disclosure relates to digitally printing silicon thin films and silicon-based electronic devices using laser-induced forward transfer (LIFT). It further relates to additive manufacturing of semiconductors, printed diodes and transistors, and digitally printed integrated circuits on rigid or flexible substrates.BACKGROUND

[0003] Additive manufacturing of electronics has leveraged organic semiconductors, metal oxides, nanoparticle inks, carbon nanotubes, and 2D materials. Despite extensive study, digital printing of silicon — the dominant semiconductor for high-mobility devices and integrated circuits — has not been available. Nozzle-based inkjet approaches are constrained by ink rheology, while solid-donor LIFT processes with thick donors can generate conductive debris that short devices and prevent reliable integrated circuitry. There is therefore a need for a debris-free digital printing process for silicon that yields semiconducting films with practical carrier mobility and supports device- and circuit-level fabrication.

[0004] The current trend in future electronics where electronic devices will be integrated and linked with sensors, communication devices, displays, wearable and flexible electronic systems to create the so called - a world of “Internet of Things” (loT), requires the development of advanced additive manufacturing techniques that are free from limitations imposed by the traditional, silicon substrate-based, semiconductor processing technology. In particular, digital printing (DP) of electronics is a highly desired technology due to its distinctive drop-on-demand features that allow custom-made production of electronics with a minimum waste of materials and premanufacturing stages. It is expected that DP of electronics would allow a whole new class of production where modeling, design, prototyping and fabrication are just few ‘clicks’ away from constructing novel objects and systems, thus making the developing stage of numerous applications simple, fast and much less expensive (e.g., allowing a development in parallel to production).

[0005] Notwithstanding extensive development in silicon processing — including microelectronics, polycrystalline silicon thin-film transistor (TFT) technology, and nanoimprint techniques — practically deployable methods for digitally printing silicon suitable for device- and circuit-level fabrication have not yet achieved broad implementation. In parallel, a variety of digital printing approaches have been investigated for other semiconducting materials, such as organic semiconductors, metal-oxide semiconductors, nanoparticle-based inks, carbon nanotubes (CNTs), and emerging two-dimensional (2D) materials. While these approaches have enabled notable demonstrations, their electrical performance and integration characteristics often do not match those attainable with silicon-based processes, particularly with respect to charge-carrier mobility, tunability of carrier concentration, and monolithic integration of complex circuitry. Accordingly, there remains a need for digital printing techniques that can produce silicon thin films with properties and manufacturing attributes compatible with reliable electronic devices and integrated circuits on a range of substrates.SUMMARY OF THE INVENTION

[0006] In one aspect, a laser-induced forward transfer (LIFT) process digitally prints polycrystalline silicon thin films from engineered thin donors having a thickness-to-optical-ab sorption-depth ratio LsJdsi less than 1 (e.g., about 0.1-0.2 at the transfer laser wavelength of -532 nm). Using pulsed laser energy on the order of -0.5 pj focused through a transparent donor substrate, individual silicon droplets are transferred to a receiver substrate to form uniform, debris-free voxels that are tiled with controlled overlap to define thin films. As used herein, “digital printing” means a maskless, digitally addressable, direct-write deposition process in which discrete, software-specified volume elements (voxels or droplets) of a material are selectively placed onto a substrate on demand, in a pattern defined by digital data (e.g., CAD or toolpath instructions), without use of physical masks or fixed printing plates. In the embodiments described, digital printing is implemented by LIFT, where individual laser pulses eject and place discrete voxels of silicon from a donor to a receiver, so that thin films and device features are built voxel-by-voxel and layer-by-layer.

[0007] In another aspect, the printed silicon is laser-sintered using a defocused beam to reduce surface roughness and significantly improve free-carrier mobility (e.g., approaching -100 cmW-s for lightly doped films) without substantially changing carrier concentration. Both / / -type (e.g., phosphorus-doped) and / / -type (e.g., boron-doped) donors are provided to realize semiconducting regions (1014— 1018cm'3) and semi-metallic regions (1019— 1021cm'3).Further aspects include digitally printed electronic devices fabricated from the LIFT-printed silicon films, including rectifying p-n diode junctions, n-p-n bipolar junction transistors (BJTs), and a differential amplifier as a proof of concept for printed integrated circuits. Thin donors mitigate debris formation enabling reliable device isolation and interconnects.

[0008] These and other embodiments of the invention are described in more detail in the description below.BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present invention is illustrated by way of example, and not limitation, in the figures of the accompanying drawings, in which:

[0010] Figure 1 A depicts schematics of the LIFT setup including a pulsed laser source, a cross-polarizer variable attenuator, and an objective lens that focuses the laser beam onto a donor substrate to eject a silicon droplet toward the receiver, in accordance with one embodiment of the invention.

[0011] Figure IB depicts an optical image of a digitally printed 200 x 200 pm2silicon square produced using a thin donor, in accordance with one embodiment of the invention.

[0012] Figure 1C depicts an optical image of a digitally printed 200 x 200 pm2silicon square produced using a thick donor, where debris formation is clearly visible, in accordance with one embodiment of the invention.

[0013] Figure ID depicts a 3D optical profilometer image of a large-area Hebrew University logo (~2 x 2 mm2) printed using a thin donor, showing no debris across the area; the inset optical image likewise shows no indication of debris, in accordance with one embodiment of the invention.

[0014] Figure 2A depicts an optical image of arrays of LIFT-printed silicon droplets arranged in columns of five, where the laser pulse energy increases by column from 0.06 pl to 9 pl; arrows indicate the selected 0.5 pl working energy used for subsequent experiments, in accordance with one embodiment of the invention.

[0015] Figure 2B depicts a scanning electron microscope (SEM) image of the same LIFT-printed droplet arrays, likewise showing columns spanning 0.06 pl to 9 pl, with arrows marking the chosen 0.5 pl condition, in accordance with one embodiment of the invention.

[0016] Figure 2C depicts a 2D optical profilometer map of several droplet columns, with a dashed square highlighting the column printed at 0.5 pl and the specific droplet selected for a line scan, in accordance with one embodiment of the invention.

[0017] Figure 2D depicts a one-dimensional profilometer scan across the diameter of the droplet highlighted in Figure 2C, in accordance with one embodiment of the invention.

[0018] Figure 3 A depicts a 2D optical profilometer image of a LIFT-printed 200 x 200 pm2silicon square formed as a single layer (IX) of overlapping droplets, in accordance with one embodiment of the invention.

[0019] Figure 3B depicts a 3D optical profilometer image of the single-layer (IX) 200 x 200 pm2silicon square depicted in Figure 3 A, in accordance with one embodiment of the invention.

[0020] Figure 3C depicts a one-dimensional profilometer scan along the single-layer (IX) 200 x 200 pm2silicon square depicted in Figure 3A, in accordance with one embodiment of the invention.

[0021] Figure 3D depicts an optical image of the single-layer (IX) 200 x 200 pm2LIFT-printed silicon square, in accordance with one embodiment of the invention.

[0022] Figure 3E depicts a 2D optical profilometer image of a LIFT-printed 200 x 200 pm2silicon square formed as a double layer (2X) of overlapping droplets, in accordance with one embodiment of the invention.

[0023] Figure 3F depicts a 3D optical profilometer image of the double-layer (2X) 200 x 200 pm2silicon square depicted in Figure 3E, in accordance with one embodiment of the invention.

[0024] Figure 3G depicts a one-dimensional profilometer scan along the double-layer (2X) 200 x 200 pm2silicon square depicted in Figure 3E, in accordance with one embodiment of the invention.

[0025] Figure 3H depicts an optical image of the double-layer (2X) 200 x 200 pm2LIFT-printed silicon square, in accordance with one embodiment of the invention.

[0026] Figure 4A depicts a bright-field optical image of a 2 pm-thick LIFT-printed silicon square before sintering, in accordance with one embodiment of the invention.

[0027] Figure 4B depicts a bright-field optical image of the 2 pm-thick LIFT-printed silicon square after 4X sintering, in accordance with one embodiment of the invention.

[0028] Figure 4C depicts a bright-field optical image of the 2 pm-thick LIFT-printed silicon square after 8X sintering scans, in accordance with one embodiment of the invention.

[0029] Figure 4D depicts a dark-field optical image of a 2 pm-thick LIFT-printed silicon square before sintering, in accordance with one embodiment of the invention.

[0030] Figure 4E depicts a dark-field optical image of the 2 pm-thick LIFT-printed silicon square after 4X sintering, in accordance with one embodiment of the invention.

[0031] Figure 4F depicts a dark-field optical image of the 2 pm-thick LIFT-printed silicon square after 8X sintering scans, in accordance with one embodiment of the invention.

[0032] Figure 4G depicts a 2D optical profilometer image of the LIFT-printed silicon square before sintering, in accordance with one embodiment of the invention.

[0033] Figure 4H depicts schematics of the sintering setup, in which the donor substrate is replaced by a transparent substrate, in accordance with one embodiment of the invention.

[0034] Figure 41 depicts a 2D optical profilometer image of the LIFT-printed silicon square after 8X sintering scans, in accordance with one embodiment of the invention.

[0035] Figure 4 J depicts a one-dimensional profilometer scan of the LIFT-printed silicon square before sintering, in accordance with one embodiment of the invention.

[0036] Figure 4K depicts a one-dimensional profilometer scan of the LIFT-printed silicon square after 8X sintering scans, in accordance with one embodiment of the invention.

[0037] Figure 5 A depicts the free-carrier concentration of LIFT-printed / / -type silicon squares versus the P / Si doping ratio, shown before sintering (square symbols) and after 8X sintering scans (circle symbols), in accordance with one embodiment of the invention.

[0038] Figure 5B depicts the electrical mobility of LIFT-printed / / -type silicon squares versus the P / Si doping ratio, shown before sintering (square symbols) and after 8X sintering scans (circle symbols), in accordance with one embodiment of the invention.

[0039] Figure 5C depicts the resistivity of LIFT-printed / / -type silicon squares versus the P / Si doping ratio, shown before sintering (square symbols) and after 8X sintering scans (triangle symbols); the inset presents an optical image of a printed silicon square on a receiver patterned with four metal pads, and the plot indicates the semiconducting and semimetallic regions, in accordance with one embodiment of the invention.

[0040] Figure 6A depicts the free-carrier concentration of LIFT-printed / / -type silicon squares versus the thickness ratio of doped-to-overall (doped + lightly doped) thin films on the donor substrate, shown before sintering (square symbols) and after 8X sintering scans (circle symbols), in accordance with one embodiment of the invention.

[0041] Figure 6B depicts the electrical mobility of LIFT-printed / -type silicon squares versus the thickness ratio of doped-to-overall (doped + lightly doped) thin films, shown before sintering (square symbols) and after 8X sintering scans (circle symbols), in accordance with one embodiment of the invention.

[0042] Figure 6C depicts the resistivity of LIFT-printed / / -type silicon squares versus the thickness ratio of doped-to-overall (doped + lightly doped) thin films, shown beforesintering (square symbols) and after 8X sintering scans (circle symbols), in accordance with one embodiment of the invention.

[0043] Figure 7A depicts schematics of a LIFT-printed p-n diode junction, in accordance with one embodiment of the invention.

[0044] Figure 7B depicts a dark-field optical image of the printed p-n junction prior to sintering, with a dashed rectangle indicating the overlapping junction region, in accordance with one embodiment of the invention.

[0045] Figure 7C depicts a dark-field optical image of the printed p-n junction after sintering, with a dashed rectangle indicating the overlapping junction region, in accordance with one embodiment of the invention.

[0046] Figure 7D depicts a 3D optical profilometer image of the LIFT-printed p-n diode junction, in accordance with one embodiment of the invention.

[0047] Figure 8A depicts a linear plot of junction current versus applied voltage for the LIFT-printed p-n diode junction, in accordance with one embodiment of the invention.

[0048] Figure 8B depicts a logarithmic plot of the absolute value of the current versus the applied voltage for the same LIFT-printed p-n diode junction, in accordance with one embodiment of the invention.

[0049] Figure 9A depicts an optical image of a printed and sintered / / -type collector extending across approximately half of the emitter-collector gap, in accordance with one embodiment of the invention.

[0050] Figure 9B depicts an optical image of the device after printing and sintering the transistor base, shown in the same field of view as Figure 9A, in accordance with one embodiment of the invention.

[0051] Figure 9C depicts an optical image after printing and sintering the emitter to form a three-terminal n-p-n transistor configuration; a schematic of the common-emitter connection is also shown, in accordance with one embodiment of the invention.

[0052] Figure 9D depicts a 3D optical profilometer image of the entire LIFT-printed bipolar transistor, in accordance with one embodiment of the invention.

[0053] Figure 9E depicts the common-emitter output characteristics, plotting collector current (Ic) versus collector-emitter voltage (VCE) for various base currents (IB), with the saturation and active regions indicated, in accordance with one embodiment of the invention.

[0054] Figure 10A depicts schematics of a LIFT-printed differential amplifier formed by two transistors (QI and Q2), showing input base currents IBI and IB2 and the commonsupply nodes Vcc (collectors) and -VEE (emitters), in accordance with one embodiment of the invention.

[0055] Figure 10B depicts a 3D optical profilometer image of the LIFT-printed differential amplifier illustrating the printed device topography, in accordance with one embodiment of the invention.

[0056] Figure 10C depicts an optical image of the LIFT-printed differential amplifier, in accordance with one embodiment of the invention.

[0057] Figure 10D depicts an optical image of the amplifier after printing n-type silicon conductive lines that interconnect the two collectors and the two emitters of the transistors; the schematic also indicates the input currents (IBI, IBI) and the common emitter and collector voltages (-VEE, VCC), in accordance with one embodiment of the invention.

[0058] Figure 11 A depicts an optical image of ( / / -type) donor substrates showing, from left to right, a transparent substrate prior to silicon deposition, the substrate after depositing a-Si thin films, and the final donor after thermal annealing to form poly-Si thin films; the inset schematically illustrates the multi-layer structure of / / -type donors prepared by serial sputtering, in accordance with one embodiment of the invention.

[0059] Figure 1 IB depicts Raman spectra of LIFT-printed silicon thin films before sintering (dotted line), after 4X sintering (solid line), and after 8X sintering scans (dashed line), in accordance with one embodiment of the invention.DETAILED DESCRIPTION

[0060] The following detailed description provides non-limiting examples and representative parameter ranges drawn from experiments. Where ranges are provided, values outside the described ranges may also be used unless otherwise noted.

[0061] The present disclosure proposes and demonstrates, a digital-printing (DP) technology for producing semiconducting silicon having electrical characteristics comparable to those reported for polycrystalline-silicon thin-film transistor (TFT) technology. Using a laser-based printing technique known as laser-induced forward transfer (LIFT), polycrystalline silicon thin films are digitally printed and doped to provide either electrons or holes as the majority free carriers over both a semiconducting regime (e.g., free-carrier concentrations of about 1014— 1018cm-3) and a semi-metallic regime (e.g., about 1019- 1021cm-3), with free-carrier mobility approaching -100 cm2 / V s, comparable to values reported for poly-Si-based TFT technology. After characterizing the electrical properties of individual (digitally printed) silicon thin films, the operation of basic electronic devices has beendemonstrated, including two-terminal rectifying p-n diode junctions and three-terminal bipolar junction transistors (BJTs) formed from the digitally printed silicon thin films. Finally, a feasibility demonstration of digitally printed, silicon-based integrated electronic circuits is provided by digitally printing and characterizing a differential amplifier comprising two BJTs. Collectively, these results demonstrate a development pathway from digitally printed unipolar semiconducting silicon thin films to digitally printed silicon-based integrated electronic circuits.

[0062] A principal advantage of LIFT over conventional inkjet techniques is that it is nozzle-free, thereby avoiding viscosity and nozzle-compatibility constraints that often limit inkjet processes to non-viscous liquid inks. In LIFT, a pulsed laser beam 12 (e.g., in the visible range) is focused through a transparent donor substrate 14 bearing a thin film 16 of the material to be printed (see, e.g., LIFT printing system 10 depicted in FIG. 1A).Absorption of the laser pulses within the thin film 16 ejects micro-droplets 18 toward a receiver substrate 20, where the droplets 18 land and solidify to form volume elements 22 (voxels) of the printed material. For inorganic semiconductors such as silicon, successful LIFT printing depends in significant part on the design and preparation of silicon-based donors, which function as the ink / cartridge of the process and enable accurate control of intentionally introduced impurities so as to set the majority free carriers (e.g., electrons or holes) in the printed silicon films. Methods for preparing doped solid silicon donors (both n-type and -type) suitable for use with LIFT are described in the Methods section below.

[0063] However, a significant limitation of using solid donors in a LIFT-printing process, particularly donors formed of metals or inorganic semiconductors, is the formation of a substantial amount of debris on the receiver substrate. This issue is especially critical for integrated electronics because the debris is conductive and can create unintended conduction paths across the receiver, which can short-circuit and destroy printed electronic devices and circuits. Reports of debris formation in LIFT printing frequently involve the use of relatively thick solid donors made of metals or inorganic semiconductors. As used herein, “thick donors” denotes a ratio LM^M »1, where LM is the thickness of the metal (or semiconductor) film on the donor substrate andis the optical absorption depth of the laser pulse. For example, in a typical LIFT process using a copper donor, the metal thickness can range from LM = 200- 1000 nm, while the visible optical absorption depth is approximately dM= 15-20 nm, yielding LMldM^ 10-50. To mitigate debris, thin silicon donors were evaluated, defined by Ls dsi « 1, where Lst is the thickness of the silicon layer on the donor and dst is the optical absorption depth of silicon. In the examples reported here, the poly-silicon thin film thickness on thedonor substrate is Lst = 100-250 nm, and the silicon absorption depth at a wavelength of 532 nm is approximately dst = 1000 nm, such that Zs / dsy ~ 0.1 -0.2.

[0064] Figures IB and 1C show optical images of two LIFT-printed Si squares: the first (Figure IB) printed using a thin Si donor (Lst / dst ~ 0.2), and the second (Figure 1C) printed using a relatively thick Si donor Lst / dst ~ 2). A significant amount of debris is evident for the square printed with the thick donor, whereas essentially no debris is observed for the square printed with the thin donor. The absence of debris across the entire receiver area is further illustrated by a 3D optical profilometer image of a large-area Hebrew University logo (area approximately 2 x 2 mm2) LIFT-printed using a thin Si donor, shown in Figure ID. Clean LIFT-printing using thin Si donors is a prerequisite for digitally printing integrated electronic circuits; accordingly, only thin Si donors were used in the remaining work.

[0065] Thin silicon donors were introduced into the LIFT setup (see Figure 1 A) to digitally print series of single silicon droplets onto a glass receiver 20 using the second harmonic of a pulsed Nd:YAG laser operating at a wavelength of 532 nm with 0.5 ns pulse width, a lOx objective lens, and a 10 pm gap between the donor 15 (including the donor substrate 14 and the thin film 16) and the receiver substrate 20. The pulse energy was tuned over the 0.06-9 pj range using a cross-polarizer variable attenuator (variable half-wave plate plus cross-polarizer). For each energy, several sets of individual droplets were printed, as shown in Figures 2A and 2B (optical top-view and SEM images, respectively). At lower pulse energies (0.25-1.5 pJ), the droplets were generally circular, uniform, and reproducible, whereas at higher pulse energies the droplets tended to spread and exhibit coffee-ring morphology. Accordingly, a pulse energy of 0.5 pj was selected as the working condition for the remainder of this work. A 2D optical profilometer map and a ID line scan across the diameter of a droplet printed at 0.5 pj are shown in Figures 2C and 2D, respectively. The corresponding laser fluence (energy density) was approximately 25-30 mJ / cm2, compared with approximately 600-1000 mJ / cm2typical for LIFT printing of metals using ns pulsed lasers. The measured average droplet diameter and height were approximately 20-25 pm and 500-600 nm, respectively.

[0066] Figures 3 A-3D show a LIFT-printed 200 x 200 pm2silicon thin-film square produced using 30 x 30 = 900 overlapping droplets (approximately 65% overlap among neighboring droplets) to form a single-layer (IX) square. Two-dimensional and three-dimensional optical profilometry (Figures 3A and 3B, respectively) and a one-dimensional scan across the square is shown in Figure 3C; a top-view optical image (Figure 3D) indicate a printed area that is dense, uniform, and free of pinholes, with no debris observed in thevicinity of the square. Figures 3E-3H present the same printing process repeated twice (2X) to form a double-layer square. The linearity and uniformity of the 2X process are evident in Figures 3E-3H, with the height of the IX square being approximately 550±125 nm (where the standard deviation of approximately 125 nm represents surface roughness), and the height of the 2X square being approximately 1000+250 nm.

[0067] To improve the mechanical and electrical characteristics of the LIFT-printed silicon thin films, a laser-based sintering process was applied. The same setup shown in Figure 1 A was used, except that the donor 15 was removed and replaced with a transparent substrate 24, as schematically illustrated in Figure 4H. The laser beam was defocused on the receiver substrate 20 to cover a large portion of the printed surface and to scan and sinter the printed silicon film. The energy density of the beam was reduced and controlled via the extent of defocusing to avoid damage to the sintered silicon surface (e.g., to avoid debris formation during sintering). A typical diameter of the defocused beam on the receiver during sintering was about 180 pm, with an energy density of about 190 pj / cm2. The influence of sintering on the printed silicon surface was evaluated by Raman spectroscopy (see Methods section below) and by dark-field optical microscopy (Olympus BX51M). The effect of sintering on the electrical properties of the printed silicon films is described in the next section.

[0068] Figures 4A-4C are bright-field optical images of an approximately 2 pm-thick LIFT-printed silicon square taken before sintering (Figure 4A), after 4X sintering (Figure 4B), and after 8X sintering scans (Figure 4C). Figures 4D-4F are dark-field optical images of the same square before sintering (Figure 4D), after 4X sintering (Figure 4E), and after 8X sintering scans (Figure 4F). Dark-field images are highly sensitive to surface roughness because rough surfaces scatter more light; accordingly, the darker appearance after sintering indicates a smoother and more homogeneous silicon thin-film surface. Two-dimensional and one-dimensional optical profilometer scans of the square surface before sintering (Figures 4G and 4J) and after 8X sintering scans (Figures 41 and 4K) indicate that the roughness was reduced from about 450 nm (before sintering) to about 150 nm after 8X sintering scans, while the film thickness remained essentially the same (about 1900 nm). No further reduction in surface roughness, nor any improvement in the electrical characteristics discussed hereafter, was observed beyond 8X sintering scans.

[0069] To measure the electrical characteristics of LIFT-printed silicon thin films, a glass receiver patterned with four aluminum (Al) pads in a square geometry was used toenable four-point probe measurements and Van der Pauw (VdP) Hall effect measurements. A silicon square approximately 2 pm in thickness and approximately 0.35 x 0.35 mm2in area was digitally printed on the patterned receiver, creating contacts to the metal pads at the edges of the square (see the inset optical image in Figure 5C). A standard VdP configuration was employed to measure electrical resistivity using a Keithley 4200-SCS parameter analyzer, and a Lakeshore 8400 Hall effect system was used to determine: (1) the polarity of the free carriers; (2) the free-carrier concentration; and (3) the free-carrier mobility. All measurements were performed at room temperature and in the dark. The quality of the ohmic contacts between the printed silicon film and the metal pads was verified to ensure reliable measurements. A magnetic field of 0.5 T was used for the Hall effect measurements.

[0070] Electrical characterizations of LIFT-printed Si thin films, before (square symbols) and after (circle and triangle symbols) 8X sintering scans, using phosphorus (P)-doped Si donors (e.g., donors made by PECVD; see the "Methods" section below), are shown in Figures 5A-5C. The polarity of the free charged carriers for all Si films printed using P-doped donors has been found to be negative, confirming that electrons are the free charged carriers for this type of donor. The doping level of the donors was controlled and tuned via the flow-rate ratio of PH3 to silane gases, P / Si (measured using mass flow controllers (MFCs) along the gas lines of the PECVD system; see the "Methods" section below). This ratio should be considered a rough estimate of the actual P / Si ratio in the printed silicon films (as other factors might affect the actual doping ratio, which was not measured directly). The density of free charged carriers (e.g., electron concentration) and the resistivity of the Si films, which are the more relevant quantities for evaluating the semiconducting properties of the Si films, have been measured and are shown in Figures 5A and 5C, respectively. Two conduction regions can clearly be seen from these figures: a semi-conducting (SC) region where the resistivity of the printed Si film varies from 1 kQ-cm down to 1 Q-cm and the corresponding free-carrier concentration varies over the 1014- 1018cm'3range. A second region of semi-metallic (SM) characteristics, having resistivities below 0.1 Q-cm down to 10'3Q-cm and corresponding carrier density in the 1019- 1021cm'3range, has been measured for heavily doped Si films (using silicon donors having P / Si » 10'4). Notice that the resistivity over the SC region, pn, varies as pn(1 / AD)Xwith x ~ 4, where ND<x (P / Si) is the donor concentration. In contrast, in the SM region, x is approximately 1, indicating an approximately linear dependence of free-carrier concentration on doping level. This result has been reported and explained for LPCVD-grown poly-Si, where trapping of free chargedcarriers across grain boundaries leads to a reduction of their concentration and to a smaller conductivity that varies as \!pnND4across the SC region. In contrast, saturation of the trapped states leads to a linear relationship of conductivity with ND(\ pnoc ND) across the SM region, similar to the conductivity of single-crystalline Si. Finally, sintering has almost no effect on the concentration of free charged carriers, as would be expected for such a low-power curing process.

[0071] In contrast, sintering has a pronounced effect on the electrical mobility of the free charged carriers, as shown in Figure 5B. After printing but before sintering, the electron mobility approaches about 10 cm2 / V-sec in the semi-conducting (SC) region and drops to about 0.3-0.5 cm2 / V-s in the semi-metallic (SM) region. These electron mobility values are comparable to, and in some cases greater than, those reported for organic semiconductors and metal oxides. Furthermore, 8X sintering of the LIFT-printed Si films yields approximately an order-of-magnitude increase in electron mobility, reaching about 100 cm2 / V-sec in the SC region and about 10 cm2 / V-sec in the SM region (see the circular points and curve in Figure 5B). These LIFT-printed Si mobility values are comparable to those reported for poly-Si used in analog thin-film transistor (TFT) technology, suggesting that LIFT printing of silicon may be exploited to develop a digital TFT printing technology.

[0072] Electrical characteristics of -type Si thin films LIFT-printed using boron (B)-doped Si donors prepared by a serial sputtering technique (see the "Methods" section below) are shown in Figures 6A-6C. Here again, the polarity of the free charged carriers has been found to be positive, verifying that holes are the majority charged carriers. The B / Si doping ratio for these donors was not measured directly, but is expected to be proportional to the thickness ratio of the deposited doped Si film (on the donor substrate) to the overall thickness of the Si film (see the "Methods" section below). From the free charged carrier density and resistivity measurements (Figures 6A and 6C, respectively), one concludes that these -type donors produced only semi-conducting (SC) -type Si, with resistivities in the range of 103-101Q-cm and a corresponding free-hole concentration of about p ~ 5 x 1015- 5 x 1017cm'3. Here again, sintering has a significant influence on hole mobility, as shown in Figure 6B, increasing mobility by about a factor of five after sintering (up to about 50 cm2 / V-sec for lightly doped -type Si), while having a smaller effect on the concentration of free holes.

[0073] Based on the electrical characteristics of the single-polarity Si thin films described above, a p-n rectifying diode junction was digitally printed on a glass receiver as follows: First, a / ?-type Si thin film having a low resistivity of about 40 Q-cm (doping level ofp ~ 5 x 1017cm'3) was printed and sintered. Next, an / / -type Si film having an electron concentration of about n ~ 1018cm'3was printed on top of the / / -type film, with an overlap area between the two films of about 50 x 200 pm2, as schematically illustrated in Figure 7A. Top-view dark-field optical images of the printed junction are shown in Figure 7B (prior to sintering) and Figure 7C (after sintering of the second, / / -type Si film). A 3D optical profilometry image of the full junction is presented in Figure 7D. Current-voltage (I-V) characteristics of the LIFT-printed (and sintered) p-n junction are shown in Figure 8A. The junction exhibits rectifying behavior, with a forward-bias turn-on voltage of about 0.6-0.65 V (with the positive terminal connected to the / / -type Si and the negative terminal connected to the / / -type Si). The reverse-bias dark current is relatively low, about 2-3 orders of magnitude smaller than the forward current, as shown by the semi-log plot of absolute current in Figure 8B. For example, Id(V = —IF) ~ 100 nA, corresponding to an estimated dark current density of about Jd(V = —IF) ~ 1 mA / cm2.

[0074] Next, a three-terminal bipolar junction transistor (BJT) in an n-p-n configuration was LIFT-printed. The transistor includes an emitter (e) and collector (c) printed from an n-type donor having an electron concentration of about n ~ 1018cm'3, and a base (b) printed from a / -type donor having a hole concentration of about p ~ 5 x 1017cm'3. The sequence of printing and sintering is illustrated in the optical images of Figures 9A-9C. First, the / / -type collector was printed and sintered, with the collector area extending to about half the gap between the collector and emitter metal pads (Figure 9A). Next, the / / -type base was printed and sintered, with a base width of about 70 pm to ensure full coverage of the collector while avoiding shorts to the emitter (Figure 9B). In this example, the base was printed as a thinner film, about 1 pm thick, and a smaller number of sintering scans (3X) were used to reduce electron transit time through the base and improve transistor performance. Finally, the / / -type emitter was printed and sintered to form the completed n-p-n BJT shown in Figure 9C, with a 3D optical profilometry image of the full device shown in Figure 9D.

[0075] The current-voltage (7C- VCE) characteristics of the n-p-n bipolar junction transistor for various base currents (7S), measured in a common-emitter configuration, are shown in Figure 9E. Typical transistor behavior is observed, including an active region (where the collector-base junction is reverse biased) and a saturation region (where the collector-base junction is forward biased), demonstrating a BJT suitable for integrated electronic circuit applications.

[0076] To demonstrate the feasibility of constructing digitally printed integrated electronic circuits using the LIFT technique, a basic differential amplifier was printed comprising two digitally printed BJTs (similar to the BJT described above). Although a BJT-based differential amplifier is a well-known building block for integrated logic circuits and is routinely used in analog design of silicon-based integrated circuits, the purpose here is not to discuss amplifier design details, but rather to provide a feasibility demonstration of digitally printing integrated electronic circuits using the LIFT technique. The structure of the differential amplifier is schematically illustrated in Figure 10A and includes two BJTs (QI and Q2) having transistor characteristics similar to those shown in Figure 9E. 3D optical profilometry and microscopy images of the printed differential amplifier are shown in Figures 10B and 10C, respectively, indicating clean, debris-free printing of the BJTs. In a final stage, the collectors and emitters of the two transistors were connected by digitally printing / / -type conducting silicon channels (having n~\ 018cm'3andw~0.2 Q-cm), as illustrated in Figure 10D. This interconnection scheme was used to measure the commonmode rejection ratio (CMRR) of the differential amplifier, which can serve as a figure of merit for reproducibility of the digital printing process by qualitatively reflecting differences between two nominally identical transistors. The measured average CMRR for the differential amplifier of Figure 10A is about CMRR- (54±2) dB over the active region of the transistors, indicating reproducible digital printing of the differential amplifier.

[0077] The capability to digitally print robust, stable, and reproducible electronic devices and circuits from highly conductive Si thin films on substrates that are not necessarily crystalline Si enables the development of new classes of applications. In particular, digitally printed Si-based TFT circuits — implemented, for example, with BJTs — on flexible and stretchable substrates (e.g., plastics, polymers, and paper) would support integration of electronic components and systems with emerging applications such as flexible displays, RFID (radio-frequency identification), internet-connected electronic tags, and Intemet-of-Things (loT) devices. In addition, printable sensors, sensor networks, and implantable or biodegradable biosensors can be realized using Si-based digital LIFT printing. Another promising direction involves LIFT-based printing of electronics on wearable textiles (electronic textiles). Because metals and metallic structures can also be printed by LIFT, metallic devices and structures (e.g., antennas, connectors, and flexible memory devices) can be integrated with LIFT-printed Si devices and circuits to create electronics that can be compressed, stretched, twisted, and conformed to curved surfaces. In the long term, digitallyprinted electronics and sensor networks on flexible, biocompatible, and implantable substrates may enable interfaces to bioelectronic devices for clinical diagnostics and treatments, and even brain-machine systems. Overall, Si-based digital manufacturing technology is expected to be low-cost, compatible with numerous other production methods and applications, and conducive to new experiences, products, and business models in areas such as digital healthcare, smart and green packaging, autonomous vehicles and the automotive industry, and information and communication technologies (ICT), particularly wireless communication.

[0078] A laser-based printing method has been presented for digitally printing semiconducting silicon with electrical characteristics comparable to those of Si-based thin-film transistor (TFT) technology. In addition, rectifying p-n diode junctions and bipolar junction transistors have been fabricated and operated, providing a feasibility demonstration of digitally printing Si-based integrated electronic circuits. This technology can be used to enable digitally printed Si-based TFT circuits containing tens, hundreds, or thousands of transistors on flexible and wearable substrates, supporting complex flexible displays, RFIDbased devices, internet-connected electronic tags, and loT devices. Integrating microelectronics with digital printing — particularly drop-on-demand deposition — using LIFT-based silicon printing is feasible and could open pathways to custom-made devices, systems, and remotely deployed applications.Methods:Preparation of Donor:

[0079] PECVD-deposited silicon donors: A first group of / / -type Si donors was prepared by depositing approximately 250 nm of silicon onto quartz substrates using a plasma-enhanced chemical vapor deposition (PECVD) system (Oxford Instruments, PlasmaLab 100). The deposition was carried out at approximately 300 °C using silane (SiEU), diluted phosphine (PEE), and hydrogen (EE) as gas sources for Si and phosphorus (P) doping, respectively. The chamber pressure was set to approximately 100 mTorr, and the RF electrode (13.56 MHz) was operated at approximately 60 W. Under these conditions — particularly the deposition temperature — the deposited film was amorphous silicon (a-Si) with a deposition rate of approximately 40 nm / min. To transform the a-Si films into dopedpoly-Si films, the donor substrates (bearing the a-Si thin films) were annealed in a dry furnace at 1100 °C for 2 hours under N2 flow; see Figure 11 A.Table 1. Doping ratio for / / -type Si donors:

[0080] Two PH3 gas cylinders with different dilutions were used to produce heavily doped and lightly doped silicon donors. The resulting donors were used to print semi-metallic Si and semiconducting Si thin films, respectively. The first cylinder (for heavily doped donors) contained 1% PH3 diluted in argon, and the second cylinder (for lightly doped donors) contained 0.01% PH3 diluted in hydrogen. The P / Si ratio was determined by the flow-rate ratio of diluted PH3 to SiF , controlled by mass flow controllers (MFCs) on each gas line. The silane flow was held approximately constant to maintain an overall deposition rate of about 40 nm / min, while the P / Si ratio was adjusted by varying the PH3 flow. Table 1 summarizes the estimated P / Si ratios for seven sets of / / -type Si donors (five lightly doped donors prepared using the highly diluted PH3 gas and two heavily doped donors prepared using the less diluted PH3 gas).

[0081] Sputtered silicon donors: Because the PECVD system was not used to prepare -type silicon donors, high-vacuum magnetron sputtering equipped with two boron (B)- doped silicon targets — one lightly doped and one heavily doped — was employed to prepare / / -type Si donors. Variation of the boron concentration in the sputtered Si film was achieved by serial sputtering from both targets to form a multilayer stack of heavily doped and lightly doped Si on a quartz substrate; see Figure 11 A (inset). The same dry -furnace anneal used above was then applied to convert sputtered Si to poly-Si and to average the dopant concentration across the film via rapid impurity diffusion. The overall multilayer thicknesswas held at 250 nm. Table 2 summarizes the thickness of the lightly doped Si sublayers, the heavily doped Si sublayers, the total Si thickness, and the ratio of heavily doped thickness to total thickness for three -type donor sets.Table 2. Thicknesses of the doped and undoped sputtered Si films and the doped-to-overall thickness ratio:

[0082] A rough estimate of the post-anneal doping concentration in the Si donors was obtained by measuring the average resistivity of the boron-doped Si targets and applying the established resistivity-doping correlation for poly-Si thin films. The lightly doped and heavily doped targets exhibited approximate doping levels of 5 x 1017cm'3(resistivity about 10 kfFcm) and 5- 1019cm'3(resistivity about 0.007 fFcm), respectively.

[0083] Raman spectroscopy of printed Si films: Raman spectra of the printed Si films, before and after sintering, were measured using a confocal Raman microscope coupled to a Raman spectrometer (In Via, Renishaw) operating at an excitation wavelength of 532 nm (CW Nd:YAG). The results, shown in Figure 1 IB, indicate a relatively broad and weak polycrystalline-Si Raman peak at a wavenumber of 514.9 cm'1prior to sintering (dotted line), and a consistent shift to higher wavenumbers with narrower and stronger Raman peaks after laser sintering (solid and dashed lines), confirming improved crystallization and stress relaxation with increasing numbers of sintering scans.

[0084] Thus, laser-induced forward transfer digital printing of silicon thin films and devices has been described. It is to be understood that the above-description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

Claims

CLAIMSWhat is claimed is:

1. A method of digitally printing silicon, comprising:irradiating a donor substrate with pulsed laser energy through the donor substrate to eject portions of a donor silicon film toward a receiver substrate as droplets that solidify on the receiver substrate as voxels, wherein the donor substrate supports the donor silicon film having a thickness Ls, andtessellating the voxels with overlap to define a printed silicon thin film on the receiver substrate,wherein dst denotes an optical absorption depth of the donor silicon film evaluated at a wavelength of the pulsed laser energy used to irradiate the donor substrate, and wherein the donor silicon film has a thickness-to-optical-absorption-depth ratio LsJdsi less than 1 so as to reduce debris formation on the receiver substrate.

2. The method of claim 1, wherein the ratio LsJdsi is between about 0.1 and about 0.2 when the optical absorption depth dst is evaluated at the wavelength of about 532 nm.

3. The method of claim 1, wherein the pulsed laser energy is between about 0.25 pj and about 1.5 pj per pulse focused to eject individual droplets having diameters of about 20-25 pm.

4. The method of claim 1, further comprising laser-sintering the printed silicon thin film using a defocused beam to reduce surface roughness and increase free-carrier mobility.

5. The method of claim 4, wherein the laser-sintering increases electron mobility to about 100 cmW'S for lightly doped / / -type silicon.

6. The method of claim 4, wherein the laser-sintering reduces surface roughness of the printed silicon thin film from about 450 nm to about 150 nm without substantially changing a thickness of the printed silicon thin film.

7. The method of claim 1, wherein the donor silicon film is doped to produce, in the printed silicon thin film, a free-carrier concentration in a semiconducting region between about 1014and 1018cm’3.

8. The method of claim 1, wherein the donor silicon film is doped to produce, in the printed silicon thin film, a free-carrier concentration in a semi-metallic region between about 1019and 1021cm-3.

9. The method of claim 1, wherein the printed silicon thin film is poly crystalline after sintering as indicated by a Raman peak shift toward higher wavenumbers and narrowing relative to the printed silicon thin film prior to sintering.

10. The method of claim 1, wherein the tessellating of the voxels comprises placing the voxels with an overlap of about 65% so as to form the printed silicon thin film as a substantially dense film having a thickness of about 500 nm to about 2 pm.

11. The method of claim 1, wherein the donor silicon film is phosphorus-doped to yield / / -type printed films or boron-doped to yield / / -type printed films.

12. The method of claim 1, wherein the receiver substrate comprises a glass substrate or a polymeric substrate suitable for flexible or wearable electronics.

13. A printed silicon thin film produced by the method of claim 1, the printed silicon thin film being substantially free of conductive debris surrounding a printed feature and exhibiting free-carrier mobility of at least about 50 cm2 / V s.

14. The printed silicon thin film of claim 13, wherein the printed silicon thin film exhibits a resistivity in a semiconducting region between about 1 kO cm and about 1 (1 cm with corresponding carrier concentration between about 1014and about 1018cm-3.

15. An integrated circuit comprising a differential amplifier including two printed bipolar junction transistors and printed doped-silicon interconnects, the integrated circuit being fabricated by the method of claim 1.

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