International Radiation Detectors, Inc.

Pulse Responsivity

Light pulses can be very destructive to sensors and optics because of the high flux levels usually encountered. Therefore, it is necessary to have a sensor with superior radiation hardness for stable responsivity. The SXUV series photodiodes were developed specifically for sources with high flux levels such as excimer lasers and the third and fourth generation synchrotrons. Unlike pyroelectric detectors which have only five or six orders of magnitude dynamic range and a significant non-uniformity of response across the surface, SXUV series photodiodes eliminate XUV exposure induced instability problems and have over eight orders of magnitude dynamic range and better than 2% uniformity. The solid state accuracy and reliability as well as the compact size and low cost of the SXUV photodiodes will provide an effective replacement for pyroelectric detectors.

When a need arises to measure light pulse sources such as lasers, certain factors must be considered. Saturation can be apparent if the energy density of the source exceeds 1 uJ/cm². Applying a reverse bias to the detector can raise the threshold of saturation as well as reduce the risetime. IRD uses a capacitively coupled bias tee (part #BT-250) to accomplish this task. The BT-250 is a low noise bias insertion tee with a DC blocking capacitor that was designed specifically for use with the SXUV/UVG series photodiodes. It should be noted that there is an approximate 5% loss of signal in the bias tee that must be accounted for when making absolute measurements.

The amount of bias needed depends on the magnitude of incident flux. The bias voltage must be increased to the point that the area under the voltage-time curve ceases to increase with increased bias voltage. This indicates that the detector is being operated in the linear region, meaning all charge generated by the incident photons is collected in the external circuit. Figure 1 shows the needed bias to avoid saturation of 100 uJ/cm² pulses. It should be noted that the applied reverse bias should not exceed the breakdown voltage of the detector.

Fig. 1: Charge seen by external circuit as a function of photodiode bias for UVG-20. 193 nm

* Excimer laser pulses with 100 uJ/cm2 energy density were used for this measurement.

Pulse Energy Measurement

The energy per pulse can be calculated as follows:

The photogenerated charge Q is proportional to the area under the voltage-time curve V(t), and is explained as

Q = ∫V(t)dt

I = V/R

Q = (1/R)∫V(t)dt

Q = A_v/R

where I is current, R is the input resistance of the oscilloscope, and A_v is the area of the time integrated voltage signal.

Certain digital oscilloscope can calculate the area under the voltage-time curve by integration to reasonable accuracy. Thus, the collected charge can be calculated by dividing the area under the V(t) curve by the shunting resistance. The shunting resistance is the input impedance on the scope or the feedback resistance of an operational amplifier if using one. The energy per pulse can then be calculated by knowing the quantum efficiency of the detector (# electrons generated in external circuit/ incident photon). We have

Energy/pulse = Q*Ep/QE

here QE is the quantum efficiency in (# electrons/photon) and Ep is the photon energy in eV.

Sample Measurement

An MPB 193 nm excimer laser (model # PSX-100) was used to compare the pulse responsivity of UVG-100 and SXUV-100 diodes and their measured CW responsivities. A capacitively coupled bias tee (IRD model BT-250) was used to reverse bias the detectors up to 120 V. The photodiode integrated voltage ∫V(t)dt was measured with a LeCroy 500 MHz digital oscilloscope with 50 ohm input impedance R and the charge Q created in the photodiode per pulse was calculated as described above.

As seen in Table 1, the correlation between the CW and pulsed 193 nm response agrees previous results for visible wavelengths [1]. This experimental verification that the CW responsivity can be used to measure pulse energy is critical to radiometric measurements of 157 nm and 13 nm pulses for which no primary standard is available presently.

Table 1: Comparison of CW and pulse responsivity of UVG* series and SXUV series photodiodes when exposed to100 nJ, 1µJ and 2.5 µJ pulses with a 3 mm diameter beam.

*Because of the high responsivity, the UVG diode was saturated at the 2.5 µJ/pulse energy level.

References:

1] R. Stuik and F. Bijkerk, "Linearity of P-N junction photodiodes under pulsed irradiation"

Nuclear Instruments and Methods in Physics Research A 489, 370-378 (2002).