Hi..use this link to see how the tone is produced.
http://arduino.cc/en/Reference/Tone
Generates a square wave of the specified frequency (and 50% duty cycle) on a pin. A duration can be specified, otherwise the wave continues until a call to noTone()
The generator is only PWM at 50%, square wave, you are not going to get any better than that out of the arduino.
Tom....... 