(Reuters) - The last of three patents that tech licensing company Rambus (RMBS.O) used to win infringement lawsuits against Nvidia Corp (NVDA.O), Hewlett-Packard (HPQ.N) and others has been declared invalid, according to legal documents.

The three patents - collectively known as the Barth patents - pertain to memory chips used in personal computers and are considered to be among Rambus' most valuable intellectual property.

An appeals board at the U.S. Patent and Trademark Office declared the patent invalid on January 24, according to a ruling posted on their website. The previous two were declared invalid in September.

The invalidation is more bad news for Rambus, whose stock shed 60 percent of its value in the weeks after a November 16 court decision in which it lost a $4 billion antitrust lawsuit against Micron Technology Inc (MU.O) and Hynix Semiconductor Inc

(000660.KS).

The Barth patents had been used to accuse a long list of tech companies of infringement, earning Rambus millions of dollars in licensing fees through settlements.

Rambus can appeal the latest decision from the PTO. We're evaluating our options, said company spokeswoman Linda Ashmore.

Rambus' share price fluctuates sharply on its successes and failures in patent litigation and licensing. The company announced Thursday that it had fourth-quarter revenue of $83.4 million and annual revenue of $312.4 million.

Rambus has aggressively used the three Barth patents to pursue infringement claims against technology companies.

It was met with success in July 2010 when the International Trade Commission, which hears patent litigation involving imported products, ruled that graphics chip maker Nvidia, Hewlett-Packard and other smaller companies had infringed the three Barth patents. Nvidia subsequently settled with Rambus on the issue.

POTENT PATENT

The patents in question were also among six that Rambus used in accusing a long list of companies of infringing in a new ITC complaint filed in early 2011. That complaint was filed against STMicroelectronics(STM.PA), MediaTek (2454.TW) and Broadcom, among others. Broadcom has since settled.

It was unclear if Rambus appealed the invalidation of the first two Barth patents from September, and the company did not immediately respond to a telephone call seeking comment.

Scott Daniels, a partner in the law firm Westerman, Hattori, Daniels and Adrian, LLP, said Rambus was unlikely to win an appeal on the third Barth patent, since it would be appealed back to the examiner, who would be highly unlikely to disagree with the higher-ranking appeals board.

Once appeals are exhausted at the patent office, companies can dispute patent invalidations to the U.S. Court of Appeals for the Federal Circuit in Washington.

Court Intellectual Property Law firm Lawsuit