1. Philips Research Laboratories, Prof. Holstlaan 4 (WA11), 5656 AA Eindhoven, The Netherlands, Philips Centre for Industrial Technology, Prof. Holstlaan 4 (WY42), 5656 AA Eindhoven, The Netherlands, XPEQT at IMEC, Kapeldreef 75, B-3001 Leuven, Belgium, and Philips Research Leuven, Kapeldreef 75, B-3001 Leuven, Belgium