Method for manufacturing quartz crystal unit and quartz crystal oscillator having the quartz crystal unit

A method for manufacturing a quartz crystal unit, comprising the steps of:

• providing a quartz crystal wafer having opposite surfaces;
• disposing at least one metal film on at least one opposite surfaces of the quartz crystal wafer;
• disposing a resist on the at least one metal film;
• forming a pattern of a tuning fork shape on the at least one of the opposite surfaces of the quartz crystal wafer;
• etching the quartz crystal wafer after the forming step of the pattern to form a quartz crystal tuning fork shape having a quartz crystal tuning fork base, and first and second quartz crystal tuning fork tines connected to the quartz crystal tuning fork base, each of the first and second quartz crystal tuning fork tines having opposite main surfaces and opposite side surfaces;
• forming an electrode on at least one of the opposite side surfaces of each of the first and second quartz crystal tuning fork tines so that the electrode formed on the at least one of the opposite side surfaces of the first quartz crystal tuning fork tine has an electrical polarity opposite to an electrical polarity of the electrode formed on the at least one of the opposite side surfaces of the second quartz crystal tuning fork tine to form a quartz crystal tuning fork resonator that vibrates in a flexural mode of an inverse phase;
• removing the resist disposed on the at least one metal film before the forming step of the electrode;
• providing a case having a through-hole and a mounting portion;
• mounting the quartz crystal tuning fork resonator on the mounting portion of the case by a conductive adhesive or a solder;
• disposing a metal or a glass into the through-hole of the case to close the through-hole thereof in a vacuum;
• and
• adjusting an oscillation frequency of the quartz crystal tuning fork resonator after the mounting step.