Durable ultraviolet sensors using highly oriented diamond films

Diamond & Related Materials 15 (2006) 792 – 796 www.elsevier.com/locate/diamond

Durable ultraviolet sensors using highly oriented diamond films Kazushi Hayashi a,⁎, Takeshi Tachibana a , Nobuyuki Kawakami a , Yoshihiro Yokota a , Koji Kobashi a , Hideaki Ishihara b , Koji Uchida b , Kenji Nippashi b , Mikihiko Matsuoka b a

Kobe Steel, Ltd., Electronics Research Laboratory, 1-5-5, Takatsuka-dai, Nishi-ku, Kobe 651-2271, Japan b Iwasaki Electric Co., Ltd., R&D Center, 1-20, Fujimi-cho, Gyoda, Saitama 361-0021, Japan Available online 18 January 2006

Abstract We fabricated vacuum ultraviolet (VUV) sensors using highly oriented diamond (HOD) films and studied their long-term stability to evaluate the feasibility of the sensors for practical use. The sensors are composed of a pair of interdigitated Pt electrodes on the HOD film surfaces, and sealed in the industrially standard can packages. Xenon (Xe) excimer lamps (λ = 172 nm) and low-pressure mercury (Hg) lamps (λ = 185 nm) were used as the radiation sources. It was demonstrated that the output signal of the sensors is reproducible and stable over 700 h under the intense VUV irradiation from Xe excimer lamps. The results of low-pressure Hg lamp measurements showed that the sensors were sensitive only to the 185-nm radiation irrespective of the presence of the 254-nm radiation that is five to ten times more intense than the 185-nm radiation. It is thus concluded that the present HOD film sensors make possible a continuous monitoring of the intense VUV radiation sources, and can be applicable to dry cleaning of silicon wafers or glass substrates in semiconductor industry and flat panel display manufacturing processes, respectively. © 2005 Elsevier B.V. All rights reserved. Keywords: Diamond film; Plasma CVD; Optical properties characterization; Sensors

1. Introduction Diamond is a semiconducting material that has excellent chemical inertness and radiation hardness [1]. Because the band-gap energy of diamond (5.5 eV) corresponds to the wavelength of 225 nm, diamond absorbs only far ultraviolet and vacuum ultraviolet (VUV) radiations. Therefore, diamond is considered to be a suitable material for solar-insensitive VUV sensors durable to an intense VUV radiation. There are a number of studies to realize practical VUV sensors [2–18]. The applications include laser power monitors, detection of extremely ultraviolet radiation for the next generation lithography, and space applications. Recently, intense VUV radiation sources such as xenon (Xe) excimer lamps (λ = 172 nm) and low-pressure mercury (Hg) lamps (λ = 185 nm) are used for dry surface cleaning of semiconductor wafers and large glass plates for flat panel displays in their manufacturing processes. In many industrial ⁎ Corresponding author. Tel.: +81 78 992 5614; fax: +81 78 992 5650. E-mail address: [email protected] (K. Hayashi). 0925-9635/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2005.11.042

processes, it is desired to continuously monitor the irradiance of the radiation sources to maintain the product quality, existing sensors such as Si photodetectors and fluorescent sensors are, however, often seriously degraded, particularly when they are continuously illuminated by the VUV radiation of high intensity. On the other hand, diamond has a potential to withstand such intense VUV radiation as high as 50 mW/cm2 and does not require any optical filter that is easily degraded only to detect the VUV radiation. However, there have been only few reports on the long-term stability, which is one of the most important specifications for the industrial use of the sensors. In the present study, we fabricated VUV sensors using highly oriented diamond (HOD) films that are superior to polycrystalline films in terms of stability and reproducibility due to confined distribution of crystal size and improved surface flatness which is particularly advantageous to fabricate sensing devices including electrode deposition. We studied the long-term stability under the irradiations of intense Xe excimer lamps and low-pressure Hg lamps to evaluate the feasibility of the sensors for practical use.

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2. Experimental procedure 2.1. Film growth Highly oriented diamond films, consisting of azimuthally oriented (001) facets [19,20], were used in this study. The films were deposited on high-resistivity Si (001) substrates using a microwave plasma chemical vapor deposition reactor capable of bias-enhanced nucleation (BEN) [21]. The conventional threestep growth was utilized for the HOD film deposition [20]: first, the surface of the Si substrates was carburized, and then the BEN treatment, in which a negatively biased substrate was immersed in the plasma, was undertaken to form epitaxial diamond nuclei on the Si substrate surface. Successively, a textured growth was carried out to form a b100N-oriented diamond film, which was followed by a lateral growth of azimuthally oriented diamond grains. Consequently, the entire surface of the diamond film was covered with (001) facets of several square micrometers. The film thickness was typically 10 μm. The surface morphology of a typical HOD film is shown in Fig. 1. 2.2. Device process After the growth, the film surface was cleaned to remove graphitic components and a surface conducting layer [22]. The treatment leads to a drastic decrease in the leakage current at the surface when the electrodes are fabricated. A pair of interdigitated Pt electrodes was fabricated on the HOD film surface by photolithography (lift-off technique). The Pt deposition was done by magnetron sputtering. The thickness of the Pt electrodes was approximately 0.2 μm, while the distance between the electrodes was 10 μm. The patterned area of the interdigitated electrodes was 2 × 2 mm2, while the total size of a sensor chip was 3 × 3 mm2. The sensors were finally mounted on a hermetic seal (standard TO5 type package) and wire-bonded to lead pins using gold wires. A sapphire window having a cutoff wavelength of around 140 nm was used as the seal cap, and the inside was purged by argon gas. Packaged diamond VUV sensors are shown in Fig. 2.

Fig. 2. Packaged diamond sensors with and without the top window.

2.3. Device characterization Responses of the VUV sensors were examined using both Xe excimer and low-pressure Hg lamps. The former has a board single peak whose center wavelength is 172 nm. The latter has a sharp peak at 185 nm in the VUV region, but the dominant peak is at 254 nm, which is five to ten times more intense than the 185-nm radiation. In some experiments, the irradiance from the radiation source was evaluated by a silicon photodiode (SXUV100, Internal Radiation Detectors) that had been calibrated by the National Institute of Standards and Technology (NIST)calibrated secondary standard photodiode. Since the present VUV sensors are photoconducting-type, a bias voltage was applied between the interdigitated electrodes. The dark currents of the sensors were less than 100 pA, typically 10–20 pA, when the bias voltage was 40 V. The I–V characteristics were linear, indicative of ohmic contacts between the Pt electrodes and the HOD film. For the temporal response measurements, an amplifier was attached to the sensors, and the output current of the sensors was converted to a voltage output. All measurements were carried out in a pure nitrogen atmosphere to prevent the absorption of the VUV radiation by oxygen. Fig. 3 shows a typical responsivity of a packaged diamond sensor as a function of the radiation wavelength [13,18]. The spectrum was obtained using a monochromatized radiation from a deuterium lamp in the wavelength

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Fig. 1. Surface morphology of highly oriented diamond film.

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Fig. 3. Typical responsivity of a packaged diamond VUV sensor as a function of radiation wavelength.

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range between 100 and 250 nm. The sensor possesses a cutoff at 225 nm that corresponds to the band-gap energy of diamond. The output of the sensor increases for shorter radiation wavelength, and exhibits a maximum at around 190 nm. The decrease in output in the wavelengths less than 190 nm is due to the absorption of the VUV radiation by the sapphire window. 3. Results 3.1. Measurements of irradiance from Xe excimer lamp

Fig. 6. Temporal response of a VUV sensor observed from a continuous irradiation of a Xe excimer lamp with 40 mW/cm2 intensity.

external quantum efficiencies of the sensors depended on the applied bias voltage, but estimated to be in the range between 10− 2 and 10− 1 for the radiation at around 190 nm [23]. 1.2

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Fig. 4. Output currents of a VUV sensor as a function of xenon lamp incident power. The irradiance of the lamp was calibrated by a NIST-calibrated secondary standard photodiode. The bias voltages of the sensor were 5, 10, and 15 V.

Fig. 4 shows the output photocurrents of a diamond VUV sensor as a function of the irradiance of incident radiation from the Xe excimer lamps [18]. The irradiance of the lamp was changed between 4 and 12 mW/cm2 and calibrated by the NIST-calibrated secondary standard photodiode. The bias voltages applied between the electrodes of the sensor were 5, 10, and 15 V. It was found that the output photocurrents of the sensor proportionally increased with the irradiance of the incident radiation. The output photocurrents also increased with the bias voltage applied between the electrodes. The

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Fig. 5. Temporal response of a VUV sensor to a Xe excimer lamp. The lamp was intermittently turned off for 45 s at an interval of 150 s.

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Fig. 7. Output signals from a continuous irradiation of a Xe excimer lamp with 10 mW/cm2 intensity measured for 30 min (a) before and (b) after a continuous exposure (210 h) of the low-pressure Hg lamp.

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3.2. Measurements of irradiance from low-pressure mercury lamps Temporal response of the VUV sensors to low-pressure mercury lamps with an intensity of 0.4 mW/cm2 was measured. The contribution of the 185-nm radiation to total output signal was evaluated by attaching two different quartz windows as an optical filter, as shown in Fig. 8. The transmittance of the

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The temporal response of the VUV sensor to the Xe excimer lamp is shown Fig. 5 [18]. The irradiance of the lamp was fixed at 10 mW/cm2. In this case, the lamp was intermittently turned off for 45 s at an interval of 150 s. The output signal of the sensors was observed to increase immediately after the lamp was turned on, and then stabilized. A fast response was also observed when the Xe excimer lamp was turned off. Note that the slight decrease of the sensor output signal immediately after the lamp was turned on is attributed to the change of transmittance of heated quartz glass tube by the arc of the lamp. Next, the long-term stability of the sensors to an intense VUV irradiation was examined. Fig. 6 shows a temporal response of the sensor obtained from a continuous irradiation of the Xe excimer lamp with an irradiance of approximately 40 mW/cm2. The output signal of the sensor was measured every 20 h, while the sensor was exposed to the radiation from the lamp throughout the test. It was clearly observed that the output signal of the sensor was stable even after 700 h. The long-term stability was also evaluated by exposing the sensor to a low-pressure Hg lamp (λ = 185 nm) with an irradiance of approximately 4.6 mW/cm2. The output signal from a continuous irradiation of a Xe excimer lamp with 10 mW/cm2 intensity was measured for 30 min, and compared before and after a continuous exposure (for 210 h) to the low-pressure Hg lamp. The results are shown in Fig. 7(a) and (b). No degradation was seen in the sensor response. These results indicate that the present diamond VUV sensors including the sapphire window are sufficiently durable for a continuous monitoring of an intense VUV radiation from Xe excimer and low-pressure Hg lamps.

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Fig. 9. Temporal response of a VUV sensor measured using an optical shutter. The shutter was intermittently closed for 45 s at an interval of 150 s.

185-nm radiation through window A was 0%, while that through window B was about 90%. Both windows act an optical filter that reduces the intensity of the 185-nm radiation during the measurements. The output signal instantly became zero when the window A was inserted in the optical path. The output signal reverted when the window A was replaced by window B. However, the output remained to be 80% of the initial value. The output signal returned to its original value when the window B was taken out. This result indicated that the photocurrent was generated only to the 185-nm radiation irrespective of the presence of the 254 nm radiation whose intensity was five to ten times higher. The temporal response of the VUV sensor was also measured using an optical shutter. The shutter was intermittently closed for 45 s at intervals of 150 s. As seen in Fig. 9, the output signal well followed the shutter action; the output signal increased immediately and become almost constant when the shutter was open. The quick response was also observed when the shutter was closed. Finally, the stability of the sensors to low-pressure Hg lamps was examined. Fig. 10 shows a temporal response of the sensor observed by a continuous irradiation from the low-pressure Hg lamp with an intensity of 0.4 mW/cm2. The output was 1.2 1

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measured every 30 min, while the sensors were exposed to the radiation throughout the test. It is clearly seen that the output signal of the sensor was stable without any influence by other intense radiation such as 254 nm. The radiation with the wavelength longer than 185 nm usually increases the temperature of conventional sensors, causing unstable responses, but the present diamond VUV sensor is very stable under such a situation. 4. Discussion The present VUV sensors were reproducible and stable for 700 h under the intense VUV irradiation from Xe excimer lamps. The excellent durability of the sensors against the intense VUV radiation can be attributed to the following. The C–C bonds in diamond are much stronger than any other semiconductors such as Si, which is widely used as VUV sensors. Due to the covalent bonds between carbon atoms, it is hard to form and propagate defects in the crystal, which could act as carrier traps and decrease sensor output. Second, the diamond films used in the present work posses the oxygen terminated surfaces that are stable at high temperature. While diamond does not form oxide layers at the surface, often arise in Si photodetectors are defects at the interface between silicon and silicon oxide, which act as recombination centers of carriers [24,25]. Another important point to be emphasized is the operation temperature. During the intense VUV irradiation, the temperature of the sensor increases. This means that some carriers are excited thermally, causing a higher leakage current in the sensor. In the case of the diamond VUV sensors, however, the number of thermally excited carriers is negligible because the band-gap energy of diamond is very large. Also, the high temperature generally degrades the junction properties of the sensors. The present HOD film sensors have only a pair of stable ohmic electrodes on the film surfaces, and hence are not virtually influenced by the high temperature. 5. Conclusions VUV sensors using HOD films were fabricated, and their long-term stability was examined to evaluate the feasibility of the sensors for practical use. It was demonstrated that the output signal of the sensors is reproducible and stable over 700 h even under the intense VUV irradiation from Xe excimer lamps. It was found that the photocurrent of the VUV sensors was generated only by the 185-nm radiation irrespective of the presence of the 254-nm radiation that is five to ten times more intense than the 185-nm radiation. The output signal of the sensors was stable to the continuous irradiation from lowpressure Hg lamps. Thus, it is concluded that the present HOD sensors make possible a continuous monitoring of the intense VUV radiation, and can be applicable to various manufacturing processes.

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