Solenoid Valve with a Pneumatic Cylinder

A valve apparatus has both solenoid and pneumatic drivers. Solenoid controls the flow rate, and pneumatic driver will provide a seal force to realize near zero sealing wherever needed. Pneumatic driver uses welded diaphragm cylinder structure so the whole system is small enough to fit into the case of a mass flow controller. The valve apparatus uses a diaphragm structure and the diaphragm is directly over the valve seat, so all other valve components will not expose to process gases. The valve apparatus is equipped with means to adjust air gap and preload force. A spacing spring is used to keep the centering of the plunger to avoid the friction between plunger and bearing as the conventional solenoid valves have. The solenoid driver can work alone with pneumatic driver removed wherever there is no close to zero sealing needed.

FIELD OF THE INVENTION

The present invention is related to solenoid valves used in mass flow controllers (MFC), more specifically in the semiconductor industry.

BACKGROUND OF THE INVENTION

MFC are used to deliver process gases for semiconductor processes. The MFCs are required to be accurate (less than 1% of full flow rate), fast response (less than 100 millisecond or faster), small footprint (about 1.125″ wide and 4.132″ long), corrosion resistant and ultra clean. The valves used in the MFCs have much to do with the requirements. Solenoid valves can satisfy most of the requirements with a relatively low cost. As a nature of solenoid valves, they cannot seal the valve absolutely, the leak-by rate can be controlled to the level of less than 1% of full flow, satisfying most of applications, but some semiconductor processes require the leak-by to be less than 0.005% of full flow. Different measures have been used to meet the requirement, such as improving the surface conditions (flatness and finishing) of the valve components, screening valve components, etc. These measures are expensive, and they are almost reached the limitation of the manufacturing equipment. Changing the material of valve components from metals to polymers, such as Polychlorotrifluoroethylene (PCTFE) will improve the sealing (may still not enough), but it will reduce the liability of the valves and increasing the cost. To reach a close to zero seal, other than the surface condition, the sealing pressure is also a key parameter. The sealing force for most of pneumatic isolation valves is usually at a level of 100 to 200 pounds. Due to the space and power limitation, the total force of the solenoids used in MFC usually is less than 15 pounds and the force used in sealing is mostly less than 0.5 pound for the reason of design and performance. Pneumatic Isolation valves are usually driven by air cylinders, although they can provide larger sealing force, but they cannot stop precisely at certain position to obtain certain flow rate, so they cannot be used to control the flow rate of MFC. It is the intension of this invention to solve the sealing issue and at the same time improve the design of solenoid valve.

SUMMARY OF THE INVENTION

In this disclosure, a pneumatic cylinder is mounted on the top of a solenoid valve, this makes the valve can control the flow rate with its solenoid valve and reach to close to zero sealing with the pneumatic cylinder. The pneumatic cylinder is removable, and the solenoid valve can work alone when the requirement of leak-by is not very strict. A diaphragm covers valve seat directly, so no other valve components will be exposed to flow medium. The air gap and valve preload are adjustable. The pneumatic cylinder is welded diaphragm structure to make it small. This makes the solenoid valve and the pneumatic cylinder can be fitted into a 1.125″ wide mass flow controller.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1is a section view of one of the embodiments of this invention andFIG. 2is an enlarged view of the valve seat area. The valve apparatus1of this invention is detachably mounted to a valve block2, which is preferably made of 316L VIM/VAR or 316L. On the valve block2, inlet port3and outlet port4are formed. The valve seat5and sealing edge7are also formed on the valve block2(FIG. 2). The center part of diaphragm61is covering the valve seat5. The periphery of diaphragm is air-tightly clamped between sealing edge7and a press ring8. The valve seat5and sealing edge7share same plane with the top surface of valve block2. This will simplify the manufacturing, only one grounding or lapping operation for the valve block top plane is needed, and keep the sealing edge, valve seat, and solenoid components in a good alignment. Diaphragm assembly6(FIGS. 2 and 3) consists of diaphragm61, plunger62, and plunger shaft63. Diaphragm61is preferably made of Inconel 718 or similar metal, to take advantage of its high strength and corrosion resistance, because it needs to endure high level of fatigue stress and corrosive environment. It is waved by stamping to obtain a larger linear stroke. It is welded to plunger62by laser or EB welding (10). The welding can be all around or spotted to 6, 8 or more points. The plunger62, which is made of magnetic metal, preferable 4xx series stainless steel, has an 90° angled top. A plunger shaft63, made of non-magnetic metal, 3xx series stainless steel or equivalent is press-fitted into the center bore of the plunger62. When the valve1is in close status, the diaphragm61is pushed toward the valve seat5by a combined forces that will be described later, the flow between inlet port3and outlet port4will be stopped. When diaphragm61is lifted by solenoid induction force, the gas will flow from inlet port3to outlet port4through valve seat5, the valve will be open. Press ring8is made of magnetic metal, preferred to be 4xx series stainless steel. It has a protruded ring9, which can help to align between solenoid part of valve1and valve block2, at the same time, it keeps diaphragm61, so as plunger assembly6centered with valve seat5.

Referring toFIG. 1again, solenoid has a body11made of magnetic metal, it can be silicon steel, 1010 steel or equivalent, if it is not magnetic stainless steel, corrosion resistant plating is needed. Body11has a step12, when solenoid valve1including body12is mounted to valve block2, step12will exert force on press ring8, which in turn applies pressure on diaphragm61, seals the valve at the sealing edge7. Body11also has a thread13to mate with the thread of upper stem14, which is made of magnetic metal, such as silicon steel, 1010 steel or equivalent. Upper stem14can be rotated by special tool anchored on holes15, while doing so, the stem14will move up and down along the thread13. Stem14has a 90° concave lower end to mate with the angled top end of plunger62. Lock nut16will lock the stem after adjusting. By adjusting the stem14, air gap17will be adjusted. Lock nut16also has its adjusting holes (not labeled). Its material is preferably magnetic metal. Coil assembly18consists of bobbin181made of glass filled Nylon or equivalent, coil182winded by copper magnetic wire, lead wires183, and isolation tape, etc. When coil18is energized by letting current flow through magnetic wire, a magnetic loop will be formed through body11, press ring8, plunger61and stem14. The tendency is to close the air gap17, which means to move the plunger upward, flex the diaphragm upward, separate diaphragm61from the valve seat5and open the valve.

Spring support washer19is resting on the shoulder of plunger shaft63, upon it is a spacing spring20(FIG. 4). The periphery of spacing spring20is clamped between a recess bore of solenoid body11and spacing spring nut21. Spacing spring20can be made of common chrome steel, such as 304 as its stress level is not very high. Its function is to keep plunger assembly6centered. Conventionally, this is done by using a bearing surrounding the plunger shaft, but that will increase the friction force, so as to increase hysteresis and slow down the valve.

A compression adjust spring22is resting on the top of spacing spring20. It is used to adjust preload force between diaphragm61and valve seat5to obtain certain leak-by flow rate. When there is no pneumatic cylinder in use, as in another embodiment described later, adjust spring22will be fully responsible to obtain required leak-by flow rate. The combination of diaphragm61, spacing spring20and adjust spring22consists the elastic response system of the valve assembly1. The top end of adjust spring22is fell into the pocket provided by spring cap23(FIG. 5), which is threaded into the through thread of spacing spring nut21. The hex flange of spring cap23is used to adjust the preload of spring22.

Pneumatic cylinder assembly24is threaded into the thread in solenoid body11, the same thread used by spacer spring nut21. In this embodiment, pneumatic cylinder24is also served as a lock nut for spacer spring nut21. The main body of pneumatic cylinder24consists of two parts, upper body241and lower body242, welded together and sandwiched a waved diaphragm243. A backup disk244is welded (245) to the diaphragm243. Backup disk244is also providing a support for diaphragm243to avoid it to be damaged under high pressure. At the bottom center of backup disk244, there is a bore246to host plunger shaft63. The gap between the bottom of bore246and the round end of plunger shaft63is around 0.015″, enough for the possible maximum stroke of the solenoid. On the center of the upper cylinder body241, a fitting247is mounted by threading or other means. The driving gas will flow in and out of the cylinder through port248. The pressure source will be set at a pressure 60 to 80 psig, which will give the cylinder 40 to 50 pounds force to push down the plunger shaft63to seal the valve.

The whole valve system is demonstrated inFIG. 6. A tube25is plugged into the port248(shown inFIG. 1) of fitting247on one end and fitting26on the other end. The fitting26is mounted to a solenoid valve27, so is another fitting28. Tube30will be connected to pressure gas source. The solenoid27is powered and controlled through connecting pins29. The power can be supplied from outside source of the mass flow controller (if the circuit of mass flow controller does not have enough power to drive solenoid valve27and the valve assembly1of this invention), but the control signal is from the control circuit of the mass flow controller to get a seamless control with the valve assembly1of this invention. When the valve starts to open from rest (assuming it is a normally close valve), the pneumatic cylinder will be depressurized first, the diaphragm244of pneumatic cylinder will return to its natural position by its resilient force of diaphragm243, and the solenoid1will take over immediately. When the valve needs to be closed, the coil of solenoid1will deenergized first and the pneumatic cylinder will be pressurized right after immediately to close the valve. Fitting26,28, solenoid27actually will be placed outside of mass flow controller cover, because there is not enough room inside. They are put on the top of the valve assembly1inFIG. 6is just for the sake of explanation.

Unless there is a strict requirement for the leak through rate, the solenoid valve1of this invention can work alone without the pneumatic cylinder24.FIG. 7shows another embodiment of this invention. In the place of pneumatic cylinder24as shown inFIG. 1, a lock nut31(also seeFIG. 8) will lock up the spacing spring nut21. In this embodiment, both of the lock nut31and spring cap23have a very good access. The makes it easy to adjust or replace spring22, if there is a need to increase or decrease the stiffness of the adjust spring22for different flow rate bins. It is also possible by change some parts to put a distance sensor on the top of the plunger shaft63to measure the valve stroke.

FIG. 9is a chart showing the valve force balance situation. The curve on the top of the curves is the force what the solenoid can provide at full power. The second curve from the top is the total resistant force that the solenoid needs to overcome to lift the valve. It is a summation of those curves below it, including resilient forces from diaphragm61, spacing spring20, adjust spring22and pressure force acting on the diaphragm61on the inside of valve seat area. It can be seen that the pressure force is negative on the chart, because it is aiding the solenoid to push the diaphragm open.

ADVANTAGES OF THIS INVENTION

1. Adding a pneumatic cylinder can make the solenoid valve reach close to zero leak-by.

2. Comparing with using a downstream pneumatic isolation valve to realize zero leak, this invention has several benefits:a) It is faster. The controls of solenoid valve and pneumatic cylinder are synchronized by using the same control system;b) When using downstream isolation valve to seal the mass flow controller (MFC), after closing the isolation valve, the connecting tube between MFC outlet and the isolation valve will contain some gas, it will flow into the downstream system uncounted when next cycle starts.

3. By using the diaphragm structure in this invention, only diaphragm is contacting process gases. This will reduce the requirements for materials of solenoid and valve. For example, only one magnetic metal, KM 45, is SEMI (Semiconductor Equipment and Materials International) approved, and its availability is scant, and the price is high.

4. As the pneumatic cylinder is making the final sealing, the solenoid valve only needs to be able to seal the valve below the flow rate of turn-down ratio, normally below 2% of full flow rate. It makes huge difference between 2% and less than 1% (worst case below 0.005%). The requirement for the manufacturing of valve components is much lower, so is the cost.

5. Air gap can be adjusted easily in this invention. Air gap adjustment is important to obtain a good performance. Many traditional valves use spacers to adjust the air gap and opening the whole valve to do so is often needed.

6. Valve preload is adjustable by tightening or loosening the adjust spring or replacing the spring.

7. The centering of plunger assembly is relying on a spacing spring, it has less friction, better hysteresis and faster comparing with traditional bearing support.

8. As there is very little leak-by concern, the gas flow direction can take normal direction for all bins. Traditionally, due to the gas pressure force, inverse flow direction is often used for higher flow rate bins. This can be explained byFIGS. 10A and 10B.FIG. 10Ais a normal flow direction andFIG. 10Bis a reverse flow direction. In a normal flow direction, the gas pressure P is pushing the valve open, when the diameter of valve bore is small at low flow rate, the gas pressure force P is ignorable comparing with the preload force. But at larger flow rate, the gas pressure force P is significant. For example, when the valve bore diameter is 0.113″ and gas pressure is 35 psid, the gas pressure force is around 0.35 pounds, this will most likely surpass the preload force which the valve can provided. To get a low leak-by flow rate, most solenoid valves are only using normal direction gas flow at low flow rate and have to use reverse flow direction at high flow. Comparing with normal direction flow, reverse flow has a larger “dead volume” (the volume between flow sensor outlet and valve inlet), large dead volume will slow down valve's response time. Also, for reverse flow, it is harder to open the valve, because the valve needs to overcome the pressure force first. At the instant the valve is open, with the pressure force suddenly reduced, the valve core will over react to cause a over shoot. For this invention, because there is little leak-by concern, normal flow direction will be used for all the flow rates, the response time will be faster.

9. The pneumatic cylinder uses weld diaphragm structure instead of traditionally piston-cylinder structure, make the whole valve small enough to fit in a 1.125″ MFC.