Nanofabrication of Active Edge Silicon Radiation Sensors

Shaday Edwards

St. Francis College, Brooklyn Heights, NY

Office of Science, Science Undergraduate Laboratory Internship (SULI)

Stanford Nanofabrication Facility

SLAC National Accelerator Laboratory

Stanford, CA

August 2012

Prepared in partial fulfillment as a requirement of the Office of Science, Department of Energy’s Science Undergraduate Laboratory Internship under the direction of Christopher Kenney of the Particle Physics and Astrophysics (PPA) division at SLAC National Accelerator Laboratory.

Participant: ___________________________________________________

Signature

Research Advisor: _________________________________________________

Signature

TABLE OF CONTENT

Abstract ii

Introduction

Materials and Method

Results

Conclusion

Acknowledgements

References

Abstract

Nanofabrication of Silicon Radiation Sensors with Active Edges. SHADAY EDWARDS (St. Francis College, NY 11201) CHRISTOPHER KENNEY (SLAC National Accelerator Laboratory, Stanford, CA)

Radiation is a form of energy; it is the movement of energetic particles or wave through a medium or space. There are two forms of radiation, charged particulate radiation and uncharged radiation. Charged radiation include, beta and alpha particles. Uncharged radiation include gamma, X and neutron rays. All these radiation have energies greater than 10 eV therefore, they are ionizing. The detection and measure of radiation is based upon the detection and measurements of its effect in a medium; since the discovery of radiation and its effects, scientists has been making tools to detect it. One of the first techniques of radiation detection was the scintillation. This is the use of a material to convert incident radiation energy into fluorescent; the light yield would be proportional to the energy deposited. This was first introduced in the 1930s. Today, semi-conductor radiation sensors are widely used. Silicon and germanium are the two semi-conductor materials that are used. We chose to work with silicon radiation sensors. There a few flaws associated with Silicon radiation sensors; there are dead regions present in the edges of the sensors which lowers the proficiency of the device. We have developed a new fabrication technique that will eliminate the dead regions in the sensors.

Introduction

Silicon radiation sensors are used in different areas of science. Thousands of them are arrange around collision points in particle accelerators to determine the energy and path of particles. They are used in X Ray Crystallography to collect the diffraction pattern from the scattered x rays to determine the structure of crystals. This technique is used in medicine to study the structure of crystallized viruses, protein and other complex molecules. They are various other applications of radiation sensors that I won’t get into.

How Silicon Radiation Sensors Work

Strips of silicon are doped with two types of impurities to improve the conductivity of the material. The n type strips are doped with pentavalent impurity which donates an extra electron to the silicon lattice that is lightly bounded to the impurity site. This results in a high electron concentration. The p type strips are doped with a trivalent impurity that is one fewer electron than the silicon atom, therefore on covalent bond is left unsaturated. This vacancy represents a hole.

Figure 1.0 shows the impurity atoms replacing a Si atom in the lattice

A junction is created between the n and p strips. Some of the free electrons in the n region will diffuse across to the junction and combine with the holes in the p region to form negative ions. The n region therefore, becomes positive. A space charge builds up leaving a depleted region between the two electrodes. When radiation is absorbed in the depleted region of the detector, conduction electron and holes are created; they are separated by the electron field.

The electrons go towards the n region and the holes go towards the p region; a signal is detected at both electrodes. Based on this, the position and energy lost by the particle can be detected.

Figure 1.1 Illustration of how the sensors detect particles a) 3D sensors b) planar sensor

Purpose of our Research

There are insensitive regions along the edges of the detector that ranges from a few hundred microns to a millimeter. These regions have dangling bonds that are conductive; this causes current to be lost and creates a shortage across the diodes. Guard rings are placed around the edges to prevent this from happening, as a result, the distance the sensors can be placed from the particle accelerators is limited.

Figure 1.2 Guard rings

The goal of our research is to remove the dead regions at the edges of the sensors which will make it more sensitive to radiation and eliminate to use of guard rings. We plan to accomplish this by making the edges of the sensors perforated/trenches. The trenches will be doped with either n or p type dopants. The dopants will be heated, causing them diffuse and touch creating a continuous electro- conducting wall around the sensor.

Figure 1.3 Perforated Edges

Figure 1.4 Diffused Perforated Edges

We are not only making the edges perforated but we are also eliminating some steps from the standard process. By doing this we will save time and money. The standard process involves bonding two wafers when doing the plasma etch to provide support to the wafer being etched. The support wafer is then etched off after the sensors made are separated by plasma dicing. We did not use a support wafer and instead of plasma dicing, we will be using a diamond saw to separate the sensors, which is a cheaper process.

Materials and Methods

Firstly, all seven silicon wafers were scribed and cleaned at the nonmetal wet bench to get rid of the scribe dust. Wet benches are cleaning stations consisting of chemically baths, dump rinsers and a spin/rinse dryer. The nonmetal wet bench consist of, a 9:1 sulfuric acid and hydrogen peroxide, 50:1 Hydrofluoric acid, 6:1 BOE and 20:1 BOE baths. The sulfuric acid/hydrogen peroxide bath is used for stripping photoresist and removing other dust, while the other baths are used for etching oxides. The wafers were placed in each chemical bath for a period of time then rinsed in the dump rinser, which contains water, before being transferred to the other baths. They were then placed in the spin/rinse dryer which is similar a washing machine. Secondly, the wafers will be cleaned at the diffusion wet bench to get rid of any impurities that may be present. This is similar to the wet bench nonmetal but the chemical baths are different. The first bath consist of 5:1:1 H2O:H2O2:NH4OH for removal of trace organics, 5:1:1 H2O:H2O2: HCl for removal of trace metal ions, 6:1 BOE for oxide etching and 50:1 HF, for oxide etching.

Following the diffusion clean, the wafers were placed in the tylan furnace for four hours, which operates at 10000C, to grow 7000A0 of thermal oxide. Using the Thermo LPCVD Nitride furnace, 3000A0 of Si3N4 was deposited unto the wafers following diffusion clean. The Si3N4 was then etched off on the front side of the wafer in the drytek2, using O2 and SF6 plasma. The thermal oxide was then stripped off in the BOE bath at the nonmetal wet bench. Buffered Oxide Etchant, BOE, is a mixture of ammonium fluoride, HF and water. The wafers were then placed in the yes oven for half an hour, which dehydrated the wafers at 1500C and primed it using Hexamethyldisilazane, HMDS. This allows for better coverage and adhesion of the photoresist. Approximately 100A0 of SPR 220-7 photoresist was dispense unto the wafer in the SVG coater.

The next step involves placing the wafers in the Karl Suss Contact aligner. The wafers are placed into contact with the photomask; a transparent plate covered with a pattern defined with a chrome metal absorbing film, then exposed to UV light, of wavelength 365nm, for nine seconds. The unexposed photoresist is then dissolved at the SVG developer and the image is formed. The wafers were baked at 900C for two hours to harden the remaining photoresist making it resistant to plasma etching. It was then etched in the STS Etcher for an hour and a half.

Results

After approximately seventy minutes of etching the first wafer, it broke. This does not mean that the technique does not work. To get positive results we will use a metal wafer holder to evenly distribute the pressure on the wafer. Also we could include another wafer in the holder to relief pressure from the wafer being etched.

Conclusion

A conclusion can’t be determined at the moment; project is still in progress.

Acknowledgements

Chris Kenney, Mentor Jasmine Hasi Julie Segal Frankie Reed SLAC National Accelerator Laboratory Department of Energy Stanford Nanofabrication Facility Steve Rock and Maria Mastrokyriakos

References

[1] Glenn F. Knoll Radiation Detection and Measurement, 3rd ed., John Wiley & Son, Inc. 2000

[2]