* ITC says none of the five patents infringed
* ITC cites document destruction as partial rationale
* Rambus share price slips in after hours trading (Updates with Rambus reaction)
WASHINGTON, July 25 (Reuters) - Chipmakers LSI Corp and STMicroelectronics NV did not illegally use Rambus Inc's patented technology to make computer chips, the U.S. International Trade Commission said in a decision on Wednesday.
Rambus' share price has been volatile, sometimes rising and falling sharply depending on the results of its court fights. It closed at $4.32 on Wednesday, up about 25 cents for the day but slid about 4 percent in after-hours trading.
The decision, which Rambus said it might appeal, affirmed an opinion by an ITC judge issued in March that said the companies did not violate Rambus' patents for high-speed chip-to-chip communication systems and memory controllers, which retrieve information when it is needed.
"We are evaluating our next steps in this matter, which may include a possible appeal to the Federal Circuit. We remain steadfast in our commitment to protecting our patented inventions from unlicensed use," said Thomas Lavelle, senior vice president and general counsel at Rambus.
The full commission gave a variety of reasons that claims asserted in the five patents were not infringed, saying some were invalid while others were not used by the accused companies.
Rambus had initially accused the companies of infringing six patents, but one of the six was dropped from the complaint.
In the last 14 months, Rambus has settled patent fights with Nvidia Corp, Broadcom Corp and Freescale Semiconductor Ltd. It settled with MediaTek in March.
Three of the five patents at issue in this case - known as the Barth patents - have been declared invalid by an appeals board at the U.S. Patent and Trademark Office. Two were declared invalid in September and a third in January.
But the patents remain valid during the appeals process, which is not exhausted. The ITC said the Barth patents were unenforceable because Rambus destroyed related documents, making them unavailable during litigation.
Rambus has aggressively used the three Barth patents to pursue infringement claims against technology companies.
The case at the International Trade Commission is Number 337-753. (Reporting By Diane Bartz; Editing by Tim Dobbyn and Steve Orlofsky)