Automated Delivery of Microfabricated Targets for Intense Laser Irradiation Experiments
January 28th, 2021
A protocol is presented for automated irradiation of thin gold foils with high intensity laser pulses. The protocol includes a step-by-step description of the micromachining target fabrication process and a detailed guide for how targets are brought to the laser's focus at a rate of 0.2 Hz.
Intense laser radiation experiments of submicrometer scale targets are currently performed at slow shot rates. Our protocol solved this challenge by placing these targets quickly at the focus of the laser in an automated manner. Our target system enables the collection of data incorporating a large number of laser shots with target parameters changed in small increments, as well as applications that benefit from a high overall radiation dose.
Visual demonstration of this protocol will show the subtleties of the wafer fabrication process and target alignment. Demonstrating target fabrication process are process engineer Nirit Porecki Shamay and Nofar Livni. To fabricate the backside, use a 250 micrometer thick 100 millimeter diameter high stress silicon wafer in a one-zero-zero crystal formation coated on both sides with silicon nitride.
Clean the wafer with acetone and with isopropanol. Then spin coat the wafer with HMDS resists to form an adhesive layer. Spin coat the wafer with an AZ 1518 positive photoresist.
Bake the wafer at 100 degrees Celsius for one minute. Photolithograph 1, 000 by 1, 000 micrometer square openings under vacuum, exposing the wafer in one four to seven-second cycle to a 400 nanometer UV lamp so that the wafer is exposed to an overall fluence of 40 joules per centimeter squared. Then use an AZ 726 developer to expose the silicon nitride and a bath of dehydrated water to stop the process.
Use a reactive ion etcher to remove the silicon nitride in the location of the squares. Place the wafer in an NMP bath for 20 minutes to remove the residual resist and photoresist, producing a replica of the mask on the silicon nitride layer. Then wash it under fresh water and let it dry.
Sink the wafer in a 30%90 degrees Celsius potassium hydroxide solution to etch the silicon through the square openings. To fabricate the front side, repeat the previously described procedure with a mask shaped as three concentric rings. Use the reactive ion etcher to remove the silicon nitride where the rings are located, followed by an NMP bath to remove resist and photoresist leftovers.
Roughen the silicon rings by sinking the wafer in nitric acid and in a solution of 0.02 molar silver nitrate and four molar hydrogen fluoride. On the etched side of the wafer, use a physical vapor deposition machine to sputter a layer of a few hundred nanometers of gold on top of a 10 nanometer film of adhesive titanium, nickel, or chrome. Block the beam and bring the first target into view under a high magnification microscope.
Point a triangulation ranging sensor to the roughened ring closest to the target and record its displacement reading. Leaving the microscope in place, move the wafer away to clear the beam path. Use the two folding mirrors and the off-axis parabolic mirror to align the beam in low power into the field of view of the microscope.
Adjust these three mirrors to correct astigmatisms in the beam. The result should be a nearly diffraction limited focal spot. Block the laser beam and bring the target back to the focus of the microscope.
Then validate its position using the microscope and the ranging sensors reading. Use software to implement a closed loop feedback between the focal axis manipulator of the target and the displacement sensor reading using the previously recorded displacement value as the setpoint. Once the closed loop positioning has reached a desired tolerance distance from the setpoint, irradiate the target with a single high-power laser pulse.
Record data from particle diagnostics and repeat the process with the next target brought into focus by the software. This target delivery system was employed to accelerate ions from the backside of 600 nanometer thick gold foils. A time series of the target displacement along the focal axis is shown here.
The values are relative to the focal position setpoint. The green dots indicate when the target displacement was within a tolerance value of one micrometer from the setpoint, which is when a laser shot was taken. Thomson parabola ion spectrometer traces were obtained from 14 consecutive irradiations of 600 nanometer thick gold foil targets.
Energy spectrums were derived from these traces. The peak-to-peak stability of the maximum proton energy was within 10%Following this procedure, investigations of ion and electron acceleration from solid foils neuron generation can be performed in a systematic manner.
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Overview
This article presents a protocol for the automated irradiation of thin gold foils using high-intensity laser pulses. It details the micromachining target fabrication process and the method for aligning targets at the laser's focus at a rate of 0.2 Hz.
Key Study Components
Area of Science
- Laser irradiation
- Micromachining
- Target fabrication
Background
- Intense laser radiation experiments are typically conducted at slow shot rates.
- Automated target placement improves efficiency and data collection.
- High overall radiation doses can be achieved with this method.
- Visual demonstrations enhance understanding of the fabrication process.
Purpose of Study
- To automate the placement of targets at the laser focus.
- To enable rapid data collection with varying target parameters.
- To demonstrate the micromachining process effectively.
Methods Used
- Utilization of a 250 micrometer thick silicon wafer.
- Coating of the wafer with silicon nitride on both sides.
- Automated system for target alignment at 0.2 Hz.
- Visual demonstrations by process engineers.
Main Results
- Successful automation of target placement.
- Increased efficiency in data collection from laser shots.
- Demonstration of the micromachining target fabrication process.
- Ability to change target parameters in small increments.
Conclusions
- The protocol significantly improves the speed of laser experiments.
- Automated systems can enhance experimental outcomes.
- Visual aids are beneficial for understanding complex processes.
Frequently Asked Questions
Executive Industry Relevance
This protocol addresses a key bottleneck in high-throughput laser-based discovery workflows by enabling automated, repeatable target delivery at the laser focus. By integrating closed-loop feedback control with precision micromachining, it supports systematic variation of target parameters and accumulation of statistically robust datasets. This capability enhances predictive confidence in mechanistic studies of ion acceleration and related phenomena relevant to preclinical model development and translational biomarker exploration.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Scientific Value: Enables interrogation of therapeutic hypotheses through controlled irradiation of microfabricated targets to probe biological response mechanisms.
- Operational Value: Facilitates target parameter variation in small increments, supporting dose-response characterization and pathway clarification.
- Predictive Value: Generates reproducible MeV-level proton beam outputs with peak-to-peak stability within 10%, enabling reliable functional target validation.
Screening & Assay Development
- Scientific Value: Produces standardized, replenishable gold foil targets suitable for high-repetition laser irradiation assays.
- Operational Value: Achieves 0.2 Hz shot rate via automated target positioning, improving assay throughput and reducing manual intervention.
- Scalability: Discusses prospects for increasing shot rates above 10 Hz, indicating potential for platform scaling in discovery pipelines.
Translational & Preclinical Research
- Translational Continuity: Supports generation of ion beams applicable to preclinical models investigating radiation-induced biological effects.
- Mechanistic De-risking: Enables systematic study of ion and electron acceleration from solid foils, reducing ambiguity in radiation response pathways.
- Biomarker Alignment: Facilitates correlation of irradiation parameters with neutron generation and other endpoints relevant to translational biomarker development.
Pipeline & Workflow Integration
The method integrates into early discovery workflows by providing a reliable, automated front-end for laser irradiation experiments, enabling progression from target validation to mechanistic screening and preclinical continuity.
- Discovery Biology: Supports hypothesis testing via precise control over target thickness, composition, and positioning at laser focus.
- Screening: Delivers quantitative, reproducible proton energy spectra from Thomson parabola ion spectrometer traces, enabling comparative analysis across conditions.
- Analytics: Provides energy spectrum derivations and displacement tolerance metrics (within 1 μm) as key readouts for experimental consistency.
- Translational Research: Connects laser irradiation outputs to downstream preclinical evaluation of ion acceleration effects in disease-relevant systems.
- Enterprise Reuse: Establishes a reusable target delivery capability applicable across multiple laser experimental campaigns and target material variations.
Operational & Enterprise Impact
- Scientific Value: Predictive confidence in ion acceleration mechanisms, reduction of experimental variability, and enhanced mechanistic de-risking.
- Operational Value: Standardized target fabrication, closed-loop alignment precision, and shot-to-shot reproducibility.
- Strategic Value: Improved capital efficiency through higher data yield per experimental session and reduced latency in target exchange.
- Portfolio Impact: Enables risk-adjusted advancement decisions by generating robust datasets for go/no-go evaluations in laser-driven biophysical studies.
Implementation Considerations
- Requires expertise in photolithography, reactive ion etching, and physical vapor deposition for target fabrication.
- Depends on triangulation ranging sensors, folding mirrors, off-axis parabolic mirrors, and software-controlled focal axis manipulators.
- Necessitates cross-team standardization between microfabrication, laser alignment, and particle diagnostics groups.
- Adaptation considerations include target material changes (e.g., gold foil thickness) and re-optimization of etching and deposition parameters.
- Practical limitations include current shot rate of 0.2 Hz, with future gains dependent on wafer handling speed and laser repetition capabilities.
