For months, companies have been sitting on the sidelines with record piles of cash. Now they're starting to deploy some of that money - not to hire workers or build factories, but to prop up their share prices. Sitting on these unprecedented levels of cash, U.S. companies are buying back their own stock in droves. So far this year, firms have announced they will purchase $273 billion of their own shares, more than five times as much compared with this time last year, according to Birinyi Associates, a stock market research firm. But the rise in buybacks signals that many companies [do not plan to] spend their cash on the job-generating activities that could produce economic growth. They don't know what they want to do with all the cash they're sitting on," said Zachary Karabell, president of RiverTwice Research. Historically low interest rates are also prompting some companies to borrow to repurchase shares. Microsoft, for instance, borrowed $4.75 billion last month by issuing new bonds at rock-bottom interest rates and announced it would use some of that money to buy back shares. The company already has nearly $37 billion in cash. A share buyback is a quick way to make a stock more attractive to Wall Street. It improves a closely watched metric known as earnings per share, which divides a company's profit by the total number of shares on the market. Such a move can produce a sudden burst of interest in a stock, improving its price.