International Radiation Detectors, Inc.

Proper Use of the IRD Photodiodes

1) AXUV/UVG/SXUV series photodiodes can be used in air, in gas ambient like helium, argon, nitrogen, etc. and under vacuum lower than 10^-10 torr. Devices with suffix EUT need to be selected for extremely low level vacuum applications like 10^-10 torr. IRD photodiodes can be vacuum baked up to 200ºC. They can be operated in the temperature range of -200ºC to 70ºC.

2) It is advisable to check the photodiode for functionality before placing it into the location of measurement such as a vacuum chamber. Do this by connecting the leads of the diode to the positive and negative terminals of a picoammeter or electrometer and exposing the diode to room light. As an example, an uncoated 1 cm x 1 cm photodiode should produce an output current of 20 to 200 micro amperes depending on room light intensity. A filtered photodiode should have an output current of less than a few nano amperes. A table lamp or flashlight can also be used to check the filtered diode functionality if room light conditions do not produce a measurable output.

3) To avoid contribution of the photo emission current to the photogenerated current, the signal should be read from the p-region (anode) of the diode with n-region (cathode) grounded. A significant photo emission current contribution has been noticed in the AXUV diodes in 70 nm to 140 nm region when the signal is read from the n-region with p-region grounded.

4) Condensables within the vacuum system in which IRD photodiodes are used must be maintained at as low a level as possible. A surface film deposited on the diode will absorb radiation and may even fluoresce. In either of these cases, a permanent film could result from interaction with XUV radiation leading to irreversible changes in device efficiencies.

5) As the most of the IRD photodiodes are windowless devices, precaution should be taken not to breathe, sneeze or touch the active area of the devices. If by accident the active area is contaminated, the surface can be cleaned by an acetone or alcohol dipped swab. As the IRD diodes have very shallow junction, the swabbing action should be gentle without any pressure. All the filtered diodes with carbon and silicon passivating coating (for example AXUV100 Mo/Si, AXUV100 Ti/C etc.) can also be cleaned by the above method. Please contact IRD for instructions on cleaning photodiodes with other directly deposited filters.

6) Care must be taken to avoid disturbing the delicate wire bonds which connect the photodiode chip to the package pins. If the wire becomes pushed down against the chip, a short will result. If this happens, use a fine wire or needle to gently lift the wire bond away from the chip surface (a microscope will usually be needed). If the wire bonds are broken the device can be sent back to IRD for repair. Most of the IRD photodiodes are assembled with multiple bonds and in most cases, redundant pins. Only one wire bond per pin is necessary for proper operation of the device.

7) Most of the AXUV/UVG/SXUV diodes can be purchased with protective epoxy on their wire bonds so that the bonds will not brake if touched accidentally. However, the diodes whose wire bonds are protected by epoxy need to be operated near room temperature, i.e., one can not heat these devices for out gassing purpose nor cool them to reduce the noise. This is necessary to avoid generation of stress on the wires because of difference in the Thermal Coefficient of Expansion (TCE) between the wire and the epoxy.

8) Linear range of AXUV/UVG/SXUV diodes can be increased several times by application of a small reverse bias. Thus, under high level radiation condition, the diode saturation can easily be checked by application of one to two volts reverse bias. In extreme cases, in excess of one hundred volts reverse bias is applied to the diodes to operate them in the linear range. A simple series circuit of the voltage source, diode and current measuring device is required when the source is a CW source. Make sure the diode is reverse biased and not forward biased as this may damage the diode. When using the diodes for pulse energy measurements usually an external bias T, like BT250 is required.

9) Care should be used while using the small active area diodes like the AXUVHS5 as absolute devices. All the AXUV products can be used as absolute devices provided that radiation beam is limited to the diode active area. When the diode is flooded (overfilled) with the beam, it is hitting the diode periphery, the quantum efficiency of which in not known. Detectors with active area as large as 24 mm x 24 mm are available from IRD to make absolute intensity measurement of large beams.

10) Since photodiodes are subject to damage by excessive heat, care must be given to soldering temperature and dwell time. As a guide, metal package devices should be soldered at a maximum of 260 ºC within 10 seconds and ceramic package devices at 260 ºC within 5 seconds at 2 mm minimum distance from package base. Use acetone or alcohol to remove solder flux. To avoid contamination of the active area, the protective window shipped with the photodiode should be left on during soldering.