For micro presentation will they use a stop watch to check time i mean is it necessary to speak for 90 seconds?

Yes the judges do check the time for micro presentation. It need not be exact 90 seconds presentation, but they judge you on how well you utilize he time, the content and your communication skills. The panel will let you know on before hand, the criterion on which they gonna judge you. All the best!

Yes. It is very important to speak for 90 seconds to compete with others. They do use stop watch