Automated Delivery of Microfabricated Targets for Intense Laser Irradiation Experiments

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.