Employees typically sign over their inventions to their employers, but this does not prevent employees from later moving on to work for a new employer or starting their own companies. When the business of the new employer or company conflicts with the patent rights that an inventor has left behind, the inventor or new employer is often tempted to claim that the patent is not valid. The validity challenge contradicts the statement that all inventors are required to make when first applying for a patent, which is that the inventor believes the invention is patentable and the listing in the application is accurate, and therefore the courts do not allow the challenge. Under the doctrine of “assignor estoppel,” the inventor, having assigned the patent, is now estopped — i.e., prevented — from avoiding liability to his or her assignee by arguing that the patent is invalid. Assignor estoppel also applies to those in close relationship (i.e., in privity) with the inventor, such as a new employer. How far does privity extend beyond the new employer? A recent case and two earlier cases address this question.
Mentor Graphics Corp.’s Patent No. 6,240,376 is directed to simulation/emulation technology that enables manufacturers of integrated circuits to detect logic errors in the...
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