LVTSCR-like structure with blocking junction under the polygate

In a LVTSCR-like structure, an additional p+ region is formed adjacent a n+ floating drain to define a p-n junction with the floating drain underneath a polygate of the structure. The polygate is used as a mask during doping of the p+ region and the n+ floating drain, and the length of the polygate is adjusted to provide the desired triggering voltage for the structure. The triggering voltage is also adjusted by biasing the polygate.

BACKGROUND OF THE INVENTION

The LVTSCR, originally developed by Texas Instruments, has found widespread application as a control device, especially in electrostatic discharge (ESD) protection. A cross-section through a typical LVTSCR is shown inFIG. 1, which shows a p+ well contact102, source104, polygate106, floating n+ drain108, p+ drain110, and n+ drain contact112. To avoid the two n− regions108,112simply acting as drains of a NMOS transistor, the p+ drain110is isolated from the p-substrate120by a n-well122, the structure being controlled by the primary snapback of the NMOS structure which is dictated by the breakdown voltage that is determined by the electric field under the polygate106.

The problem with the LVTSCR100is that it has a rather high breakdown voltage, thus having a high triggering voltage. Also, a cascaded structure is typically required when using a LVTSCR as overvoltage clamp where the voltage typically exceeds the maximum gate voltage.

The present invention addresses the above shortcomings of the LVTSCR structure ofFIG. 1.

SUMMARY OF THE INVENTION

The invention relates to a new LVTSCR structure, which will also be referred to as a LVTSCR-like structure to distinguish it from the LVTSCR structure known in the art.

According to the invention, there is provided a method of forming a LVTSCR-like structure with improved parameters over a conventional LVTSCR, the LVTSCR-like structure defining an anode and a cathode and having a polygate, the method comprising forming an additional p+ region in the cathode to define a p-n junction with a n+ floating drain. Preferably the p-n junction is formed to lie beneath the polygate of the structure, and the polygate may be used as a mask during doping of the p+ region and the n+ floating drain. Typically, the p+ region is formed in a PLDD region and the n+ floating drain is formed in a NLDD region. A p-well/n-well junction may be formed underneath the polygate, with the p+ region formed in the p-well, and the n+ floating drain formed in the n-well.

The polygate may be biased by connecting it to the anode or the cathode or to a separate biasing circuit. As a further aspect of the invention, the p+ region and n+ floating drain may be formed during a high voltage portion of a CMOS process and the polygate during a low voltage portion. In fact any one or more of the n+ floating drain, p+ region, NLDD, PLDD, n-well, or p-well may be formed during a high voltage portion of the process and the remaining regions during a low voltage portion, or vice versa. Triggering voltage of the structure may be controlled by adjusting the polygate length or by adjusting the bias of the polygate e.g., to about half the anode voltage.

Further, according to the invention, there is provided a method of forming a LVTSCR structure with improved parameters over a conventional LVTSCR, the LVTSCR structure defining an anode and a cathode, and having a polygate, comprising forming a n+/p+ junction beneath the polygate. Typically, the n+/p+ junction is formed by forming an additional p+ region adjacent a n+ floating drain of the LVTSCR structure, and the polygate may be used as a mask when forming the p+ region and n+ floating drain. In addition, a p-well/n-well junction may be formed underneath the polygate.

The distance between the p+ region and the n+ floating drain may be adjusted to vary the triggering voltage of the LVTSCR structure. Also, the polygate may be biased by connecting it to the anode or the cathode or to a separate biasing circuit. According to one aspect of the invention, the method may comprise forming the p+ region and n+ floating drain during a high voltage portion of a CMOS process and the polygate during a low voltage portion of the process.

An important feature of the invention is that it allows adjusting the polygate length to adjust the triggering voltage of the LVTSCR structure. In one application, the polygate may be biased to about half the anode voltage.

Still further, according to the invention, there is provided a LVTSCR structure that includes a n+ floating drain and a polygate, as well as an additional p+ region formed adjacent the n+ floating drain, so as to define a p/n junction with the n+ floating drain underneath the polygate. The n+ floating drain and additional p+ region are typically formed in a NLDD and a PLDD region, respectively, and the NLDD and PLDD regions may be formed in a n-well and a p-well, respectively, the n-well/p-well junction lying beneath the polygate. The polygate may, in addition, be connected to a cathode or an anode of the structure or to a separate bias circuit.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 2shows one embodiment of a LVTSCR structure of the invention. The structure200includes a p+ well contact202, source204, polygate206, floating n+ drain208, p+ drain210, and n+ drain contact212. In addition, the structure200includes a new floating p+ region220in the cathode to define a blocking junction under the polygate206between the floating n+ drain208and the new p+ region220.

The present invention makes use of the polygate206to serve as a mask during doping of the n+ drain208and new p+ region220. This takes account of misalignment between doping masks during doping since the doping masks can overlap the polygate206without effecting the spacing between the doping regions. The polygate thus, in effect, acts as a self-aligned mask. This allows an abrupt p-n junction to be created and results in a breakdown voltage that can be adjusted by adjusting the polygate206length. Thus the breakdown voltage can be reduced by simply reducing the gate length. In practice, using standard CMOS process steps, the highly doped n+ drain208and p+ region220are typically formed in lightly doped regions referred to as n-lightly doped drain (NLDD) and p-lightly doped drain (PLDD), respectively (not shown). Thus the new p-n junction is, in effect, a junction between NLDD and PLDD of the n+/NLDD and p+/PLDD combinations. It will, however, be appreciated that the present invention could be implemented with one or both LDD regions blocked out, thereby defining a blocking junction between NLDD and p+, or between PLDD and n+, or between p+ and n+, as the case may be. In one embodiment, the doping of the n+ drain208and p+ region220is performed during a high voltage phase of the process. A typical semiconductor circuit may include a core and an I/O structure. These two portions typically operate at different voltages. The core typically operates at a lower voltage dictated by the process, e.g. for a 0.18 μm process the voltage is 1.8V±10%, while the I/O structure may operate at a higher voltage of 3.3V or 5V. For a 0.251 μm process the core voltage is 2.5V±10%, while the I/O voltage will again be at a higher voltage of 3.3V or 5V. These different portions will be implemented by varying the process steps in order to accommodate the low and high voltage levels, respectively. For instance, in the case of a high voltage structure, the gate oxide has to be thicker and is typically implemented by making use of a dual or triple oxide. For example in the case of a 0.18 μm process, the gate for the low voltage part has a length of 0.18 μm and a gate oxide thickness (for the oxide layer underneath the gate) of 30 Å, while the gate for the high voltage part has a length of approximately 0.35–0.4 μm and a gate oxide thickness of 70 Å. Also, the doped regions will be adapted to the different operating voltage. During a high voltage process, more dopant extends under the gate from either side of the gate230. Thus, for example in a 0.18 μm process, a junction width between the p+ and n+ regions210,220of approximately 0.15 μm is achieved even with a polygate length of 0.35 μm. In contrast, for a low voltage implantation in a 0.18 μm process the junction width will remain rather large (approximately 0.1 um) even with a polygate length of only 0.18 μm.

The present invention proposes making use of a combination of high and low voltage process steps for a single device. For example, by making use of high voltage process steps in forming the n+ drain208and p+ region220, while forming the gate using low voltage process steps a shorter gap length to be achieved under the polygate, between the n+ drain208and p+ region220. Results indicate that a negative gap length, i.e., overlap, can be achieved. This results in very low breakdown voltages of the order of about 3V for a 0.18 μm process. As the length of the polygate206is increased, the junction between the doping regions smoothes out and the breakdown voltage increases. Thus breakdown voltage can be carefully controlled by adjusting the polygate length. Simulation results show that the breakdown can be increased in this way up to the well—well breakdown voltage level.

While the embodiments discussed above dealt with high voltage regions for the n+ and p+ regions, it will be appreciated that the formation of the NLDD and PLDD could also involve different voltages process steps to that used for the polygate. A CMOS process could also combine two different voltage levels for different doped regions, e.g., 1.8 V and 3.3 V CMOS process to provide different combinations of high and low voltage LDD's and n+ and p+ regions and even different combinations of high and low voltage n-well222and p-well224. For example a high voltage process may be used to form the p-well224, and a low voltage process to form the n-well222.

As can be seen fromFIG. 2, the junction between the n-well222and the p-well224is also shifted in this embodiment to lie below the polygate206.

The invention further includes adjusting the breakdown voltage by biasing the polygate206. This can be done by connecting the gate206to the anode or cathode of the LVTSCR structure (either directly or through a resistor) or by biasing the gate206using a separate bias circuit. (It will be appreciated that the anode of the structure inFIG. 2is defined by the drain/emitter side while the cathode is defined by the well/source side.

This biasing of the gate206facilitates an important application of the new LVTSCR structure, namely for use as an overvoltage cell without the need to use a cascoded structure. For example, if a 5V tolerant cell with 4V gate oxide is realized, the gate potential can be provided at about 2.5V through the use of a divider circuit. In contrast to single gate NMOS devices this new structure will ensure low leakage operation. Also, the self-aligning effect realized by the polygate will ensure stable parameters. Also, the structure provides for gate dynamic control during ESD pulses.

It will be appreciated that although the invention was described with respect to a specific embodiment, it can be implemented in different ways, defining a p-n junction under the polygate, without departing from the scope of the invention as defined by the claims.