micromachined arrayed dip pen nanolithography ... - Semantic Scholar
MICROMACHINED ARRAYED DIP PEN NANOLITHOGRAPHY PROBES FOR SUB-100nm DIRECT CHEMISTRY PATTERNING David Bullen1, Xuefeng Wang1, Jun Zou1, Seunghun Hong2, Sung-Wook Chung2, Kee Ryu1, Zhifang Fan1, Chad Mirkin2, Chang Liu1 1
Micro Actuators, Sensors, and Systems Group, Micro and Nanotechnology Laboratory, University of Illinois at UrbanaChampaign, Urbana, IL, USA 2
Nanoscale Science and Engineering Center, Northwestern University, Evanston, IL, USA
ABSTRACT We report the development of micromachined passive and active probe arrays for parallel dip pen nanolithography (DPN). DPN is a soft lithography method that allows direct, mask-less deposition of chemicals onto substrates with sub 100-nm resolution. An active DPN probe with thermal bimetallic actuation has been developed and tested. Lithographic results and an analytical tool for optimizing active probe designs with respect to a number of performance criteria are also demonstrated.
INTRODUCTION Dip Pen Nanolithography (DPN) is a recently introduced [1] method of scanning probe lithography that is uniquely capable of depositing chemicals onto substrate surfaces with fine spatial resolution. A schematic of the DPN ink-transfer processes is shown in Figure 1. In this method, an atomic force microscope (AFM) probe tip is coated with a chemical of interest by dipping the probe in a solution containing the chemical or by evaporation from a heated source. When the probe is placed in contact with a surface, a meniscus forms and provides a pathway for the chemical to diffuse from the probe tip to the surface where it may bind, depending on its chemistry. The extreme sharpness of the probe tip allows fine features with a linewidth smaller than 100 nm to be routinely produced. The DPN method offers several advantages over other nanolithographic methods based on scanning probes.
Figure 1. Schematic diagram of the DPN process. Molecular “inks” diffuse along an AFM probe (stationary or traveling) to the tip, then through the water meniscus and onto a substrate where they self-assemble.
0-7803-7744-3/03/$17.00 ©2003 IEEE
Chief among them is its ability to directly deposit chemical substances on a surface without high temperatures, electric fields, or requiring a vacuum. In addition, DPN is a maskless process, which distinguishes it from microcontact printing which uses silicone elastomer stamps. DPN has been used to pattern biological macromolecules such as thiol-modified ssDNA [2] and proteins such as collagen [3] and rabbit immunoglobulin G [4]. The meniscus has been used as a nanoscale chemical reactor to pattern sol-gel precursors [5], several metals [6,7], conducting polymers [8], silanes [9], and organic dyes [10]. DPN generated alkanethiol patterns have been found to be robust enough to mask against gold etchants [11]. Conventional DPN applications use a single AFM probe and, as a result, are relatively slow and serial. The process can be accelerated by a factor of n if an array of n identical pens is used. Ideally, the tip-to-tip spacing between probes should be minimized. The first attempt at multiprobe patterning [12] used a one-dimensional array of undiced commercial probes. However, the array spacing (~1.5mm per probe) was far from optimal. The focus of our work is to develop high-density, one-dimensional, passive and active probe arrays that satisfy the need for true highdensity pattern generation. Passive probes were first developed, followed by active probe arrays.
PASSIVE DPN PROBES Passive probes with small tip-to-spacing have been produced from silicon nitride and silicon and are shown in Figure 2 and Figure 3 [13,14]. The silicon nitride probes are 375µm long (base to writing tip), 50µm wide, 600nm thick, and have a tip-to-tip spacing of 100µm. The spacing is small enough to allow the patterns of adjacent tips to touch when using a 100µm AFM scanner. The probes have an analytically estimated spring constant of 0.018 N/m. The silicon probes are 15µm wide and 5µm thick. The tip-to-tip spacing is 310µm. The suspension is folded to increase the beam’s effective length and lower its spring constant to an FEA estimated value of 0.315 N/m. Both designs have been used for topographic imaging and lithography with line widths down to 60nm. The fabrication of these devices has been previously described [14] and is summarized here. The probe tips are first formed by anisotropicly etching the surface of a silicon wafer, followed by oxidation sharpening [15]. For
4
Figure 2. SEM of a passive, 32-probe silicon nitride DPN array. Figure 4. A 10 probe thermally actuated DPN array.
Figure 3. SEM of a passive 8-probe silicon DPN array. silicon nitride probes, the wafer is coated with low pressure chemical vapor deposited (LPCVD) silicon nitride and the beams are etched out using reactive ion etching. The beams are then released by anisotropic wet etching from the front side of the wafer. For silicon devices, the beams are etched out of an ion implanted boron etch stop layer and the remaining silicon is removed by anisotropic wet etching.
ACTIVE DPN PROBES In active arrays, each probe contains an actuator capable of lifting its tip off the substrate independently of the others. This allows all the probes to travel the same path but write different patterns. There are a number of possible actuation methods including thermal bimetallic bending, electrostatic actuation, piezoelectric actuation, etc. For our application, thermal actuation was chosen for its large deflection, fabrication simplicity, and robust operation. As shown in Figure 4, each active probe consists of a cantilever beam with a joule heater, a silicon nitride/gold bimetallic thermal actuator, and a tip. The operating principle is demonstrated in Figure 5. The actuator is configured to lift the tip from the surface when heated, eliminating the concern that a heated probe may alter the ink transfer characteristics when writing. The fabrication process for the active array is an extension of the process used to make the passive silicon nitride devices. After the beams are patterned, a chromium adhesion layer and gold layer are thermally evaporated and patterned to form the heater, power leads, and actuator. The
Figure 5. Schematic diagram of the operating principle of a thermally actuated DPN probe. (Top) When the probe is un-heated, the tip contacts the writing surface and allows DPN writing. (Bottom) When the probe is heated, the tip lifts from the writing surface and suspends writing. beams are then released from the substrate as before. The probes in Figure 4 are 295µm long, 80µm wide, and have a tip-to-tip spacing of 100µm. They have a silicon nitride thickness of 9300Å and a gold thickness of 3650Å leading to an analytically estimated spring constant of 0.154 N/m. The chromium layer has a greater intrinsic strain than the silicon nitride and gold layers. As a result, its thickness can be used to control the curvature of beam after release. The thermal performance of a thermally actuated DPN probe is plotted in Figure 6. The typical operating power is approximately 2.5mW, corresponding to a tip deflection of 10µm during operation. Analytical modeling indicates that, at this power, the probes reach an average temperature of approximately 28ºC above ambient, which is supported by uniform temperature testing. In our experiments, the binding and friction characteristics of the 1-octadecanethiol (ODT) ink were unaffected by these temperature regimes. A demonstration of pattern variation using a thermally actuated probe is shown in Figure 7. In this set of lateral force microscopy (LFM) images, an active DPN probe was passed through the same figure-8 pattern ten times while being activated as necessary to create the numbers 0-9.
5
(a)
Figure 8. An LFM image of an 80nm wide ODT line made by an active probe writing at 20µm/sec.
ACTIVE PROBE ARRAY MODELING (b) Figure 6. Active DPN probe deflection vs. (a) uniform change in temperature and (b) heater power. The difference between the deflection in vacuum (
Micro Actuators, Sensors, and Systems Group, Micro and Nanotechnology Laboratory, University of Illinois at UrbanaChampaign, Urbana, IL, USA 2
Nanoscale Science and Engineering Center, Northwestern University, Evanston, IL, USA
ABSTRACT We report the development of micromachined passive and active probe arrays for parallel dip pen nanolithography (DPN). DPN is a soft lithography method that allows direct, mask-less deposition of chemicals onto substrates with sub 100-nm resolution. An active DPN probe with thermal bimetallic actuation has been developed and tested. Lithographic results and an analytical tool for optimizing active probe designs with respect to a number of performance criteria are also demonstrated.
INTRODUCTION Dip Pen Nanolithography (DPN) is a recently introduced [1] method of scanning probe lithography that is uniquely capable of depositing chemicals onto substrate surfaces with fine spatial resolution. A schematic of the DPN ink-transfer processes is shown in Figure 1. In this method, an atomic force microscope (AFM) probe tip is coated with a chemical of interest by dipping the probe in a solution containing the chemical or by evaporation from a heated source. When the probe is placed in contact with a surface, a meniscus forms and provides a pathway for the chemical to diffuse from the probe tip to the surface where it may bind, depending on its chemistry. The extreme sharpness of the probe tip allows fine features with a linewidth smaller than 100 nm to be routinely produced. The DPN method offers several advantages over other nanolithographic methods based on scanning probes.
Figure 1. Schematic diagram of the DPN process. Molecular “inks” diffuse along an AFM probe (stationary or traveling) to the tip, then through the water meniscus and onto a substrate where they self-assemble.
0-7803-7744-3/03/$17.00 ©2003 IEEE
Chief among them is its ability to directly deposit chemical substances on a surface without high temperatures, electric fields, or requiring a vacuum. In addition, DPN is a maskless process, which distinguishes it from microcontact printing which uses silicone elastomer stamps. DPN has been used to pattern biological macromolecules such as thiol-modified ssDNA [2] and proteins such as collagen [3] and rabbit immunoglobulin G [4]. The meniscus has been used as a nanoscale chemical reactor to pattern sol-gel precursors [5], several metals [6,7], conducting polymers [8], silanes [9], and organic dyes [10]. DPN generated alkanethiol patterns have been found to be robust enough to mask against gold etchants [11]. Conventional DPN applications use a single AFM probe and, as a result, are relatively slow and serial. The process can be accelerated by a factor of n if an array of n identical pens is used. Ideally, the tip-to-tip spacing between probes should be minimized. The first attempt at multiprobe patterning [12] used a one-dimensional array of undiced commercial probes. However, the array spacing (~1.5mm per probe) was far from optimal. The focus of our work is to develop high-density, one-dimensional, passive and active probe arrays that satisfy the need for true highdensity pattern generation. Passive probes were first developed, followed by active probe arrays.
PASSIVE DPN PROBES Passive probes with small tip-to-spacing have been produced from silicon nitride and silicon and are shown in Figure 2 and Figure 3 [13,14]. The silicon nitride probes are 375µm long (base to writing tip), 50µm wide, 600nm thick, and have a tip-to-tip spacing of 100µm. The spacing is small enough to allow the patterns of adjacent tips to touch when using a 100µm AFM scanner. The probes have an analytically estimated spring constant of 0.018 N/m. The silicon probes are 15µm wide and 5µm thick. The tip-to-tip spacing is 310µm. The suspension is folded to increase the beam’s effective length and lower its spring constant to an FEA estimated value of 0.315 N/m. Both designs have been used for topographic imaging and lithography with line widths down to 60nm. The fabrication of these devices has been previously described [14] and is summarized here. The probe tips are first formed by anisotropicly etching the surface of a silicon wafer, followed by oxidation sharpening [15]. For
4
Figure 2. SEM of a passive, 32-probe silicon nitride DPN array. Figure 4. A 10 probe thermally actuated DPN array.
Figure 3. SEM of a passive 8-probe silicon DPN array. silicon nitride probes, the wafer is coated with low pressure chemical vapor deposited (LPCVD) silicon nitride and the beams are etched out using reactive ion etching. The beams are then released by anisotropic wet etching from the front side of the wafer. For silicon devices, the beams are etched out of an ion implanted boron etch stop layer and the remaining silicon is removed by anisotropic wet etching.
ACTIVE DPN PROBES In active arrays, each probe contains an actuator capable of lifting its tip off the substrate independently of the others. This allows all the probes to travel the same path but write different patterns. There are a number of possible actuation methods including thermal bimetallic bending, electrostatic actuation, piezoelectric actuation, etc. For our application, thermal actuation was chosen for its large deflection, fabrication simplicity, and robust operation. As shown in Figure 4, each active probe consists of a cantilever beam with a joule heater, a silicon nitride/gold bimetallic thermal actuator, and a tip. The operating principle is demonstrated in Figure 5. The actuator is configured to lift the tip from the surface when heated, eliminating the concern that a heated probe may alter the ink transfer characteristics when writing. The fabrication process for the active array is an extension of the process used to make the passive silicon nitride devices. After the beams are patterned, a chromium adhesion layer and gold layer are thermally evaporated and patterned to form the heater, power leads, and actuator. The
Figure 5. Schematic diagram of the operating principle of a thermally actuated DPN probe. (Top) When the probe is un-heated, the tip contacts the writing surface and allows DPN writing. (Bottom) When the probe is heated, the tip lifts from the writing surface and suspends writing. beams are then released from the substrate as before. The probes in Figure 4 are 295µm long, 80µm wide, and have a tip-to-tip spacing of 100µm. They have a silicon nitride thickness of 9300Å and a gold thickness of 3650Å leading to an analytically estimated spring constant of 0.154 N/m. The chromium layer has a greater intrinsic strain than the silicon nitride and gold layers. As a result, its thickness can be used to control the curvature of beam after release. The thermal performance of a thermally actuated DPN probe is plotted in Figure 6. The typical operating power is approximately 2.5mW, corresponding to a tip deflection of 10µm during operation. Analytical modeling indicates that, at this power, the probes reach an average temperature of approximately 28ºC above ambient, which is supported by uniform temperature testing. In our experiments, the binding and friction characteristics of the 1-octadecanethiol (ODT) ink were unaffected by these temperature regimes. A demonstration of pattern variation using a thermally actuated probe is shown in Figure 7. In this set of lateral force microscopy (LFM) images, an active DPN probe was passed through the same figure-8 pattern ten times while being activated as necessary to create the numbers 0-9.
5
(a)
Figure 8. An LFM image of an 80nm wide ODT line made by an active probe writing at 20µm/sec.
ACTIVE PROBE ARRAY MODELING (b) Figure 6. Active DPN probe deflection vs. (a) uniform change in temperature and (b) heater power. The difference between the deflection in vacuum (
