By Noel Randewich

SAN FRANCISCO (Reuters) - Intel Corp's forecast quarterly revenue that disappointed Wall Street, while a sharp increase in capital spending it plans for 2013 unnerved investors already concerned about slow demand for personal computers.

Shares of the world's leading chipmaker slid more than 5 percent in after-hours trade on Thursday after it projected this year's capital spending at $13 billion - plus or minus $500 million - exceeding many analysts' estimates for about $10 billion.

Intel said $2 billion of its increased expenditures would go toward expanding a facility for researching future manufacturing technology. But some analysts worried that with PC sales already slow, expanding too quickly may create excess capacity that could hurt the bottom line.

"People are starting to freak out about the capex," said Sanford C. Bernstein analyst Stacy Rasgon. "The concern is that if I spend a lot of money and I build up my factories, I don't have enough demand to fill them. They have very high fixed costs, and it pulls your margins down."

Outgoing Chief Executive Paul Otellini, who plans to retire in May after a successor is identified, said the investment in manufacturing would lower costs in the long run.

"The leading edge capacity is the lowest cost for us on a per unit basis," Otellini told analysts on a conference call. "Regardless of what you think the size of the market is, the leading edge fabs are the single greatest asset that we have."

Otellini said the higher capex is not intended to bankroll a foundry or contract chipmaking business. But he did not rule out manufacturing semiconductors for other chip companies, as long as that did not empower a rival.

In the fourth quarter, Intel's revenue was $13.5 billion, compared with $13.9 billion a year earlier. Analysts had expected $13.53 billion.

It estimated first-quarter revenue of $12.7 billion, plus or minus $500 million. Analysts expected $12.91 billion.

STRUGGLING IN MOBILE

Intel is used to being king of the personal computer market, particularly through its historic Wintel alliance with Microsoft Corp, which has led to breathtakingly high profit margins and an 80 percent market share.

But it has struggled to adapt its technology for smartphones and tablets, a market dominated by Qualcomm Inc, Samsung Electronics Co Ltd and Nvidia Corp. PC makers are struggling to stop a decline in sales as consumers hold off on buying new laptops in favor of more nimble mobile gadgets.

Microsoft's long-awaited launch of Windows 8 in October brought touchscreen features to laptops but failed to spark a resurgence in sales that Intel and many PC manufacturers had hoped for.

Intel's hefty investment plans reflect its confidence in the future, even as Wall Street worries about the chipmaker's struggle to gain traction in the mobile market.

"Our core advantage really is our manufacturing leadership," Chief Financial Officer Stacy Smith told Reuters. "450 will give us a significant cost advantage relative to others."

Intel is expanding its research fab in Hillsboro, Oregon, to develop technology for manufacturing chips on 450 mm silicon wafers, a complicated step up from the current 300 mm wafer standard.

Larger wafers can translate into big savings because more chips can be etched onto each of them. But building 450 mm plants is expected to be so expensive that only a few industry leaders, including Intel, Samsung Electronics and TSMC, are expected to have the necessary scale.

Some Wall Street analysts gave Intel high marks for expected operating efficiency this year.

"The revenue isn't going to be there, but the margin and expense control is going to stabilize the bottom line," said Cody Acree, an analyst at Williams Financial. "I think it's probably a success if you can be flat in an industry that most people expect to be flat to down."

Intel foresees first-quarter gross margins of 58 percent, plus or minus two percentage points. Analysts on average expected gross margins of about 56 percent for the current quarter, according to Thomson Reuters I/B/E/S.

It estimated a 2013 gross margin of 60 percent, plus or minus a few percentage points. Analysts on average had expected 59 percent.

Net earnings in the December quarter were $2.5 billion, or 48 cents a share, compared with $3.4 billion, or 64 cents a share, year-ago period.

Analysts had expected 45 cents, and said the surprisingly strong performance was partly due to a lower effective tax rate of 23 percent. This was below Intel's forecast of about 27 percent.

Still, shares of Intel fell 5.6 percent in after-hours trade to $21.43, after closing up 2.58 percent at $22.68 on the Nasdaq.

"This is a company that is continuing to spend money to participate in the market. That may concern some investors," said Doug Freedman, an analyst at RBC Capital.

(Reporting by Noel Randewich; Editing by Richard Chang and Steve Orlofsky)