Patents are designed to help inventors protect their ideas. But a new research paper from the Boston University School of Law argues that the growing wave of patent litigation is costing our economy dearly - as much as half a trillion dollars over the past two decades. “It’s a tax on innovation,’’ said paper co-author Michael Meurer, a BU Law professor. The paper examined lawsuits filed by companies that don’t produce products, services, or inventions of their own.