Preliminary R&D on Resistive DLC in China Yi Zhou1, You LV1, Lunlin Shang2, Guangan Zhang2， Jianxin Feng1, Jianbei Liu1, Zhiyong Zhang1 State Key Laboratory of Particle Detection and Electronics, University of Science and Technology of China State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences 28-09-2017

Good optical and electric performance DLC and it’s applications Diamond-like carbon (DLC): A class of amorphous carbon material that displays some of the typical properties of diamond. DLC is usually applied as coatings to other materials that could benefit from some of those properties. Advantages of DLC Applications High hardness Blades, drills, moulds… Thermal conductive Cooling fins… Low friction factor Bearings… Good optical and electric performance MPGD Useful links of DLC: https://en.wikipedia.org/wiki/Diamond-like_carbon https://www.azom.com/article.aspx?ArticleID=623

DLC deposition methods Physical Vapor Deposition (PVD): A variety of vacuum deposition methods which can be used to produce thin films and coatings. PVD is characterized by a process in which the material goes from a condensed phase to a vapor phase and then back to a thin film condensed phase. Cathodic Arc Deposition Electron beam physical vapor deposition Evaporative deposition Pulsed laser deposition Sputter deposition (We are using Magnetron Sputtering) Pulsed electron deposition Sublimation sandwich method Useful links of PVD: https://en.wikipedia.org/wiki/Physical_vapor_deposition Chemical Vapor Deposition (CVD): A Chemical process used to produce high quality, high-performance, solid materials. The process is often used in the semiconductor industry to produce thin films. In typical CVD, the wafer (substrate) is exposed to one or more volatile precursors, which react and/or decompose on the substrate surface to produce the desired deposit. Microwave plasma-assisted CVD (MPCVD) Atomic-layer CVD (ALCVD) Combustion Chemical Vapor Deposition (CCVD)  Hybrid Physical-Chemical Vapor Deposition (HPCVD) Metalorganic chemical vapor deposition (MOCVD) Rapid thermal CVD (RTCVD) Vapor-phase epitaxy (VPE) Photo-initiated CVD (PICVD) Useful links of CVD: https://en.wikipedia.org/wiki/Chemical_vapor_deposition

Magnetron sputtering deposition of DLC Goal: Surface resistivity is between 50MΩ/□ and 100MΩ/□, repeatable and uniform Sample clamping Vacuum Pumping1 The Magnetron sputtering system (Teer 650) Hydrogen+DLC4 Isobutane Start Pumping for Residual Gas2 Deposition Start Pure DLC Deposition Stop Isobutane Stop The Chamber for deposition Cooling in Vacuum3 Sample taking down Remove the air in the chamber (should be less than 3×10-5Torr); Remove the residual gas to clean the gas-supply line; Cooling in vacuum helps the DLC release the inner stress uniformly Add hydrogen into the DLC to increase the resistivity Target Sample Support Chamber size: Φ650mm×700mm Best Sample size (up to): 250mm×200mm (Rigid), 250mm × 1320mm (Flexible)

The 1st test (Sample: 25μm Kapton) We used a “general” setup of the system If all the other parameters of the DLC are the same: The pure DLC has a very low resistivity, we have to add hydrogen into the DLC to increase the resistivity; The higher isobutene rate, the higher resistivity of the DLC layer; The longer of the deposition time, the DLC layer will be thicker and has a lower resistivity; Shows opposite results Deposition Time Isobutane rate Resistivity Thickness Sample1 10 min 3.0 sccm 48 MΩ/□ ~80 nm Sample2 3.1 sccm 76 MΩ/□ Sample3 20 min 3.2 sccm 46 MΩ/□ ~160 nm Sample4 15 min 40 MΩ/□ ~120 nm These test results are not repeatable due to: When the isobutene is close to 3.0 sccm, the flow rate become unstable; this is because the full scale of the flowmeter (it’s a MKS flowmeter) is 100sccm, so the typical accuracy is about: ±1%×100sccm= ± 1sccm; When the flow rate of isobutane varies 0.1 sccm, the resistivity changes a lot; The inner stress of the DLC is too much and make the foil roll itself; Conclusions: It is too difficult to control the resistivity by using the isobutene rate; We want to try to use the pure DLC (inner stress is much smaller) and control it’s resistivity by using the thickness;

The 2nd test (Sample: 25μm Kapton) Part I: We noticed the rotation speed and the target current Deposition Time Resistivity Sample1 10 min 35~70 MΩ/□ Sample2 17~40 MΩ/□ Sample3 5 min 90~360 MΩ/□ Sample4 16~20 MΩ/□ Sample5 44~54 MΩ/□ Sample6 130~160 MΩ/□ Sample7 22~26 MΩ/□ Rotation Speed： 5r/min Current on the Target： 3.5A Uniformity is bad, results are not repeatable, this maybe caused by: The deposition time is too short, the thickness of the DLC is too small and maybe become discontinuous; The current on the target is too large, some carbon-atoms emit out as big clusters, this may make the uniformity be worse due to the DLC layer is not thick enough; The slow rotation speed of the sample support may cause the bad uniformity; The deposition is not repeatable, there are some unknown factors which make the resistivity unstable!!! Conclusions: Increase the deposition time; Increase the rotation speed: 5r/min→10r/min(max); Decrease the current on target;

The 2nd test (Sample: 25μm Kapton) Part II: We noticed the initial vacuum degree We have to : 1. Choose a reasonable deposition time and current to let the DLC have uniform resistivity; 2. Find out the unknown factors and let the coating process be stable; Deposition Time Current Resistivity Sample1 20 min 1.50 A 108~120 MΩ/□ Sample2 1.65 A 78~82 MΩ/□ Sample3 1.70 A 19~22 MΩ/□ Sample4 1.80 A 29~35 MΩ/□ Sample5 40 min 1.0 A 11~13 MΩ/□ Sample6 11~12 MΩ/□ Sample7 4~5 MΩ/□ Sample8 9~10 MΩ/□ If the DLC is continuous, uniform, and all the other parameters are the same, the higher current should give the DLC a lower resistivity. But Sample3 and 4 show us the opposite result! There should be a unknown factor! When the deposition time is longer than 20 min, the resistivity of the DLC is uniform. To find out the unknown factor, we set a time-table for the coating process and do each step in a certain time strictly…except Sample 7 If we use long deposition time and a low target current, we will get good uniformity of the resistivity; Sample 5, 6 ,8 show us a very good repeatability on resistivity; Sample 7 has a much longer pumping time for vacuum than the others, the initial vacuum degree is much higher than sample 5, 6, 8; Conclusion: The initial vacuum degree affects the resistivity, because there are hydrogens in the air !!!

The 3rd test Resistive DLC deposition method: We tried to control the resistivity by adjusting the initial vacuum degree and deposition time Initial vacuum degree Deposition Time Current Resistivity Sample1 ~1.4×10-6Torr 40 min 1.0 A 0.9~1 MΩ/□ Sample2 ~2.0×10-6Torr 10~12 MΩ/□ Sample3 14~15 MΩ/□ Sample4 15~17 MΩ/□ Sample5 ~2.5×10-5Torr 60~67 MΩ/□ Sample7 2.5×10-5Torr 90~100 MΩ/□ Sample8 ~100 MΩ/□ Sample9 1.8×10-5Torr 20~50 MΩ/□ Sample10 38 min 90~120 MΩ/□ Sample11 39 min 50~75 MΩ/□ Sample12 60~80 MΩ/□ Sample13 60~70 MΩ/□ 25μm Kapton We observed the resistivity of the DLC increased while the initial vacuum degree decreased and good repeatability 50μm Apical +5μm Cu Goal reached!!! Resistive DLC deposition method: Use the “Pure DLC” steps for deposition, the hydrogen can be added into the DLC by using the residual air in the chamber; Set the deposition time to 40min and the current to 1.0A, adjust the initial vacuum degree to let the DLC resistivity be close to the goal; Slightly adjust the deposition time to optimize the resistivity if necessary; Deposition Time Tuning

New sputtering system (Hauzer 850) will be ready next year Summary & Outlook Summary: New sputtering system (Hauzer 850) will be ready next year We have made several samples with the size about 15cm×15cm, the resistivity of them is between 60MΩ/□ to 80MΩ/□. We have found a method that can control the DLC resistivity to a reasonable degree; for more precise, repeatable and uniform deposition, we still have lot of R&D works to do. Outlook: We are planning to make some 30cm×30cm samples in October. We will make a μRWELL detector using our samples to see the performance of the DLC. Thanks Special Thanks to: Antonio Teixeira, Rui De Oliveira, Giovanni Bencivenni Chamber size: Φ800mm×900mm Best Sample size (up to): 500mm×500mm (Rigid), 500mm × 1900mm (Flexible)