warning!
befor change fuse bits to obtain 9.6 MHz internal oscillator please read below note:
(this content is originating in ATtiny13 Errata Sheet)

Device may lock for further programming:
	Special combinations of fuse bits will lock the device for further programming effectively
	turning it into an OTP device. The following combinations of settings/fuse bits
	will cause this effect:
	 128 kHz internal oscillator (CKSEL[1..0] = 11), shortest start-up time
	(SUT[1..0] = 00), Debugwire enabled (DWEN = 0) or Reset disabled
	RSTDISBL = 0.
	 9.6 MHz internal oscillator (CKSEL[1..0] = 10), shortest start-up time
	(SUT[1..0] = 00), Debugwire enabled (DWEN = 0) or Reset disabled
	RSTDISBL = 0.
	 4.8 MHz internal oscillator (CKSEL[1..0] = 01), shortest start-up time
	(SUT[1..0] = 00), Debugwire enabled (DWEN = 0) or Reset disabled
	RSTDISBL = 0.
Problem fix/ Workaround:
Avoid the above fuse combinations. Selecting longer start-up time will eliminate the
problem.