Field of the Disclosure
The present disclosure generally relates to semiconductor devices, and more particularly, to a lateral double-diffused metal oxide semiconductor transistor and a method for manufacturing the same.
Description of the Related Art
In various electronic systems, voltage regulators, such as DC-DC power converters, are used for providing a stable supply voltage. A high-efficient DC-DC power converter is a critical factor in battery management in a low-power apparatus such as a notebook, a mobile phone and the like. A switching regulator converts a DC voltage to a high-frequency pulse voltage and then filters it to generate a DC output voltage. Specifically, the switching regulator includes a power switch for coupling a DC voltage source such as a battery to a load such as an integrated circuit and decoupling the DC voltage source from the load repeatedly. An output filter typically includes an inductor and a capacitor, which are coupled between the input voltage source and the load, for filtering an output signal of the power switch and providing a DC output voltage. A controller, such as a PWM (i.e. pulse width modulation) controller, a PFM (i.e. pulse frequency modulation) controller, or the like, may be used for controlling the power switch to maintain stable DC output voltage.
The power switch may be a semiconductor switch, such as a metal-oxide-semiconductor field effect transistor (MOSFET), an insulated gate bipolar transistor (IGBT), and the like. A lateral double-diffused metal oxide semiconductor (LDMOS) transistor has a source region which is formed in a body region having a doping type opposite to that of the conductivity type of the LDMOS transistor, and a drain region which is formed in a high-resistance drift region having a doping type identical to that of the conductivity type of the LDMOS transistor. Due to the existence of the drift region, the drain region of the LDMOS can withstand a high voltage. Accordingly, the LDMOS transistor can provide advantages of a large driving current, a low on resistance and a high breakdown voltage, and widely be used in the switching regulator.
In a conventional process for manufacturing the LDMOS transistor, both a field oxide FOX and a high-voltage gate oxide HVGOX are formed by local oxidation of silicon, for defining an active region and as a gate dielectric respectively. A gate conductor is used as a hard mask in ion implantation, for forming doped regions in a self-aligned manner. In the self-aligned process, the gate conductor is a hard mask and defines location of the doped regions.
However, the gate conductor has a smaller width and thickness when the power switch is scaled down. If the gate conductor has a small thickness, implanted ions may penetrate the gate conductor into the semiconductor substrate. The gate conductor may be insufficient to be used as a mask layer during ion implantation, and thus fails to define location of the doped regions.
Moreover, the field oxide cannot be scaled down due to so-called “bird beak” phenomenon. With scaling down of the power switch, the field oxide still occupies a field region which has an increased ratio in an area of the active region, and becomes a critical factor which impedes size reduction of the power switch. The field oxide also introduces a step at a surface. In a case that the gate conductor is too thin to have a uniform thickness, the gate conductor cannot be easily patterned during patterning. Thus, a conventional process for forming a LDMOS transistor having a thin gate conductor has disadvantages of poor yield and reliability.