Great idea. I'd chat to your HR / personnel department but you could do it on company performance or personal performance (or a mix of the two).
For company performance, you could chose quality measures but I wouldn't chose it for everything. So, for example if you want to have a <x CPMU figure for year end, perhaps also have a measure of profitability? It's important not to be a functional silo IMO, and quality personnel as much as anyone have to realise they contribute to the whole company.
Alternatively (or additionally), you could have a personal performance thing. I.e. set objectives for each of them, they are likely to be different for each person and include personal targets as well as project work outside of their normal role. So, for example you could have for one person "To develop your presentation skills by doing xxx by this date" and they could also have "Reduce light weight complaints by 20% by year end" or something similar. You then review this regularly and at the end of the year and then how well they've done against these measures counts to how much bonus they get.
The latter way is a lot more work but I suspect more effective in motivation. After all, believe it or not, money does not motivate people. In today's workplace and financial pressures, perhaps things have changed but spending time with their manager and being told "well done" or "you need to work on this, here's what we can do" is far more motivating so the second option allows you to do that (which keeps them happy) and also allows you to award the bonus. Lots of work but that's what management is all about!