A thermal mass flow controller having a thermal mass flow
meter with an orthogonal thermal mass flow sensor includes a base defining a
primary fluid flow path therein for carrying a flow of fluid to be metered. A
pressure dropping bypass is positioned in the primary fluid flow path. A flow
measuring portion of a thermal mass flow sensor is oriented substantially
transversely or orthogonally with respect to and is in communication with the
primary fluid flow path. The flow measuring portion includes a portion of an
electrical bridge for determining a temperature of the sensor and produces a
mass flow rate signal in response thereto. A valve is connected to an outlet of
the primary flow path to control the flow of fluid in response to the mass flow
rate signal.
Descrizione
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation of application Ser.
No. 08/608,218 filed Feb. 28, 1996 now abandoned, which is a continuation of
application Ser. No. 08/461,398 filed Jun. 5, 1995 now abandoned, which is a
continuation of 08/361,855 filed Dec. 22, 1994 now abandoned, which is a
continuation of application Ser. No. 08/137,879 filed Oct. 15, 1993 now
abandoned, which is a continuation-in-part of U.S. application Ser. No.
07/962,290, filed Oct. 16, 1992 for Thermal Mass Flow Controller Having
Orthogonal Thermal Mass Flow Sensor now abandoned.
BACKGROUND OF THE INVENTION
The invention relates generally to a thermal mass flow
controller and in particular relates to a thermal mass flow controller having a
portion of a sensing element oriented transversely with respect to a bypass
flow path extending through the thermal mass flow controller.
Thermal mass flow controllers and thermal mass flow
meters are employed in the semiconductor industry and in other industries for
measuring the rate of flow of a quantity of gas employed in a piece of
equipment for the manufacture of a semiconductor wafer and the like. Such
thermal mass flow controllers often are used in gas shelves of diffusion
furnaces, chemical vapor deposition systems, plasma etching systems, sputtering
systems and the like to meter precisely amounts of reactant and carrier gases
to a working chamber of the equipment. The thermal mass flow controllers are
used to meter precisely the amounts of reactant and carrier gases to be
delivered to a treatment chamber of the equipment. Such treatment chambers may
comprise process tubes or process chambers. Such gases may include hydrogen, oxygen,
nitrogen, argon, silane, dichlorosilane, ammonia, phosphorus oxychloride,
diborane, boron tribromide, arsine, phosphine, sulfur hexafluoride and the
like. Oftentimes, multiple gas sources are employed in conjunction with a
particular treatment chamber. For instance, silane may be used in the treatment
chamber for chemical vapor deposition of polycrystalline silicon, also known as
polysilicon, in combination with one or more doping agents. As a result, each
of the process tubes or process chambers in a particular piece of equipment may
have multiple reactant gas delivery lines connected and must, of necessity,
have multiple mass flow controllers connected in the gas lines to meter
appropriate amounts of the reactant and carrier gases process gases to the
treatment chamber. The use of such multiple mass flow controllers, of course,
expands the size of the gas shelves used for these types of equipment.
The manufacture of modern semiconductors having finer and
finer microelectronic features has necessitated that the acceptable
contamination levels within clean rooms in which such manufacturing takes place
have continuously been reduced in order to provide adequate wafer yields. As a
result, the expense involved in the construction of such clean rooms has steadily
increased and is anticipated to continue increasing. As such clean rooms are
expanded in size due to the relative amount of floor space or foot-prints
occupied by equipment, their corresponding cost of course also increases. Thus,
the equipment size for a given throughput through a particular clean room is an
economic consideration which is always of importance to a wafer fabricator.
Concomitant with the space requirements for clean rooms
is a requirement that footprint considerations often require that mass flow
controllers be capable of use in a variety of orientations. Unfortunately, in
most cases, conventional thermal mass flow controllers may only be used with
their bypass and sensors both positioned substantially horizontally to avoid
introducing unwanted convective effects into the sensor which would result in
perturbation of the mass flow controller readings.
One approach to solving the convection problem is to
allow a flow controller for instance to be oriented vertically, as set forth in
PCT application PCT/US91/04208, published Dec. 26, 1991, corresponding to U.S.
application Ser. No. 07/537,571, filed Jun. 14, 1990 now abandoned and
corresponding U.S. application Ser. No. 07/614,093, filed Nov. 14, 1990 now
abandoned all for Thermal Mass Flow Meter, assigned to the instant assignee.
Those applications disclose a thermal mass flow meter having a sensor which
allows the bypass flow path to be oriented in a substantially vertical
direction without the necessity of the sensor being oriented in a substantially
horizontal direction. The mass flow controller, however, like other prior art
mass flow controllers may only be used in a vertically oriented direction. That
is, it has a single preferred direction in which it may be oriented. It may not
be used in a variety of attitudes other than with the bypass position
substantially vertically.
What is needed then is a thermal mass flow controller
which is compact and may be positioned in a variety of orientations with
introducing convective perturbations in the flow controller reading.
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