NRL_CVD_specs.docx

The Naval Research Laboratory (NRL) requires a metal-organic chemical vapor deposition system (MOCVD) for the growth of single- to few-monolayer transition metal dichalcogenides and their heterostructures, such as MX2 (M = Mo, W; X = S, Se) and other 2D materials, on epitaxial graphene films on SiC substrates. The system must provide substrate temperatures of up to 1500 C for a 3 inch diameter sample, a sample load lock to isolate the growth chamber from atmosphere, and operating growth pressures of 100 m T to 760T. The system must provide for the controlled delivery of source precursors in the form of gases ( H2S, H2Se, WF6 ), high vapor pressure liquids ((C2H5)2S and (C2H5)2Se ), and high vapor pressure solids ( W(CO)6 and Mo(CO)6 ). The system must operate both in the conventional CVD mode as well as an atomic-layer-deposition (ALD) mode. The system must include all hardware and software for the operation of the system, the safe handling of source materials (e.g. gas safety cabinets and alarms), and abatement of exhaust gases.

Sample handling

Sample loadlock

The system shall include a sample loadlock chamber which isolates the process chamber from ambient and keeps the sample in an inert gas environment while a substrate or grown sample is introduced or removed. The loadlock chamber shall include all pumps, gauging, valves and other hardware / software compatible with the specified process gases and necessary to evacuate the chamber from ambient to less than 100 mTorr pressure in 15 minutes. Only dry pumps shall be used to prevent any contamination from oil lubricated pumps. The loadlock system shall provide for fully automatic backfill and pumpdown of the loadlock chamber using a pump/backfill/repeat process to insure complete isolation of the process chamber and process gases from ambient, transfer of the substrate to the process chamber for MOCVD growth, and removal of the grown sample from the process chamber to the loadlock chamber. The loadlock chamber itself shall be contained within a glovebox purged with an inert gas flow (NRL house nitrogen) to prevent exposure of the sample to the ambient atmosphere.

Process chamber

The process chamber shall be of the hot wall design and include all pumps, gauging, valves and hardware compatible with the specified process gases as necessary to provide a base pressure of 50 mTorr or less, and operate the process chamber at pressures of 200 mTorr to 500 torr. Only dry pumps shall be used to prevent any contamination from oil lubricated pumps.

The substrate holder shall include a heating system to control and maintain a 3” diameter substrate wafer up to 500 microns thick (typically sapphire, silicon carbide or silicon) at a user-defined temperature in the range 100 to 1500 degrees Celsius ± 2˚C at pressures ranging from 200 mTorr to 100 Torr, with a temperature uniformity of ±2˚C across the 3” diameter substrate wafer. The heating system shall be capable of maintaining a SiC wafer at 1500˚C for 30 minutes for the growth of epitaxial graphene on SiC at a pressure of 1 Torr. The substrate holder shall provide for sample rotation during growth to optimize the uniformity of temperature, film thickness and composition across the substrate wafer. The sample temperature shall be controlled by software that includes user-defined heating rates, soak times at a specified temperature, and cooling rates.

The process chamber shall include a remote upstream radio-frequency (RF) plasma source controlled by the operating software to assist in conditioning the source gas flows for optimum CVD growth on the sample substrate. The source shall be at least 300 watts in power as selected by the user, and include automatic matching network, shielding and safety interlocks.

Source materials and delivery

The system shall include source delivery hardware and software for the controlled delivery of the following source materials at user-specified flow rates and chamber operating pressures from 100 mT to 760T. Separate injector points shall be used for the metal (Mo, W) and chalcogen (S,Se) sources to minimize pre-reactions. All sources shall be configured to operate in both run and vent modes, and provide rapid on/off switching of the source material to the process chamber.

Gases:

H2S hydrogen sulfide H2Se hydrogen selenide WF6 tungsten hexafluoride These gases will be provided by NRL in an industry standard size gas cylinder.

The vendor shall provide the necessary gas safety cabinets, regulators, sensors and alarms as described below.

Liquid bubbler vapor delivery (3 independent source channels):

· One configured for Diethyl sulfide (C2H5)2S

· One configured for Diethyl selenide (C2H5)2Se

· One spare channel The bubbler vessels shall be provided by NRL. The temperature of each bubbler vessel shall be user defined within the range 25-120˚C ±1˚C . The bubbler pressure shall be controlled and maintained at a fixed constant value for all operating pressures of the process chamber. The delivery lines from the bubbler to the process chamber shall be heated as selected by the user from 25-130˚C to prevent condensation of the source within the line. The bubbler temperature, pressure, gas flow, inlet / outlet / bypass valves shall be user selected through the control software.

Solid source vapor delivery (3 independent source channels):

· One configured for molybdenum hexacarbonyl Mo(CO)6

· One configured for tungsten hexacarbonyl W(CO)6

· One spare channel The solid source vessels with capacity in the range of 100 to 250 cubic centimeters shall be included in this procurement. The temperature of each solid source vessel shall be user defined within the range 25-120˚C ±1˚C . The vessel pressure shall be controlled and maintained at a fixed constant value for all operating pressures of the process chamber. The delivery lines from the source vessel to the process chamber shall be heated as selected by the user from 25-130˚C to prevent condensation of the source within the line. The source vessel temperature, pressure, gas flow, inlet / outlet / bypass valves shall be user selected through the control software.

Gas lines and fittings All gas lines shall be of 316L stainless steel, cleaned and electro-polished, and assembled with butt-welded seams only. All fittings shall be of the Cajon VCR type to minimize dead space for trapped gas or impurities. After assembly, the lines shall be helium leak to insure that any leakage is less than 5x10-9 atm cm3/sec. All reactive gas lines shall be automatically purged to vent and abatement with inert gas whenever the reactive gas is not being used.

System control

The system shall be controlled by a computer (Windows or MacIntosh operating system) with a graphical user interface that provides

· control over all of the relevant growth parameters, including

· substrate temperature and heat / cool rates

· all gas flows

· loadlock chamber pressure

· process chamber pressure

· solid and liquid precursor vessel temperatures

· all vacuum and pumping subsytems

· continuous data logging of same

· graphical representation of same

· real time system status

· the ability to write, edit, store, recall and execute process run streams,

· information on status of precursor delivery sources, lines, valves, plasma

· information on status of all valves (vacuum system, source delivery, etc)

· information on status of gas cabinets and abatement system

· internet connection to allow remote operation, monitoring and diagnostics

· standard continuous CVD mode of operation

· a cyclic atomic layer deposition (ALD) like mode of operation

Gas handling, safety cabinets and panels The gas delivery system must be in accordance with ASME 31.1 Power Piping.

Gas cabinets shall be included for the hazardous process gases. No more than three cylinders must be placed in a single gas cabinet. The cabinets for hydrogen sulfide, hydrogen selenide and tungsten hexafluoride shall include a nitrogen purge cylinder and be fully automated to provide nitrogen purge of the lines and connections for cylinder change or other maintenance. The cabinet for hydrogen shall include a nitrogen purge cylinder and be semi-automated to provide nitrogen purge of the lines and connections. The cabinets shall include adjustable cylinder shelves to accommodate smaller size gas cylinders, and an independent sprinkler head to be connected to house water. These cabinets shall be interfaced to the control software, and include leak detection, excess flow sensors and alarms appropriate for the respective gas. Electrical fittings inside the gas cabinets must be suitable hazardous location. The cabinets shall include provisions to connect to an external water supply for the sprinkler head, and for electrical connection to an external gas monitoring / alarm system (provided by NRL).

Gas line connection between gas cabinet and process system: gas line enclosed within duct which is maintained at negative pressure.

Exhaust gas abatement system All of the hazardous process gases shall be directed to an abatement / treatment module from any vent valve and after exiting the process chamber. The abatement module will be configured to reduce the concentration of process gases and/or chemicals in the final exhaust stream to a level below the maximum exposure level identified on the MSDS data sheet for each respective gas, specifically including each of the source materials noted above. The module shall be interfaced to the system control software so that no hazardous process gas can flow unless the abatement module is functioning. The final exhaust shall be monitored with appropriate sensors and alarms to insure that the concentration of process gases and/or chemicals in the final exhaust stream are at a level below the maximum exposure level identified on the MSDS data sheet for each respective gas (specifically including H2S, H2Se and WF6).

Installation The vendor shall install the system with the source materials at NRL and demonstrate full operation to include active flow of all source materials specified here. NRL shall provide the source materials (H2S, H2Se and WF6 in standard gas cylinders, (C2H5)2S and (C2H5)2Se as liquid sources in stainless valved bubblers, and Mo(CO)6 and Mo(CO)6 in solid source form. NRL shall further provide all electrical connections, water cooling, water for sprinkler heads, compressed air, and connections for external alarms.

System components should be sized with the following physical building constraints in mind which impact delivery and installation:

· a freight elevator is available at the building loading dock

· the freight elevator has maximum dimensions of width x depth x door height of 72” x 87” x 81” (door height can be increased to 84” if necessary)

· the doorway to the lab room in which the system will be installed is 71” wide by 81” tall

· the lab room is rectangular with dimensions of 15 ft x 20 ft

Training The vendor shall provide onsite training for 4 NRL personnel for system start-up, operation and routine maintenance (e.g. changing gas source cylinder, changing bubbler vessel, replacing process chamber consumables, servicing abatement module, etc).

Warranty The vendor shall provide a 12 month warranty from system installation that covers all parts other than routine expendables.