Stuart P. Lansley

Diamond photodetectors for next generation 157-nm deep-UV photolithography tools

Abstract Next generation photolithography stepper tools will operate at 157 nm and require robust solid state photodetectors to ensure efficient operation and facilitate direct beam monitoring for photoresist exposure dosimetry. There is currently no commercial detector system able to fully meet all the demanding requirements of this application. Diamond, which is intrinsically visible blind and radiation hard, is an obvious candidate for consideration. In this paper we report the results of the first study to assess the viability of thin film polycrystalline diamond photodetectors for use in 157 nm F 2 –He based laser lithography tools. Co-planar inter-digitated gold photoconductor structures were fabricated on free standing thin film diamond and exposed to pulses from an industrial F 2 –He laser in the fluence range 0–1.4 mJ cm −2 . The electrical and optical characteristics of the devices have been measured and are compared to the response of a standard vacuum photodiode. The suitability of the diamond devices for use at 157 nm is discussed.

Diamond photoconductors: operational lifetime and radiation hardness under deep-UV excimer laser irradiation

Abstract The first study of long term pulse exposure and fluence level on the performance of CVD diamond photodetectors subjected to 193 nm excimer laser radiation has been performed. Whilst diamond is considered ‘radiation hard’ it is shown that damage to detector performance can be provoked at laser fluence levels considerably below that required for graphitisation or ablation. However, the application of defect passivation treatments prior to device use acts to considerably reduce the damaging effect of the radiation, such that devices suitable for stable laser monioring applications can be realised.

Imaging deep UV light with diamond-based systems

Diamond grown by chemical vapour deposition (CVD) techniques has shown great promise for the fabrication of high sensitivity, low dark current, fast and visible-blind deep UV photodetectors. In addition to the careful choice of substrate material, defect passivation treatments applied to the diamond after growth have been found to considerably enhance the detector characteristics achieved. In this paper, we report on the first purposefully designed one-dimensional CVD diamond imaging array for the detection of nanosecond 193-nm excimer laser pulses using this approach. It is shown to perform extremely well, giving less than 2% pixel-to-pixel variation in signal response, and is fast enough to avoid any sign of charge build-up during prolonged operation.

Diamond deep UV photodetectors: reducing charge decay times for 1-kHz operation

Abstract Diamond grown by chemical vapour deposition (CVD) methods is thought to be ideal for the fabrication of visible blind, fast deep UV photodetectors. However, careful device design and selection of high-quality CVD thin film diamond is, in itself, insufficient for the realisation of high performance devices. Post-growth device treatments are capable of transforming the optoelectronic properties of the material such that commercially interesting devices result. In the present study we have shown that sequentially applied methane–air treatments continue to modify both the gain level and speed of the device. Three such treatments give an optimal gain level, whilst more treatments than this lead to an improved turn-off speed. For the first time we have demonstrated the successful operation of a CVD diamond photoconductive device at at least 1 kHz at 193 nm, a frequency that is required for state-of-the-art excimer laser applications at this wavelength.

A thin-film diamond phototransistor

A phototransistor fabricated from thin-film diamond is reported. Polycrystalline diamond grown by chemical vapor deposition, which is p-type by virtue of near-surface hydrogen, has been used to realize optically activated metal–semiconductor field-effect transistors (FETs). Devices with thin (30 nm) Al Schottky gates and Au source and drain contacts operate as effective enhancement-mode metal–semiconductor field-effect transistors at room temperature; illumination of an electrically isolated gate leads to increased channel current, although saturation is still evident. At deep UV wavelengths (<220 nm), a photodetector gain of around 4 has been measured; the mechanism of operation has been identified as photodiode-like turn-on followed by FET amplification.

High-speed diamond photoconductors: a solution for high rep-rate deep-UV laser applications

Beam monitoring of excimer lasers operating at high powers in the deep ultra-violet (DUV) is becoming increasingly important, due to the rapid proliferation of these systems in micromachining, photolithography, and other areas of industrial interest. This task requires radiation-hard detectors able to operate effectively for extended periods at high laser rep-rates. DUV-visible-blind photoconductors can be fabricated on polycrystalline CVD diamond, a material that is intrinsically radiation-hard and visible-blind. However, the performance of detectors fabricated on as-grown material is insufficient to meet the requirements of many excimer laser applications. In this paper, we show that sequentially applied post-growth treatments can progressively change both the gain and speed of these devices. Charge-sensitive deep-level transient spectroscopy (Q-DLTS) and transient photoconductivity (TPC) has been used to study the effect of these treatments on the defect structure of our thin-film diamond detector material. For the first time, we report the successful operation of a diamond photoconductive device with linear bias and fluence-response characteristics at more than 1 kHz at 193 nm.

AN OPTICALLY ACTIVATED DIAMOND FIELD EFFECT TRANSISTOR

Abstract Thin film diamond photodetectors are one of the most promising classes of diamond devices for commercial exploitation. Already, photoconductive devices which display extremely high levels of selectivity between deep UV and visible light, allied to good sensitivity, are becoming available. However, more advanced device designs are required if high speed operation is to be achieved alongside high sensitivity. Phototransistors are ideally suited to this application, but until recently room temperature operation of diamond field effect transistors (FETs) was not possible. This has changed with the emergence of p-type hydrogenated diamond and this paper describes the fabrication and operation of the first MESFET based diamond photodetectors (OPFETs) to be made. Optical modulation has been demonstrated and the wavelength selectivity measured. Devices respond significantly more to UV light than visible wavelengths, with gain levels of around 4. The mechanism(s) of operation are discussed.

Diamond photodetector response to deep UV excimer laser excitation

Abstract Diamond photoconductive detectors have previously been shown to be suitable for detection of deep ultraviolet nanosecond-scale pulses from excimer lasers, which are to be used in next generation lithographic systems. Using simple test circuitry and low bias voltages easily measurable responses were observed when these detectors were illuminated with laser pulses at typical laser fluence levels. However, as the laser fluence is increased the detector response appears to broaden and change shape as well as increase in magnitude. In this paper we present analysis of the response of these detectors. This analysis shows that over the laser fluence range used the magnitude of the detector response exhibited a linear response. By approximating the temporal evolution of the laser pulse and simulating the transfer characteristics of the test circuit it can be seen that the pulse broadening and shape change seen with increasing fluence can be explained by the test circuit. The shape of the detector response, therefore, closely resembles the temporal shape of the laser pulse, demonstrating that these devices are fast enough to give a true representation of 10–15 ns laser pulses at a wavelength of 193 nm.

Diamond-based deep-UV sensors for lithography applications

Next generation photolithography stepper tools will operate at 157 nm and require robust solid state photodetectors to ensure efficient operation and facilitate direct beam monitoring for photoresist dosimetry. There is currently no commercial detector system able to fully meet all the demanding requirements of this application. Diamond has a band gap of 5.5 eV. This implies that detectors fabricated from this material may be intrinsically visible blind and radiation hard. In this paper the results of the first study to assess the viability of the use of thin film polycrystalline diamond photodetectors for use in 157 nm F2-He based laser lithography tools are presented. Co- planar interdigitated electrode structures were fabricated on free standing polycrystalline diamond to realise photoconductive devices. These were exposed to pulses from an F2-He laser in the fluence range 0 - 1.4 mJcm-2. The electrical and optical characteristics of the devices have been measured and are compared to the response of a standard vacuum photodiode. The diamond devices appear to be ideally suited for use at 157 nm in lithography applications.

Diamond-based 1-D imaging arrays

Diamond has shown great promise for the fabrication of high sensitivity, low dark current, fast and visible-blind deep UV photodetectors. In addition to careful choice of substrate material, defect passivation treatments applied to the diamond after growth have been found to considerably enhance the detector characteristics achieved. In this paper we report on the first purposefully designed 1-D CVD diamond imaging array for the detection of nanosecond 193 nm excimer laser pulses using this approach. It is shown to perform extremely well, giving less than 2% pixel-to-pixel variation in signal response, and is fast enough to avoid any sign of charge build up during prolonged operation.