Abstract:
High voltage and low voltage devices are provided on a common semiconductor substrate. An integrated semiconductor circuit includes a semiconductor substrate of a first conductivity type. Well regions of a first conductivity type and well regions of a second conductivity type are formed in the substrate. Low voltage devices are formed in well regions of the first conductivity type. A high voltage device includes source/drain regions of the second conductivity type formed, respectively, in well regions of the second conductivity type, an oxide region disposed on a surface of the substrate located above a region of the substrate that serves as a channel for the high voltage device, and a gate region disposed on the oxide region.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of Ser. No. 09/006,918, filed Jan. 13, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates generally to integrated metal oxide semiconductor (MOS) devices and, in particular, to combining high voltage and low voltage devices on a single semiconductor substrate. 
     Refinements in the design and fabrication of integrated circuits (ICs) have provided circuits having more than one million transistors. With continued research and development, the microelectronics industry continues to push transistor densities even higher. One consequence of increasing transistor densities is that device operating voltages are generally getting lower. Whereas, older transistor-transistor logic (TTL) circuits were designed to operate at 5 volts (v), newer circuits such as memory devices and central processing units (CPU) are being designed to operate at 3.3v, 2.5v, and 1.8v. 
     SUMMARY OF THE INVENTION 
     The invention generally provides methods and apparatus to fabricate high voltage transistors (operating for example at approximately 12 volts and above) on a substrate of higher density, low voltage (operating for example at approximately 5 volts and below) devices. In one embodiment, a high voltage transistor is fabricated on a substrate of low voltage devices without any additional photolithographic or process steps. In other embodiments, photolithographic and process steps are added to allow operational refinements of the high voltage transistor. 
     Including high voltage and low voltage devices on a common substrate allows the fabrication of integrated circuits having increased functionality at a reduced cost over (functionally) equivalent combinations of circuits, some incorporating high voltage devices and some incorporating low voltage devices. Examples of high voltage circuits that can be implemented on a substrate of predominately low voltage devices in accordance with the invention include, but are not limited to: (1) on-chip programming devices for use in programmable read-only memory devices; (2) high voltage sense circuits; (3) liquid crystal display drivers; (4) servo control circuits; and (5) disk drive read channel circuitry. Other features and advantages of the invention will become apparent from the following detailed description. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 shows a high voltage device fabrication process flow. 
     FIGS. 2A through 2D illustrate semiconductor substrate cross-sections corresponding to the process flow of FIG.  1 . 
     FIGS. 3A and 3B show test data for one illustrative high voltage device. 
     FIG. 4 shows another high voltage device fabrication process flow. 
     FIGS. 5A and 5B illustrate semiconductor substrate cross-sections corresponding to the process flow of FIG.  4 . 
     FIG. 6 shows another high voltage device fabrication process flow. 
     FIG. 7 illustrates a semiconductor substrate cross-section corresponding to the process flow of FIG.  6 . 
     FIG. 8 shows another high voltage device fabrication process flow. 
     FIGS. 9A and 9B illustrate semiconductor substrate cross-sections corresponding the process flow of FIG.  8 . 
     FIG. 10 shows yet another high voltage device fabrication process flow. 
    
    
     DETAILED DESCRIPTION 
     New low voltage devices are more susceptible to high voltage signals that prior devices. For example, a circuit implemented in 1.8v technology can be damaged when exposed to a high voltage signal, i.e., a signal of approximately 12v and above. It would be beneficial to integrate both low voltage and high voltage/drive capability into a single circuit on a single substrate. The combined integration of low and high voltage devices would make possible the design and manufacture of complete functional circuits; circuits having both logic and direct input-output (I/O) control capability. 
     Recent complimentary metal oxide semiconductor (CMOS) technology advances have focused on using energy implant technology to form retrograde twin-well structures within which devices are constructed. Retrograde twin-well technology is characterized by reduced well thermal drives, well defined well edges, thinner field oxide requirements, reduced well-to-well spacing design rules, and greater accuracy and flexibility in defining the n-well/p-well doping profiles. Retrograde twin-well processing technology may be exploited to fabricate high voltage transistors (designed to operate at approximately 12v and above) on a substrate predominantly populated with devices designed to operate at approximately 5v and below. 
     While detailed embodiments of the present invention are described, it should be understood that the disclosed embodiments are merely exemplary of the invention. The details disclosed herein are not to be interpreted as limiting, but merely as the basis of the claims and as a basis for teaching one skilled in the art how to make and/or use the claimed invention. 
     FIGS. 1 and 2 illustrate formation of a high voltage NMOS device in a p-type substrate having low voltage devices. FIG. 1 shows an illustrative process flow and FIG. 2 substrate cross-sections corresponding to some of the process flow steps of FIG.  1 . High voltage device fabrication begins with a p-type substrate (step  100 , element  200 ) having an ionic concentration of approximately 5e 14  to 5e 15  Boron (B) atoms/cm 3 . Field and active regions of both the high voltage and low voltage devices are defined using an isolation technology (step  105 ). Suitable isolation technologies include local oxidation of silicon (LOCOS) and its variants such as semirecessed, sidewall-masked isolation (SWAMI), self-aligned planar-oxidation technology (SPOT), sealed-interface local oxidation (SILO), and fully recessed oxide (FUROX) as well as non-LOCOS techniques such as trench technology. FIG. 2 illustrates a general LOCOS technique using a field oxide  205  of silicon dioxide (SiO 2 ), typically 2000 Å to 5000 Å thick. P-well regions  210  may be used to form low voltage devices. 
     Following isolation, n-well structures  215  are formed in the substrate (step  110 ) to provide high voltage device drain extensions and regions in which low voltage p-type devices can be formed. N-well regions  215  may be formed using one or more energy implants. For example, a first implant of arsenic (As) or phosphorous (P) at 50 KeV to 200 KeV (preferably 100 KeV), followed by a second implant of As and/or P at 250 KeV to 700 KeV (preferably 400 KeV), followed by a third implant of As and/or P at 500 KeV to 2 MeV, preferably 800 KeV, may be designed to create a retrograde n-well having approximately 2e 11  to 2e 13  ions/cm 2 . An additional layer of photoresist (not shown in FIG. 2) may be used to shield areas of the substrate where no implantation is desired. 
     For additional isolation between the low and high voltage devices a p-well block can be constructed (step  115 ). The p-well block does not require an additional process step (e.g., implantation), but rather a change in existing low voltage device p-well masking to ensure a physical separation between low voltage device p-wells and adjacent high voltage device n-wells. Thus, p-well blocks are formed during low voltage device p-well formation at no additional process cost. The p-well block ensures there are three distinct regions at the surface of the substrate: p-well; n-well; and bulk. The electrically decoupled bulk regions (i.e., the bulk regions are not used to set any electrical characteristic of the low voltage devices) enable formation of useful and stable high voltage devices. 
     P-well block formation (step  115 ) is followed by a relatively short and low temperature anneal phase designed to activate the implantation regions (step  120 ). A typical activation anneal uses an 850 C. to 1000 C. bake for 15 to 60 minutes. The anneal&#39;s relatively low temperature and short time period limits the lateral diffusion of the implanted ions, resulting in relatively sharp well edges and well ionic concentration of approximately 1e 16  to 5e 17  ions/cm 3 . 
     High voltage gate regions  230  are formed and source  220 , drain  225 , and gate  230  regions are doped in the normal course and at the same time as the source, drain, and gate structures of the substrate&#39;s low voltage devices (step  125 ). Source  220  and drain  225  regions can be n+ regions by implantation of As and/or P ions at a dose of approximately 1e 15  to 6e 15  ions/cm 2  to yield a concentration of approximately 1e 20  ions/cm 3 . Gate structure  230  can be made of polysilicon and is typically 1700 Å to 4000 Å thick. During low voltage device fabrication, field oxide  205  is preferably made as thick as practical yet thin enough to support the desired low voltage device spacing (e.g., design rules). Generally, as gate oxide thickness decreases a high voltage devices drive capacity increases while stray capacitance associated with the low voltage devices&#39;increases. The increased capacitance can limit the operational speed of the low voltage devices. The high voltage device&#39;s threshold voltage (V th ) may be adjusted by the silicon band-gap voltage (approximately 1.2 volts) by using either n+ or p+ material for the gate structure instead of polysilicon. 
     Finally, conductive paths are formed to allow use of both the high voltage and low voltage devices (step  130 ). Materials suitable for forming device interconnections include, but are not limited to, aluminum (Al), molybdenum (Mo), tungsten (W), titanium nitride (TiN), and titanium-tungsten combinations (Ti:W). 
     FIGS. 3A and 3B show test data for a high voltage device having an approximately 50 μm channel width and an approximately 5 μm channel length. The substrate&#39;s low voltage device characteristics were as follows: gate breakdown voltage of approximately 7.5 volts; drain breakdown voltage of approximately 6.0 volts; device operating voltage of approximately 3.3 volts; device-to-device spacing of approximately 0.6 μm; and a gate oxide thickness of approximately 70 Å. As shown, no aberrant behavior (breakdown, snap-back, or punch-through) was detected even at high voltage device gate voltages of 20 volts and well voltages (bulk voltage to source voltage) of 4 volts. 
     A high voltage device in accordance with FIGS. 1,  2 , and  3  may be used to, for example, implement a circuit for sensing high voltage signals and/or for generating drive currents in excess of 6 mA per device without negatively impacting the operational characteristics of the low voltage devices or incurring any additional processing steps. Several high voltage devices may be combined to implement circuits having output drive capability in excess of 50 mA without requiring large amounts of substrate area. The low voltage devices could be used to implement any desired conventional functional circuit such as a microprocessor or a disk drive control unit. 
     Referring to FIGS. 4 and 5, an alternative embodiment is shown which uses an additional mask to expose the high voltage device gate oxide region (step  400 ) and a timed etch (step  405 ) to selectively reduce the gate oxide&#39;s thickness  500 . With the addition of this one operation (one mask to open a photoresist window over the high voltage device&#39;s gate oxide and one etch), the drive capacity of the high voltage device may be increased approximately five times without affecting low voltage device characteristics. The etch (step  405 ) may be performed, for example, by a timed buffered oxide etch (BOE) or buffered hydrofluoric acid (HF) etch. The precise time of the etch depends at least upon the gate oxide&#39;s initial thickness and concentration of the etchant (e.g., HF). The gate oxide  500  may be taken as thin as desired (within control limits of the timed etch), typically 400 Å to 1000 Å. The effect of this operation is to increase high voltage device transconductance which, also has the effect of reducing the high voltage device&#39;s gate breakdown voltage. For a gate oxide thickness of 400 Å, the high voltage device gate breakdown voltage is in excess of 40 volts. This embodiment retains all of the advantages, and can be modified in the same manner, as the prior embodiment while incurring only a minimal added fabrication cost. 
     Referring to FIGS. 6 and 7, the prior embodiment is modified to provide a high voltage device gate implantation step ( 600 ) after the gate oxide has been exposed (step  400 ) but before it has been etched (step  405 ). Implant energies may be between approximately 1e 11  to 1e 12  ions/cm 2 , generally using B or boron-fluoride (BF 2 ) ions. Shallow implantation energies of approximately 100 KeV to 200 KeV, and deep implantation energies of approximately 100 KeV to 1 MeV may be used. Gate region implantation may also be performed before the gate etch (step  405 ) and, if so, implantation energies as low as 25 KeV for a shallow implant, and 300 KeV for a deep implant may be used. If the modified p-type gate material  710  is annealed at the same time the source and drain regions are annealed (step  120 ), no additional processing is necessary. Gate region implantation (step  600  and region  705 ) can allow the high voltage device source and drain regions to be more closely spaced which, in turn, tends to reduce the effect of the device&#39;s channel length and increases the high voltage device&#39;s current drive capability. Implantation (step  600 ) also allows an increased flexibility in setting the high voltage device&#39;s electrical characteristics, such as its threshold voltage. 
     In yet another embodiment, the timed etch-back operation of FIGS. 4 through 7 is replaced with an oxide growth process, see FIGS. 8 and 9. Following step  110 , a layer of silicon nitride  900  is deposited on the surface of the semiconductor (step  800 ). After the high voltage gate regions are exposed (step  400 ), the field region is etched and a layer of oxide is grown (step  805 ), and is typically 300 Å to 1000 Å using any convenient dry or wet oxide growth process. Following gate oxide formation, the silicon nitride is stripped (step  810 ) using any convenient stripping agent (e.g., phosphoric acid) and processing continues as described above (e.g., at step  115 ; see FIGS. 1,  4 , and  6 ). One benefit of this approach is that the gate oxide thickness  905  can be better controlled than in an etch process. This added control, in turn, allows the design of a high voltage device with more tightly controlled electrical characteristics such as threshold voltage, current drive capability, and input capacitance. This embodiment retains all of the advantages, and can be modified in the same manner as the prior embodiments. 
     In yet another embodiment, see FIG. 10, a gate region implantation (step  600 ) is performed after the gate oxide has been grown, but before the silicon nitride has been stripped (step  810 ). Following the nitride strip, fabrication continues in accordance with step  115  through  130  as discussed above (step  1000 ). 
     In those embodiments using a gate region implantation (see, e.g., element  710  in FIG.  7 ), a typical silicon nitride layer of 800 Å to 2500 Å is sufficiently thick to stop implantation energies of up to approximately 100 KeV. If higher implantation energies are desired, the photoresist used to pattern the silicon nitride, which may be up to approximately 3 μm thick, may be left in place during implantation. (The gate region must be kept essentially clear of both the silicon nitride and photoresist.) The protection afforded the substrate by the photoresist in combination with the silicon nitride can shield the substrate from ion energies of up to approximately 2 MeV. Following implantation, the photoresist and silicon nitride may be stripped (using any convenient technique) and processing continued. 
     One of ordinary skill will recognize that many variations of the described processes may be made while still achieving fabrication of a high voltage device. For example: isolation (step  105 ) and well formation (step  110 ) steps may be done in reverse order—step  110  followed by step  105 ; a standard p-buried layer for epi replacement may be formed in the substrate  200  before any other steps (i.e., before step  105 ); channel stop implants can be formed in the high voltage device gate region (i.e., under the high voltage device gate filed oxide  205 ) to allow finer control over device threshold voltage or, alternatively, the substrate&#39;s bulk dopant level may be adjusted prior to fabrication to fulfill the same goal; gate region ion implantation may be performed either before or after a gate oxide is formed. In addition, a PMOS high voltage device may be formed by stating with an n-type substrate material, forming p-wells, and p+ source and drain regions. The embodiments described herein constitute exemplars only, other embodiments are within the scope of the following claims.