Semiconductor wafer metrology apparatus and method
A semiconductor wafer metrology technique which corrects for the effect of electrostatic forces on an atmospheric buoyancy compensated weight force measurement of a semiconductor wafer. In one aspect a wafer is weighed in a faraday cage whose is measured independently. A change in the measured weight of the faraday cage can be used to correct the measure weight the wafer. In another aspect a direct electrostatic measurement can be converted into a weight correction using a predetermined correlation between an electrostatic charge measured by the charge meter and a weight error force. In another aspect the electrostatic measurement may be indirect, e.g. derived from varying the distance between the wafer and a grounded plate parallel to the wafer to effect a change in an electrostatic force between the grounded plate and the wafer.
1. A method of measuring the mass of a semiconductor wafer, the method including:
placing the wafer on a sensing element of a first weighing instrument, the sensing element of the first weighing instrument being inside a grounded faraday cage which is on a sensing element of a second weighing instrument whereby the weight of the faraday cage is measurable independently of the weight of the wafer;
determining a change in a measured weight of the faraday cage indicated by the second weighing instrument when the wafer is placed inside the faraday cage; and
correcting a measured weight of the wafer indicated by the first weighing instrument using the determined change in measured weight.
2. A method according to claim 1 , including compensating for buoyancy exerted on the wafer by the atmosphere in the faraday cage.
3. A method according to claim 1 , including obtaining a baseline measured weight of the faraday cage indicated by the second weighing instrument immediately before the wafer is placed inside the faraday cage, wherein the change in measured weight of the faraday cage is determined by calculating a difference between a weight indicated by the second weighing instrument after the wafer is placed inside the faraday cage and the baseline measured weight.
4. A method according to claim 1 , wherein the measured weight of the wafer is corrected by subtracting the determined change in measured weight indicated by the second weighing instrument from the measured weight of the wafer indicated by the first weighing instrument.
5. A method according to claim 1 , including positioning the wafer substantially equidistant between two parallel grounded conductive plates.
6. Apparatus for measuring the mass of a semiconductor wafer, the apparatus having:
a first weighing instrument for measuring the weight of the wafer;
a faraday cage for containing the wafer during measurement; and
a second weighing instrument for measuring the weight of the faraday cage independently from the wafer when the wafer is contained within the faraday cage.
7. Apparatus according to claim 6 , having monitoring means arranged to determine the buoyancy exerted on the wafer by the atmosphere in the cage.
8. Apparatus according to claim 6 , wherein the first weighing instrument includes a weighing pan for supporting the wafer during weight measurement, the weighing pan being made from or having a surface layer made from an electrically insulating material.
9. Apparatus according to claim 6 , including two parallel grounded conductive plates locatable equidistant from a wafer when its weight is measured by the first weighing instrument.
10. Apparatus according to claim 6 , including a grounded conductive guard ring locatable around a periphery of a wafer when its weight is measured by the first weighing instrument.