Tuesday, February 22, 2005

Another science setback in IPT: rf inductance heating analogized to microwave ovens

The article in the Feb. 2005 issue of Intellectual Property Today by Michael M. Carlson ("An approach to effective opening statements in patent cases") also has an interesting discussion of rf (radiofrequency) heating wherein rf heating is analogized to microwave heating.

At page 11, the author explains how his client "patiently explained that a microwave sends out RF waves at exactly the right frequency to vibrate water molecules. When the water molecules vibrate, they get hot, and, in turn, the food, which contains water molecules heats up.

The RF heating on a semiconductor reactor works the same way, except that the RF waves are tuned to vibrate graphite molecules in the pedestal, which gets hot, and in turn, heats the wafer sitting on the pedestal."

Actually, rf induction heating uses an AC current in a coil (not "tuned" to "vibrate" graphite "molecules") to generate a magnetic field which generates eddy currents in the electronic conductor material (susceptor, here graphite).

RF induction heating does not work for non-electronic conductors (eg, most foods) and microwave heating does not work well when an electronic conductor is placed within the microwave, as some people have learned.

Of the comment about "vibrate," microwave heating of materials is produced by the microwave electromagnetic field causing the molecules of the water to rotate back and forth and the
resulting friction between molecules produces heating through out the material.

Of the relevant frequency, microwave ovens used in the home generate between 500 watts and one thousand watts of microwave power at a frequency of about 2400 megahertz (MHz or 2400 million cycles per second), corresponding to a wavelength of 12.5 cm (4.9 inches)in air. This frequency of a domestic microwave oven (=2.45GHz) is not selected so that it is at the maximum absorbency for water (which is something like 10GHz). This high frequency explains why it is not good to place metals in a microwave. If a metallic conductor is placed in a microwave cavity, most of the microwave energy is reflected with relatively little energy penetrating beyond a few microns into the surface. However, colossal surface voltages may still be induced, and these are responsible for the dramatic electrical discharges that can be observed when a metal is placed in a microwave oven.

In contrast to microwave ovens, rf induction operates at a much lower frequency: low frequencies of 5 to 30kHz are effective for thicker materials requiring deep heat penetration, while higher frequencies of 100 to 400kHz are effective for smaller parts or shallow penetration.

Although many simplifications may be necessary to introduce technical material to a judge (or a jury), a lawyer has to be careful in oversimplifying, lest his opponent jump on an obvious error and harp on the error to attack the lawyer's credibility. Here, the (false) tennis ball analogy, as well as the microwave analogy, could have been attacked by a skilled opponent.

An earlier issue of IPT pointed to the adverse consequences when an expert witness erroneously referred to 35 USC 112 as discussing one of ordinary skill in the art. Although the difference between the artisan of 112 and the one of ordinary skill may be ephemeral, the credibility of the expert was destroyed.

***Basics of induction heating***

The basic components of an induction heating system are an AC power supply, induction coil, and workpiece (material to be heated or treated). The power supply sends alternating current through the coil, generating a magnetic field. When the workpiece is placed in the coil, the magnetic field induces eddy currents in the workpiece, generating precise amounts of clean, localized heat without any physical contact between the coil and the workpiece.

The induced current flow within the part is most intense on the surface, and decays rapidly below the surface. So the outside will heat more quickly than the inside; 80% of the heat produced in the part is produced in the outer "skin". This is described as the "skin depth" of the part. The skin depth decreases when resistivity decreases, permeability increases or frequency increases.

***Practical example

Of an asserted analogy between microwave heating and rf heating, from US 4,439,267:

FIG. 1 schematically shows a system with which the instant methods may be practiced and includes reactor 10 in which the crystals are grown, with susceptor 11 therein. The reactor is surrounded in part by coil 12 fed by radio-frequency generator 13. The walls of reactor 10 are made of a material (such as glass or quartz) transparent to the output energy of 13, and susceptor 11 is made of a material such as carbon (graphite) which will absorb the output energy of 13 and which is non-reactive with the materials to be placed on or in it.

The substrate is placed on a susceptor pedestal made of carbon for example, and heated by R.F. induction.