Scalable Plasmonic Nanolithography: Prototype ...

Proceedings of the ASME 2016 International Manufacturing Science and Engineering Conference MSEC2016 June 27-July 1, 2016, Blacksburg, Virginia, USA

MSEC2016-8671

SCALABLE PLASMONIC NANOLITHOGRAPHY: PROTOTYPE SYSTEM DESIGN AND CONSTRUCTION

Yuan Wang Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94709, USA

Mohamed E. Saad Center for Precision Metrology, The University of North Carolina at Charlotte, Charlotte, NC 28223, USA

Kang Ni Center for Precision Metrology, The University of North Carolina at Charlotte, Charlotte, NC 28223, USA

Yen Chi Chang Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90024, USA

Cheng-Wei Chen Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90024, USA

Chen Chen School of Mechanical Engineering Purdue University, West Lafayette, Indiana 47907, USA

Liang Pan School of Mechanical Engineering Purdue University, West Lafayette, Indiana 47907, USA

Tsu Chin Tsao Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90024, USA

Adrienne S. Lavine Mechanical and Aerospace Engineering, University of California, Los Angeles, CA 90024, USA Xiang Zhang* Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94709, USA

David B. Bogy Mechanical Engineering, University of California, Berkeley, Berkeley, CA 94709,USA * Contact Author

ABSTRACT Maskless nanolithography is an agile and cost effective approach if their throughputs can be scaled for mass production purposes. Using plasmonic nanolithography (PNL) approach, direct pattern writing was successfully demonstrated with around 20 nm half-pitch at high speed. Here, we report our recent efforts of implementing a high-throughput PNL prototype system with unique metrology and control features, which are designed to use an array of plasmonic lenses to pattern sub-100 nm features on a rotating substrate. Taking the advantage of air bearing surface techniques, the system can expose the wafer pixel by pixel with a speed of ~10 m/s, much faster than any conventional scanning based lithography system. It is a low-cost, high-throughput maskless approach for the next generation lithography and also for the emerging nanotechnology applications, such as nanoscale metrology and imaging.

INTRODUCTION The conventional projection-type photolithography approach for nanoscale manufacturing is facing possibly insurmountable challenges, especially to invent novel technical solutions that remain economical for the next generation of semi-conductor integrated circuits [1]. Although extreme ultra violet (EUV) lithography with the next generation photo-masks and 193-nm immersion lithography with multiple patterning can deliver 22 nm and smaller nodes, it still cannot effectively address the reliability and cost issues required for mass production. Maskless nanolithography approaches, including electron-beam, focused ion-beam and scanning-probe lithography (SPL) offer a path to overcome these obstacles by reducing mask costs and shortening the cycle time for nanoscale device validation. However limited throughput remains a challenge for these approaches for applications of high volume

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manufacturing. We have recently demonstrated a 22-nm halfpitch direct pattern writing using plasmonic nanolithography (PNL), where an array of plasmonic lenses are used to pattern a rotating surface by concentrating short wavelength surface plasmons into tiny spots [2-10]. This nano-fabrication scheme has the potential of a few orders of magnitude higher throughput than current maskless techniques, and it opens the way for a new cost effective approach towards the next generation lithography for nanomanufacturing. However, developing a set of nanoscale metrology and positioning system for this unique PNL technology is a challenging task because of its high scanning speed at the order of 10 meter per second. Here, we report our recent progress of implementing this scalable nanolithography technology.

half-pitch resolution (Figure 2B). And the resolution can be further improved by plasmonic design and guiding mechanisms. Application of ultrafast laser becomes another key to achieve the high resolution and throughput. An ultrafast laser source can greatly lower the required operating power level, and improve the pattern uniformity and feature size. Application of ultrafast laser also significantly reduces the heat accumulation effect in PNL head, and allows the employment of the close-packed plasmonic lens array for parallel pattern purpose. The nanoscale optothermal process design and optimization have been published elsewhere [13]. Since the scope of this paper is to report the status of system integration, the detailed plasmonic, control and precision designs will be reported later in their specific fields.

TECHNOLOGY OVERVIEW PNL performs nanoscale patterning on a rotating surface. As shown in Figure 1, the firing of the laser in the PNL system is synchronized with the position of the plasmonic lens (carried by plasmonic flying head) to enable lithography of arbitrary patterns. The ultrahigh speed firing of the laser is controlled by a high speed optical modulator from a pattern generator, according to the object to be written on the resist. The relative position of the head to the disk can be obtained from the angular position of the disk from the spindle encoder and the radial position of the nanostage, which holds the plasmonic flying head. The plasmonic lens serves for the function of concentrating the incident laser pulse into a nanoscale spot of tens of nanometers in size [2, 3, 11]. A precision airbearing spindle is used to mount a resist-coated substrate (typically a 4 inch silicon wafer) and to spin it at high-speed equivalent to a linear velocity of ~10 meters per second. The working distance between the plasmonic lens and the resist surface is maintained at the close proximity gap by the use of the advanced airbearing surface (ABS) technique [12]. Similar to most other maskless lithography approaches, it is a challenge to implemental large scale arbitrary patterning with good reliability, high throughput and high resolution. In particular for PNL, achieving good pattern definition at nanoscale requires the considerations of real-time sensing and compensating positioning errors in a rotating framework. The major motion errors are from the relative motions between the plasmonic lens and the rotating substrate, which are mainly caused by the windage acting on the head assembly and the motion errors of the spindle including both synchronized (or repeatable) and asynchronized (or non-repeatable) motion errors. Figure 1 shows a schematic of the PNL experimental setup and an illustration of the motion error resources decomposed into XRZ (off-track, down-track and off-plane) coordinates and some of the motion errors are coupled.

Figure 1: A schematic of the PNL experimental setup. (Top) The plasmonic lens focuses ultraviolet laser pulses onto the rotating substrate by concentrating optical energy into nanoscale spots. An advanced airbearing surface (ABS) technology is used to maintain the gap between the lens and the substrate at ~10 nm. A pattern generator is used to pick the laser pulses for exposure through an optical modulator according to the angular position of the substrate from the spindle encoder and the radial position of the flying head from a nanostage. (Bottom) Illustration of components of relative position between the plasmonic flying head.

Figure 2A shows the SEM image of a plasmonic lens array made of chromium thin film. In experiments, we have successfully demonstrated high-speed patterning with 22 nm

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Figure 2: A hybrid plasmonic lens (HPL) design that enables high-throughput maskless parallel plasmonic nanolithography at deep sub-wavelength resolution. (A) The SEM image shows a loosely packed HPL array made in 60 nm thick Cr film at 2micron spacing. (B) The AFM image of a closely packed dots with 22 nm half pitch size. PROTOTYPE SYSTEM CONFIGURATION The prototype PNL system consists of a central stage for sensing and controlling and a set of preferential components including the light source, the optical modulator, the patterning generator, and the light delivery optics. As shown in Figure 3, the central stage is composed of a base and carriage and shares some metrology features of the multi-scale alignment and positioning system (MAPS) system previously developed by the team [14]. Two Halbach linear array motors move the carriage in the radial direction. The advantage of using the Halbach linear array is that it provides smooth contactless linear motion that is directly proportional to the current applied without generating much heat. Vacuum preloaded air bearings are used to carry the carriage at a 5-micron air gap on diamondturned surfaces located on the machine base and two shoulders. The air bearings have high angular stiffness, which makes them a good choice to reduce pitch and yaw errors. Eddy current dampers are used on the central patterning stage to provide the necessary damping during the carriage motion. The main advantage of these dampers is that they provide a smooth linear damping directly proportional to the carriage speed. At 5 μm/s carriage moving speed, the dampers provide about 4.28 mN damping force. The central patterning stage has a set of microscale metrology and control devices (called pre-focusing stage) that tracks the position of plasmonic lens and keep the incident laser beam aligned to the plasmonic lens. Similar servo technology is also used in the optical pickup heads of CD/DVD drives [15].

Linear Stage (r) Substrate

Rotary Stage () Figure 3: The high precision linear stage being developed by the team. High speed plasmonic nanolithography testbed. (Top) 3D mechanical model of PNLM; Top (Middle) and side (Bottom) views of thecentral patterning stage installed in a Class-100 clean environment.

The carriage motion is constrained by two Hall effect limit switches that provide a contactless means to stop the writing process when necessary. Also, there are pressure limit switches to monitor the pressure of the air bearings’ air supply. These switches can automatically stop the writing process when triggered and give an alarm to users.

Similar to many precision motion control machines, the PNL prototype system adopts the multi-stage strategy to compensate the relative error motion between the plasmonic

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lens and the rotating substrate [16]. In this system, linear motors are used for long range low bandwidth tracking (in a range of a few inches, at a sub-1 kHz bandwidth and 100s of nm accuracy), and piezoelectric actuators are used for short range high frequency compensation (in a range of a few microns, at a few 100s of kHz bandwidth and a few nm accuracy). In particular, two parallel linear motors are used to control the coarse movement of the system in R and X directions. Mounted on the course linear motor stage, the plasmonic lens is attached to a two-dimensional piezoelectric actuated stage that compensates for the high bandwidth movement in R and X directions. The Z error motion is automatically compensated by the self-adaptive ABS technology with an accuracy of ~1 nm.

while long-term noises are being mitigated from the perspective of motion control.

HIGH PRECISION LINEAR STAGE DEVELOPMENT The lithography performance of the system relies on the tracking performance of the multi-stage system, which is dominated by yaw control of the linear motor stage and coordination between the motor stage and piezoelectric stage. Dimensional metrology systems for the central patterning stage shown in Figure 4a and 4b have devised to identify the motion characteristics of the linear stage system, which is necessary for system control and calibration. The linear motor closed-loop control has been implemented and verified within the desired velocity range to achieve desirable tracking and yaw performance. Long-time behaviors of environmental noises such as thermal fluctuations and mechanical vibrations around the PNLM system have been monitored efforts to minimize their magnitudes have been made. A laboratory environmental measurement system was built to precisely configure, control and manage the data acquisition system of the multichannel temperature and pressure information. Five thermistors for a thermal model were calibrated in the range of 18 to 24 degree Celsius to the agreement level of 0.008 Kelvin. The displacement measurement interferometer was functionally tested. Monthly room temperature variation was improved with a standard deviation of 0.4°C (monthly average temperature: 23.8°C) as shown in Figure 4c. A first order one dimensional thermal expansion approximation model was derived to numerically study the relationship between temperatures of lab, machine components’ geometry and their thermal response time constants in terms of the change of sensitive direction of lithography motion.

Figure 4: (Top) Schematic of DMI (displacement measuring interferometer) setup. (Middle) Angular interferometer setup for measurements of pitch (middle left) and yaw (middle right) of the linear stage. (Bottom) Room temperature measurements around the PNLM system over a month. Figure 5 shows the measured motion characteristics of the central patterning stage. Measurement uncertainty associated with the metrology work, including intrinsic instrument errors, was also quantitatively evaluated to assess the effectiveness of the measured information. A motion trajectory for lateral straightness measurement was programmed and the straightness error data acquisition program was updated and synchronized with the real time control system. The straightness error of carrier motion was tested without using an analog filter. The

The compressed air and vacuum supply were modified such that they pass through capacitive air tanks for low pass filtering of pressure fluctuations, prior to their delivery to the system air bearings. The active vibration isolators in the antivibration table where the system is installed have been further tuned. Short-term noises were reduced by these measurements

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temperature and humidity measurement functionally demonstrated in the field.

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speed mode is suppressed to be less than 50 nm. Both averages of error are to zero mean, as shown in Figure 6 and 7.

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Figure 7: Positioning error in constant velocity mode (top) and at the speed of 5 µm/sec (bottom).

Figure 5: Pitch (top) and yaw (middle) measurements of stage carriage motions measured at the carriage speed of 33 μm/sec. (Bottom) Straightness measurements of the stage carriage motion.

Trajectory smoother has also been developed to reduce spikes as the moving speed of the stage is changed. Before applying the trajectory smoother, the positioning error is increased when the speed of the stator suddenly changed. This development successfully decreases the transient error and keeps the stage moving precisely and smoothly. The experimental result is shown in Figure 8. The smooth trajectory is particularly important for the multi-stage positioning control strategy. It provides a round of zero-mean error motions which can be compensated by the fast piezo actuator without noticeable positioning errors caused by hysteresis or drift. After these treatment, the frequency and amplitude of left-over errors

A set of feed-back/feed-forward controllers working under different frequency range have been used. A PID cascaded with a lag compensator controller has been designed and implemented to improve the performance under constant scanning speed mode and to achieve zero steady state error. By tuning the displacement between the additional pole and zero at the compensator, the novel controller is able to track ramp signal without steady state error. This flexible design keeps the original performance the same in the stationary mode, and solves the tracking delay when the stage starts to move. After this updating, the tracking error in both stationary and constant

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motion can be well compensated by the fast piezoelectric actuators at the next stage.

the central patting stage. Analysis of these results confirmed that the overall error motion of the stage is well below 100 nm.

METROLOGY AND CONTROL OF THE PLASMONIC FLYING HEAD As shown in Figure 10, the two fiber optic interferometer tips were mounted in stiff mechanical brackets built into the carrier of the central patterning stage and a Faraday isolator was used to seal the detection electronics from noises. The fiber interferometers provides the relative motions between the interferometers probe tip and the slider body (plasmonic flying head) to monitor the in-plane vibrations of the lens during the writing process. A set of high-speed shear piezo actuators are used to compensate the in-plane head vibration based on sensor measurements, while the off-plane vibration is automatically compensated by the ABS.

Figure 8: Positioning error after adding the trajectory smoother. The error in transient is decreased since the gradual change of the input reference. The overlaying input reference and measured translation motion (top) and their errors for inplane translation (middle) and yaw off-plane (bottom) error motions.

Figure 10: Two fiber optic interferometers for 2D metrology of the plasmonic writing head. An advanced magnetic encoder technique for high resolution angular measurement and positioning is under the final developed stage. Uniform magnetic gratings with 80 nm pitch have been successfully written onto the magnetic disk. This magnetic disk can be used as an encoder mask to provide a high angular resolution better than 2 μrad. This magnetic mask can have more than one million counts per revolution while the conventional optical mask count is limited to several thousand counts per revolution. The magnetic gratings have been successfully read out by a flying magnetic head. The analog signal is translated to an encoder signal with digital transistor–transistor logic (TTL) format by a high-speed comparator. The encoder signal will be used to trigger a fieldprogrammable gate array (FPGA) and then the FPGA calculates the in-site angular position. The pattern generator provides the laser pulse modulation signal to the optical modulator simultaneously according to the angular position.

Figure 9: Optical image of exposed line pattern by the assembled system. The line width is about 1 μm and the pitch is 6 μm. The plasmonic lens is not used here in order to generate optically visible images. The scalable plasmonic nanolithography prototype system has been assembled successfully and is located in a Class-100 clean air environment. Figure 9 shows exposure lines using the assembled system. Here, plasmonic lens is not used in this test in order to generate optically visible features. The exposure results provides the in-situ measurement of the error motion for

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A pattern generation system based on the high-resolution magnetic encoder has been developed. Advanced clock management techniques for synchronous massive parallel pattern generation have been used based on the FPGA electronics. Our system synchronizes the triggering of the electro-optic modulators to the magnetic encoder and the laser clock events in order to minimize angular positioning errors with ultrafine timing resolution. Finite State Machines (FSM) based on Look-up tables (LUTs) allow for independent, parallel task completion and adaptability to different user needs. The skew and jitter of the clock has been reduced using a digital lock loop and phase lock loop. Figure 12 shows the 01 series pattern output from the pattern generator, triggered by the magnetic encoder. The optimization of the multi-stage coordination is under development using a model-based constrained optimal control algorithm. This algorithm generates reference trajectories for the coarse stage and secondary stage by optimizing the absolute tracking performance. The benefit of this algorithm is that the system constraints, such as stage velocity, saturation, displacement, and signal saturation are considered during the optimization process. More results about control optimizations will be reported later.

Figure 11: Magnetic encoder system. (top) magnetic gratings on the magnetic mask (scale bar: 1μm). (bottom) experimental analog and digital magnetic encoder signal from the encoder system.

CONCLUSION In the paper, we presented the development of a novel nano-manufacturing system, including high-throughput maskless nano-lithography utilizing a plasmonic optical lens flying at a speed of ~10 m/s for high-throughput nano-fabrication with nanoscale overlay accuracy. We have experimentally demonstrated the capability of high speed arbitrary nanopatterning with the overall error motion well below 100 nm. The further development of the prototype PNL system will be centered to the whole system integration with improved GUI (graphic user interface) for end users to easily operate the system. With fully operational prototype system, the process for nanolithography will be further developed together with an assessment of process capability of the machine for various potential applications.

Figure 11 shows 2 magnetic gratings on the magnetic mask disk captured by magnetic force microscopy. The pitch of each grating is 80 nm, corresponding to 2 μrad angular resolution per magnetic mark. The resolution can be further improved using the phase lock concept. The magnetic head follows this grating and the magnetic readback signal is generated by the magnetic transducer.

ACKNOWLEDGMENTS This work is financially supported by NSF Nano-scale Science and Engineering Center (NSEC) for Scalable and Integrated Nanomanufacturing (SINAM) (Grant No. CMMI-0751621), Grant No. CMMI-1405078 and No. CMMI-1554189.

REFERENCES Figure 12: Experimentally acquired time-evolution of the synchronized clock signals, the generated pattern signal (here periodic '01' in red) and magnetic encoder trigger (black).

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