Miniature silicon based thermal vacuum sensor and method of measuring vacuum pressures

A pressure sensing device for sensing atmospheric pressures, the pressure sensing device comprising,

• a silicon chip substrate having a gas chamber formed therein, the gas chamber extending through a cross-section of the chip substrate from a bottom surface of the chip substrate to a top surface of the chip substrate,
• a top opening of the gas chamber covered by a membrane on a top surface of the chip,
• an opening in a side wall of the chip into the gas chamber,
• a first resistive element mounted on the membrane,
• a second resistive element mounted on the chip and not over the gas chamber,
• a cover attached to the chip and covering the membrane and first resistive element, an underside of the cover spaced from the membrane a distance not greater than 10 microns,
• electrical leads connected to the first and second resistive elements,
• a voltage source connected to a bridge containing the first and second resistive elements, and
• a microprocessor for receiving digitally converted analog values of voltages of the bridge applied to the first and second resistive element and for computing power values based upon voltage outputs of the bridge indicative of power required to maintain a predetermined temperature differential between the surface upon which the first resistive element is mounted and the surface upon which the second resistive element is mounted.

A method of measuring vacuum pressures of a known gas in the approximate pressure range of 760 Torr to 1.times.10.sup.-5 Tort, comprising the steps of:

• providing a gas pressure sensor having a silicon chip substrate with a gas chamber which extends through a cross-section of the chip substrate from a top of the chip substrate to a bottom of the chip substrate, a top opening of the gas chamber covered by a membrane attached to a top surface of the chip substrate, a first resistive element mounted on the membrane, a second resistive element mounted on the chip substrate and not over the gas chamber, an opening through a side wall of the chip substrate to the gas chamber, and a cover attached to the chip substrate and covering the membrane and first resistive element, an underside of the cover spaced from the membrane a distance not greater than 10 microns,
• applying a voltage to the first resistive element,
• applying a voltage to the second resistive element,
• controlling the applied voltages to maintain a temperature differential between the membrane and the chip substrate, and
• measuring the voltages and currents applied to the first and second resistive elements required to maintain the temperature differential, the measurements being indicative of pressure about the membrane.

A gas pressure sensing structure for sensing vacuum pressures, the pressure sensing structure comprising:

• a base having a gas chamber cavity which extends from a top of the base to a bottom of the base,
• a membrane covering a top opening to the gas chamber cavity,
• a cover attached to the base and covering the membrane, a bottom surface of the cover spaced from a top surface of the membrane a distance not grater than 10 microns, and
• a first resistive element disposed at least partially on the membrane,
• a second resistive element disposed at least partially on the base,
• a voltage source connected to the first and second resistive elements, and
• a measuring circuit for measuring voltages and currents applied to the first and second resistive elements, the measuring circuit comprising a bridge circuit having four resistors, the bridge circuit connected to an amplifier, a capacitor and a transistor.