Rapid solar blind ultraviolet imaging detector based on gallium oxide Schottky diode array

Abstract

The invention discloses a rapid solar blind ultraviolet imaging detector based on a gallium oxide Schottky diode array, which comprises a Pt metal layer, a passivation layer, a beta-Ga2O3 single crystal and a Ti / Au contact electrode, and is characterized in that the Pt metal layer is connected with the upper surface of the beta-Ga2O3 single crystal; the passivation layer is embedded and matched with the Pt metal layer, the passivation layer is connected with the upper surface of the beta-Ga2O3 single crystal, and the lower surface of the beta-Ga2O3 single crystal is connected with the Ti / Au contact electrode. The detector has excellent performance under the irradiation of solar blind ultraviolet light, and also has excellent optical image recognition capability. The method can be widely applied to the field of photoelectric detectors.

Description

Technical Field

[0001] The present invention relates to the field of photodetectors, and particularly to a fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array. Background Art

[0002] Since the solar radiation in the solar-blind ultraviolet band (200 - 280 nm) is completely absorbed by the ozone layer during the process of passing through the atmosphere, there is almost no naturally generated solar-blind ultraviolet light signal on the earth's surface. Compared with visible light and infrared detection, solar-blind ultraviolet detection has lower background noise, higher anti-interference ability and higher detection accuracy when capturing target signals, and is widely used in many fields such as missile tracking and flame detection and shows great advantages.

[0003] Silicon-based photodetectors are currently widely used commercial solar-blind ultraviolet photodetectors due to their mature integration technology. However, silicon has a small bandgap width and responds in a wide spectral range. In the field of solar-blind ultraviolet detection, it needs to be used in combination with a heavy filter to shield the visible light background, which not only increases the complexity of the system, but also attenuates the solar-blind ultraviolet light signal, resulting in a reduction in detection efficiency. In addition, based on the characteristics of silicon materials, silicon-based photodetectors cannot cope with the harsh application environments that may appear in the field of solar-blind ultraviolet detection, such as high temperature, high pressure and high radiation. Therefore, silicon-based photodetectors face difficult problems such as low responsivity in the solar-blind ultraviolet band and unsuitability for application in harsh environments. Summary of the Invention

[0004] In view of this, in order to solve the technical problems that the existing silicon-based photodetectors have low responsivity to the solar-blind ultraviolet band and cannot cope with the harsh application environments that may appear in the field of solar-blind ultraviolet detection, the present invention proposes a fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array, including a Pt metal layer, a passivation layer, a β-Ga2O3 single crystal and a Ti / Au contact electrode, wherein:

[0005] Using metal Pt as the top Schottky contact electrode, the Pt metal layer is connected to the upper surface of the β-Ga2O3 single crystal;

[0006] In order to improve the device response speed and reduce the device dark current, an amorphous SiO2 is covered as the passivation layer in the area except the Pt metal layer, the passivation layer is inlaid and matched with the Pt metal layer, and the passivation layer is connected to the upper surface of the β-Ga2O3 single crystal

[0007] Ti / Au is used as the bottom ohmic contact common electrode, and the lower surface of the β-Ga2O3 single crystal is connected to the Ti / Au contact electrode.

[0008] The overall preparation structure is a vertical device of Pt / β-Ga2O3 / Ti / Au.

[0009] In some embodiments, it further includes:

[0010] Covering a layer of fine-mesh Ti / Au electrodes on top of the Pt metal layer can reduce the incident light blocking area and increase the collection efficiency of photo-generated carriers.

[0011] Based on this, the array elements are then led out for wiring interconnection, which is convenient for testing the electrical performance of individual array elements.

[0012] Based on the above scheme, the present invention provides a fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array, with a structure of Pt / β-Ga2O3 / Ti / Au. This device exhibits an extremely fast response speed and low dark current, and each array element has high uniformity and repeatability. At the same time, this work also demonstrates the imaging ability of the device and obtains a clear target imaging pattern. This work is the first to attempt the integration of an on-chip vertical Schottky barrier structure photodetector, proving the application potential of the fabricated device in high-speed imaging and providing a direction for the development of high-precision integration of solar-blind ultraviolet imaging detection technology from laboratory research to practical applications. Description of the Drawings

[0013] Figure 1 (a) is a conceptual diagram of solar-blind ultraviolet fast imaging application, Figure 1 (b) shows the electron affinities of Pt, β-Ga2O3, and Ti materials, Figure 1 (c) is a schematic diagram of the device structure and the top grid electrode design, Figure 1 (d) shows the optical transmittance of 10 nm thick Pt in the solar-blind ultraviolet band, Figure 1 (e) is the transmittance of the photoresist in the ultraviolet band;

[0014] Figure 2 (a) is the XRD pattern of β-Ga2O3 single crystal, Figure 2 (b) is the high-resolution TEM image of β-Ga2O3 single crystal and the corresponding SAED pattern, Figure 2 (c) is the Raman spectroscopy test result of β-Ga2O3 single crystal, Figure 2 (d) is the optical transmittance of β-Ga2O3 single crystal, Figure 2 (e) is the AFM result of the 10 nm thick Pt electrode on the surface of β-Ga2O3;

[0015] Figure 3 is a schematic diagram of the preparation process of the 8×8 planar array detector in the specific embodiment of the present invention;

[0016] Figure 4 (a) is the microscopic image of the planar array detector, Figure 4(b) is the semi-logarithmic I-V curve of the device under dark conditions and 246 nm UV illumination, Figure 4 (c) is the IV curve with variable power, and the illumination intensity varies in the range of 24.1 - 481.4 μW / cm 2 , Figure 4 (d) is the transient response of the photodetector under 257 nm pulsed laser excitation, Figure 4 (e) is the I-T curve at different voltages, Figure 4 (f) is the energy band diagram after the top electrode Pt, back electrode Ti / Au are in contact with the β-Ga2O3 single crystal;

[0017] Figure 5 (a) is the spectral response and EQE curve of the photodetector at 0 V. The inset shows the suppression ratio of the responsivity of the device at 213 nm and 350 nm wavelengths, Figure 5 (b) is the 1 / f noise power spectrum of the device in the test environment at 0 V, Figure 5 (c) is the stability test of the device at 0 V after 8 months, Figure 5 (d) is the performance comparison radar chart of the Ga2O3-based solar-blind UV detector, Figure 5 (e) is the comparative analysis of the response time of solar-blind UV detectors made of different materials;

[0018] Figure 6 (a) is the schematic diagram of the photodetector array imaging system, Figure 6 (b) and Figure 6 (c) are the dark current uniformity distribution maps at 0 V and -1 V respectively, Figure 6 (d) is the schematic diagram of testing the photocurrent of each array element under bias voltages of 0 V and -3 V, Figure 6 (e) and Figure 6 (f) are the distributions of photocurrent and dark current at 0 V respectively, Figure 6 (g) and Figure 6 (h) are the imaging results at 0 V and -3 V. Detailed implementation manners

[0019] Next, the technical solutions in the embodiments of the present application will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present application. Obviously, the described embodiments are only a part of the embodiments of the present application, rather than all the embodiments. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present application.

[0020] It should be noted that for the convenience of description, only the parts related to the relevant invention are shown in the accompanying drawings. Without conflict, the embodiments in the present application and the features in the embodiments can be combined with each other.

[0021] As shown in this application and the claims, unless the context clearly indicates otherwise, words such as "a", "an", "one", and / or "the" are not specifically singular and may also include the plural. Generally speaking, the terms "comprising" and "including" only indicate the inclusion of the steps and elements that have been clearly identified, and these steps and elements do not constitute an exclusive list. A method or device may also include other steps or elements. An element defined by the statement "comprising one..." does not exclude the existence of other identical elements in the process, method, product, or device that includes the element.

[0022] In the description of the embodiments of this application, "a plurality" means two or more than two. The following terms "first" and "second" are only used for descriptive purposes and cannot be understood as indicating or implying relative importance or implicitly specifying the quantity of the indicated technical features. Thus, the features defined with "first" and "second" may explicitly or implicitly include one or more of such features.

[0023] In addition, flowcharts are used in this application to illustrate the operations performed by the system according to the embodiments of this application. It should be understood that the operations before or after do not necessarily need to be executed precisely in sequence. On the contrary, they can be executed in reverse order or simultaneously. At the same time, other operations can also be added to these processes, or one or several operations can be removed from these processes.

[0024] In order to achieve more precise synchronization between the detected target and the detection result, thereby improving the working efficiency of the application scenario, a fast and sensitive solar-blind ultraviolet photodetector is a basic element in the above advanced application fields. With the development of optoelectronic integration technology, the demand for solar-blind ultraviolet detectors with imaging capabilities is increasing. In this context, a high-performance solar-blind ultraviolet array detector with a fast response speed and low power consumption is a key step in realizing the practical application of solar-blind ultraviolet detection technology. Its application concept is as Figure 1 (a) shown. Specific Embodiment 1:

[0026] A fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array, characterized in that it includes a Pt metal layer, a passivation layer, a β-Ga2O3 single crystal, a Ti / Au contact electrode, and a fine grid Ti / Au electrode, wherein: the Pt metal layer is connected to the upper surface of the β-Ga2O3 single crystal; the passivation layer is fitted and matched with the Pt metal layer, the passivation layer is connected to the upper surface of the β-Ga2O3 single crystal, the lower surface of the β-Ga2O3 single crystal is connected to the Ti / Au contact electrode, and the fine grid Ti / Au electrode covers the top of the Pt metal layer.

[0027] To form a Schottky barrier metal-semiconductor contact type with β-Ga2O3 single crystal, in this work, metal Pt with the largest work function and relatively high transmittance to solar-blind ultraviolet light was selected as the top Schottky contact electrode, and Ti / Au was used as the bottom ohmic contact common electrode to fabricate a vertical device with the structure of Pt / β-Ga2O3 / Ti / Au. Figure 1 (b) is a schematic diagram of the theoretical energy band structure before metal-semiconductor contact. To improve the device response speed and reduce the device dark current, an amorphous SiO2 was covered as a passivation layer in the area except for the Pt metal layer, which can also reduce the influence of the metal layer fabricated in the subsequent wiring process on the electric field distribution between the array elements. On the basis of this structural design, a Ti / Au electrode with a fine grid was covered on the top of the Pt metal layer, which can reduce the incident light blocking area and increase the collection efficiency of photo-generated carriers, and then the array elements were led out for wiring interconnection to facilitate the electrical performance test of individual array elements. The device structure and the schematic diagram of the top grid electrode design are shown as Figure 1 (c).

[0028] By analyzing the results of multiple experiments, when the thickness of the Pt layer is approximately 10 nm, it has both relatively high transmittance and relatively uniform film quality. The transmission of solar-blind ultraviolet light is shown as Figure 1 (d). In addition, to reduce the interference of photo-generated carriers in the non-photosensitive area on the device response signal, in this work, a photoresist with strong absorption of solar-blind ultraviolet light was used to cover the non-photosensitive area of the device. The transmittance of this photoresist is shown as Figure 1 (e).

[0029] Based on the above scheme, the present invention also described the characterization of the basic properties of the material: the existence of crystal defects will affect the separation of photo-generated carriers, so the crystal quality of the photo-absorbing layer is extremely important for the detection performance. Figure 2 (a) is the XRD of β-Ga2O3 single crystal. The results show that this β-Ga2O3 single crystal is the (100) plane, and 30.107° and 45.848° correspond to the (400) and (600) planes respectively. The inset is the double crystal rocking curve of β-Ga2O3 single crystal, and the full width at half maximum is only 61 arcsec, indicating relatively high crystal quality. Figure 2 (b) shows the low magnification and high resolution transmission electron microscope (TEM) images of β-Ga2O3 single crystal, and the single crystal diffraction pattern. The consistent orientation also proves the relatively high crystal quality of β-Ga2O3 single crystal. The Raman spectrum is shown as Figure 2 c shows 11 characteristic peaks belonging to β-Ga2O3 in the range of 100 - 1000 cm-1, indicating the preparation of β-Ga2O3. From the UV-Vis transmission spectrum of β-Ga2O3 single crystal, an obvious absorption cut-off edge can be observed in the solar-blind ultraviolet band, asFigure 2 As shown in (d), its optical band gap is about 4.71 eV, indicating that the β-Ga2O3 single crystal has solar-blind ultraviolet detection ability. To ensure the stability of the Pt layer manufacturing process, the thickness and uniformity of the Pt layer were obtained using the AFM method. Figure 2 As shown in (e).

[0030] Based on the above scheme, the present invention also provides a method for fabricating an 8×8 planar array detector based on β-Ga2O3 single crystal. On the basis of completing the structure design, the 8×8 planar array detector based on β-Ga2O3 single crystal is gradually fabricated. The device manufacturing process mainly includes photolithography process and metal electrode plating, as Figure 3 shown. First, an 8×8 Schottky metal pattern needs to be completed. The pattern is made using photolithography, and then Pt metal is sputtered using an ion beam. After stripping, a Pt array with a size of 600×600 μm is obtained. Next, a SiO2 passivation layer is made on the part other than the Pt array. Then, the top metal grid pattern is constructed by photolithography, and a Ti / Au (10 nm / 100 nm) pattern electrode is made using electron beam evaporation. The grid size of the photosensitive area is 595×595 μm, the line width is 5 μm, and the non-photosensitive area is provided with leads to connect the array elements to the Pad electrodes, with a width of 20 μm. The Pad electrodes connected externally to the leads are for facilitating testing and packaging, and the electrode size is 200×300 μm. Similarly, a bottom Ti / Au (10 nm / 100 nm) ohmic contact is obtained using electron beam evaporation as the bottom common electrode of the device. Finally, the area other than the photosensitive array elements and Pad electrodes is covered with photoresist to ensure that only the photosensitive array elements can respond to solar-blind ultraviolet light. The surface structure of the device is observed using a microscope as Figure 4 (a), which is consistent with the layout ( Figure 1 c), indicating that the surface device manufacturing process is reliable.

[0031] Based on the structure of the above embodiments, the present invention also conducted basic electrical performance tests on the device:

[0032] On the basis of the device structure design and fabrication, the optoelectronic properties of the device were systematically measured for any one of the array elements. Figure 4 (b) shows the I-V curves of the device under light-dark and light irradiation conditions with a wavelength of 246 nm. The results show that the device has typical Schottky barrier contact characteristics, and the open-circuit voltage is 0.85 V. It is worth noting that the photo-to-dark current ratio (PDCR) of the device at 0 V is about 5.04×10 4, the dark current is as low as 0.58 pA. These results indicate that the device can operate without applying an external bias. At the same time, the large PDCR and low dark current can significantly reduce the influence of noise and improve the sensitivity of the device. These factors are the key indicators for describing the solar-blind ultraviolet imaging ability. To analyze the dependence of the incident light intensity and photocurrent, the I-V curves at different optical power densities were measured as shown in Figure 4 (c). As the incident optical power density decreases, the photocurrent and open-circuit voltage of the device also tend to decrease. This is because the reduction of the incident optical power leads to a decrease in the number of incident photons, and fewer photo-generated carriers result in lower photocurrent and open-circuit voltage at the same bias voltage. As a key parameter of the detector, the response speed was measured using a 257 nm pulse laser, Figure 4 (d) shows a single-cycle response signal. The response rise time τ r and the fall time τ d are defined as the time required to change from 10% to 90% of the saturated photocurrent. By analysis, the rise time is only 5.9 ns, and the fall time is fitted to the falling-edge curve using the bi-exponential relaxation equation 21 :

[0033]

[0034] where I0 is the steady-state photocurrent, t is the time, A and B are fitting constants, and τ d1 and τ d2 are the two components of the relaxation time respectively. The fitting results are in good agreement with the experimental data, and τ d1 is 21.4 ns, and τ d2 is 78.6 ns. Therefore, the solar-blind ultraviolet device fabricated in this work has an extremely fast response speed, indicating that the incident photons rapidly excite photo-generated carriers, which are quickly collected by the electrodes without applying an external bias, resulting in a short response time. The response speed in the nanosecond range is much faster than the reported Ga2O3 devices, and the ability to follow fast optical signals demonstrates the potential for applications in the field of fast solar-blind ultraviolet imaging. By Figure 4 (e), the current values of the periodic switching with time at different operating voltages were observed. The stable photocurrent indicates that the response performance of the device is repeatable. The energy band diagram of this Schottky barrier detector with excellent performance is shown in Figure 4As shown in (f), the different work functions of Pt and β-Ga2O3 result in band bending when they are in contact, forming a Schottky barrier and generating a rectifying effect. On the contrary, the work functions of Ti and β-Ga2O3 are similar, and the interface barrier is low, so a good Ohmic contact can be formed. When exposed to solar-blind ultraviolet light, the electrons in β-Ga2O3 are excited from the valence band to the conduction band, and the generated photo-generated electron-hole pairs are separated under the action of the built-in electric field, and finally a photocurrent is formed.

[0035] Responsivity, external quantum efficiency and detectivity are also key indicators to describe the detection performance of the device. Through the formula the responsivity can be calculated, where P and S are the incident optical power and the effective area of the device, respectively. The external quantum efficiency can be calculated by the formula where h is Planck's constant, c is the velocity of light, R is the responsivity, q and λ are the electron charge and the incident light wavelength, respectively. From the Figure 5 spectral response characteristics of (a), the device shows a relatively good light response and the response cut-off wavelength is about 253 nm (the cut-off wavelength is the wavelength corresponding to the responsivity dropping to 1 / e of the maximum value). The responsivity reaches a maximum of 0.025 A / W at 0 V, and the EQE reaches a maximum of 14.5%. The rejection ratio R 213 nm / R 350nm is as high as 8.9×10 3 , proving the excellent spectral selectivity of the detector. The detectivity is described by the formula and the detectivity is maximum when the incident light wavelength is 213 nm, reaching 7.7×10 10 Jones at 0 V bias.

[0036] The noise level is one of the important factors affecting the detection performance of the device. The environmental background noise and the noise spectral density of the detector at 0 V were measured as Figure 5 (c). The noise spectral density shows an inverse proportional relationship with the frequency, showing good frequency dependence characteristics. The measured results of the noise density varying with frequency can be expressed by Hooge's law as where S I is the noise spectral density, I is the total current of the device, f is the frequency, and α and β are fitting parameters 22 . By fitting the experimental data of the device noise spectral density, α≈0.95 is obtained. α being approximately 1 is a typical 1 / f noise characteristic, so 1 / f noise is the main mechanism of the noise spectrum 23 . Compared with the environmental background noise, the noise spectral density of the device hardly changes and is less than 10 -23 A2 / Hz, indicating that the device fabricated in this work has a low noise level. To verify the long-term working stability of the device, the I-T test was carried out on the device after storing it for eight months under ambient conditions, with a 5 s interval between each switching cycle. As can be seen from Figure 5 (c), no obvious degradation of the device performance was observed within 100 cycles, and it could work normally. The good performance repeatability indicates that the device can be practically applied in industrial production.

[0037] Based on the above performance analysis, the β-Ga2O3-based detector fabricated in this work has good detection performance. Among various β-Ga2O3-based solar-blind ultraviolet detectors (Ref. 24-29), it has obvious advantages in dark current and response speed at 0 V, and the performance comparison is as Figure 5 (d) shows. As shown in Figure 5 (e), compared with the solar-blind ultraviolet detectors fabricated with other materials (Ref. 4, 25-35), the response speed of the device fabricated in this work has also been greatly improved. In addition, the PD has great advantages in the on-off ratio and ultraviolet-visible light rejection ratio at 0 V, as shown in Table 1.

[0038] Table 1. Performance comparison of solar-blind ultraviolet detectors

[0039]

[0040]

[0041] Considering that it is necessary to further verify the imaging potential of the array detector, an imaging system as shown in Figure 6 (a) was built. The incident light passed through the mask pattern and reached the PD, and the covered area was in a dark state. Before the imaging experiment, the uniformity of all elements in the 8×8 array was verified by testing the dark current and photocurrent of each element. Figure 6 (b) and Figure 6 c show the dark current distribution of each element of the device at 0 V and -1 V. Figure 6 As shown in (d), the photocurrent of each element was tested under the bias voltages of 0 V and -3 V. In addition, the dark currents and photocurrents of the 64 photodetector units all showed good uniformity, indicating that they can work independently as pixels in the photodetector array, and the distribution situations are shown in Figure 6 (e) and Figure 6 (f) respectively. Finally, the imaging performance was demonstrated by identifying the mask projected onto the array detector. The different light intensities at different positions led to numerical differences in the photocurrent, and the three-dimensional current distribution of all elements is as shown in Figure 6 (g) and Figure 6(h). The test results show that the β-Ga2O3-based detector array can recognize obvious and clear images under both 0V and -3V bias voltages, and the similarity with the mask image is very high. The excellent recognition ability of the detector verifies the potential of the β-Ga2O3-based detector array in the field of solar-blind ultraviolet fast imaging in the future. In the future, the resolution can be improved by increasing the number of detector elements, and the solar-blind ultraviolet imaging detection technology can be further developed.

[0042] Finally, the present invention proposes an 8×8 solar-blind ultraviolet detector array based on β-Ga2O3 single crystal. This array is a Schottky barrier photodetector with a Pt / β-Ga2O3 / Ti / Au structure and uses SiO2 as a passivation layer. In order to ensure the integration of the device, a vertical structure is used, and the bottom is a common cathode electrode. The detector has excellent performance under solar-blind ultraviolet light irradiation. The response speed reaches 15.2 ns at the fastest when the voltage is 0V. The dark current of the device is as low as 0.58 pA, and the PDCR is about 5.04×10 4 ,R 213nm / R 350nm The rejection ratio is as high as 8.9×10 3 ,and the responsivity at 213 nm is 0.025 A / W. In addition, the imaging ability of the detector is further verified. The elements in the array have high uniformity and excellent optical image recognition ability. This work provides a way for the solar-blind ultraviolet fast imaging detection technology and may also have potential inspiration in the design of other photodetectors.

[0043] The above is a specific description of the preferred embodiment of the present invention. However, the present invention is not limited to the described embodiment. Those skilled in the art can make various equivalent deformations or substitutions without departing from the spirit of the present invention, and these equivalent deformations or substitutions are all included in the scope defined by the claims of this application.

Claims

1. A fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array, characterized in that: It includes a Pt metal layer, a passivation layer, a β-Ga2O3 single crystal and a Ti / Au contact electrode, wherein: The Pt metal layer is connected to the upper surface of the β-Ga2O3 single crystal; The passivation layer is embedded and matched with the Pt metal layer, the passivation layer is connected to the upper surface of the β-Ga2O3 single crystal, and the lower surface of the β-Ga2O3 single crystal is connected to the Ti / Au contact electrode.

2. A fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array according to claim 1, characterized in that: It also includes a fine-grid Ti / Au electrode, which covers the top of the Pt metal layer.

3. A fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array according to claim 1, characterized in that: The thickness of the amorphous SiO2 layer of the passivation layer is 80nm~150nm The thickness of the top Schottky contact electrode Pt layer does not exceed 15 nm.

4. A fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array according to claim 1, characterized in that: Here’s how it works: The different work functions of Pt and β-Ga2O3 lead to band bending when the two come into contact, forming a Schottky barrier and producing a rectifying effect; The work functions of Ti and β-Ga2O3 are similar, and the interface barrier is low, forming a good ohmic contact; When exposed to solar-blind ultraviolet light, the electrons of β-Ga2O3 are excited from the valence band to the conduction band, and the photogenerated electron-hole pairs are separated under the action of the built-in electric field, eventually forming a photocurrent.

5. A fast solar-blind ultraviolet imaging detector based on a gallium oxide Schottky diode array according to claim 2, characterized in that: The preparation method is as follows: Fabricating an array pattern on the β-Ga2O3 single crystal and sputtering Pt metal using an ion beam to form a Pt metal layer; On the β-Ga2O3 single crystal, a SiO2 passivation layer is formed outside the Pt metal layer; The top metal grid pattern is constructed by photolithography and the Ti / Au pattern electrode is made by electron beam evaporation to obtain a fine grid Ti / Au electrode; constructing a Ti / Au contact electrode at the bottom of the β-Ga2O3 single crystal by electron beam evaporation; Cover with photoresist.

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