Adapting deep proton lithography to air

There are several technologies for cheap mass fabrication of microelements with large structural depth. One method uses a type of ion-track lithography, specifically deep proton lithography, which exploits the fact that some polymers are sensitive to proton radiation. Irradiating the sample with a proton beam breaks the polymer's main-chain bonds and, as a consequence, changes the way the polymer reacts to various chemical substances. During the developing process that follows irradiation, one can operate selectively on the irradiated parts of the polymer to obtain microstructures with high aspect ratios.

So far, all deep-proton-lithography systems have operated in a vacuum. In these systems, a beam of accelerated protons is patterned using a mask before illuminating a target. Working in a vacuum chamber presents several drawbacks, however: it limits the size of the target, lengthens the time needed to replace the target, and requires a relatively small vacuum-compatible steering setup.

These limitations motivated us to propose a novel proton lithography system that operates in air.¹ We found that locating the system outside the vacuum simplifies the whole system, makes the process faster and cheaper,² and allows for quick access to the target. The parameters of the proton beam change, however, when moving from vacuum into the air, which affects the way protons interact with the target. This forces us to add several new elements to the conventional proton-lithography setup.

The first element of the system transfers the high-energy proton beam from the vacuum chamber into the air. In order to do this, we placed an aluminum mask at the end of the tube driving the protons. The mask includes a hole 1mm in diameter, which is covered with a thin foil. The foil seals the vacuum chamber but lets the protons out, while causing the least possible change to their energy and other properties. Beyond the foil, we use three other masks to improve the characteristics of the proton beam. Apart from these elements, the setup resembles other deep proton lithography systems. The essential difference is that locating the system in the air limits neither the materials used nor the size of the elements.

The system works in the following way: the accelerator produces a beam of protons that travel through the thin foil and into the air (see Figure 1). After a few millimeters, the protons pass through a mask, which limits the width of the beam. Next, the beam encounters a shutter, which works like a switch. Farther on, a mask placed on a moving table forms the beam that irradiates the sample. This mask is a matrix of holes with various diameters. We choose the hole depending on the beam size we need. The next part of the setup is the target, which is located on a metal base placed on two translation stages that are joined, providing two degrees of freedom. The metal base and a picoammeter act as a current detector that measures the dose of radiation to the target. The computer accepts information from the picoammeter and decides whether to stop or to continue the radiation. The computer also positions all the moving elements.

An experimental system has been built (see Figure 2). The deep proton lithography setup in air is no longer limited to elements that can fit within the vacuum chamber, nor to equipment that is vacuum-compatible. Our modifications have addressed the changes in the proton beam traveling in air. In spite of the added elements, our setup is cheaper and easier to use than the conventional one and we have shown that it produces microelements of similar quality.

The author would like to  acknowledge the help and advice of Professors Maria and Stefan Kufner.