PMOS depletable drain extension made from NMOS dual depletable drain extensions

Abstract

In accordance with an embodiment of the invention, there is an integrated circuit device having a complementary integrated circuit structure comprising a first mos device. The first mos device comprises a source doped to a first conductivity type, a drain extension doped to the first conductivity type separated from the source by a gate, and an extension region doped to a second conductivity type underlying at least a portion of the drain extension adjacent to the gate. The integrated circuit structure also comprises a second complementary mos device comprising a dual drain extension structure.

Description

CROSS-REFERENCE TO RELATED APPLICATION

FIGS. 7a and 7b is a cross-section of an integrated circuit device according to various embodiments of the present invention.

where E is the electric field, q is the electron charge, N_a is the doping of P-type drain extension (in this case the doping is assumed to be uniform), x₁ is the thickness of P-type drain extension, and ∈ is the dielectric constant of silicon.

Further,
Q=N_ax₁  [2]

where Q is the integrated dose of the P-type extension (in units of ions/cm²). After substituting:
E=qQ/∈  [3]

According to various embodiments, the absolute voltage in the depletion layer on one side of the junction can be given by the second integral of Possion's equation;
V_p=qN_ax₁²/2∈=qQ²/2∈N_a  [4]

For the P-type drain extension side,
V_n=qN_dt_n²/2∈  [5]

where V_n is the voltage in the depletion layer in the N-type extension, t_n is the width of depletion layer in N-type extension at a voltage that just totally depletes the P-type extension, and N_d is the doping level of the N extension (which in this case is assumed to be uniform). The integrated dose in the two sides of a depletion layer are equal, therefore:
x₁N_a=t_nN_d  [6]

Solving for t_n:
t_n=x₁N_a/N_d  [7]

and substituting into the expression for V_n:
V_n=qN_d[(x₁N_a)/(N_d)]²/2∈=q(x₁N_a)²/N_d2∈  [8]

Substituting
x₁=Q/N₈  [9]
V_n=qN_a²(Q/N_a)²/N_d2∈=qQ²/N_d2∈  [10]

And the total applied voltage is the sum of the voltages in the depletion layers on the two sides of the junction:
V=V_n+V_p  [11]
=qQ²/N_d2ε+qQ²/N_a2ε  [12]
=qQ²/2ε((1/N_d)+(1/N_a)).  [13]

According to various embodiments, another PMOS device 500 is shown in FIG. 5. PMOS device 500 can comprise a P-type substrate 502, an N-type well 504, a P-type drain extension 510, a gate 512 having sidewall spacers 514, a P+ drain contact 520, an N+ body contact 522, a P+ source 524, a gate oxide 530, an N-type extension 540 surrounding the P-type drain extension 510, a P+ link layer 550 that links the P-type drain extension 510 to the channel under the sidewall spacer 514, and field regions 560, such as LOCOS, STI, and/or other structures as will be known in the art adjacent to the P-type drain extension.

According to various embodiments, the PMOS device 500 can be formed in the well 504 formed from an N-type extension 540. Further, according to various embodiments, the well 504 can be the body of the PMOS device. Moreover, the PMOS device 500 can be made using a layer that is also used to make the bodies of low voltage PMOS devices, i.e., an N well. The well 504 can also be a shallow N body layer optionally self aligned to the gate, similar to that of the NDMOS body 242 shown in FIG. 2. The depletable N-type extension layer 540 can also be used as part of the body in the region under a portion, or most of the gate 512. According to various embodiments, the N-type extension 540 can provide an N-type layer under the P-type drain extension 510 that depletes the P extension under reverse drain to body voltage. According to various embodiments, the N-type extension 540 can be formed from the same N-type extension that is used to form the N-type extension in the complementary NMOS device.

For example, when combining the PMOS device 500 shown in FIG. 5 with the NMOS device 200 of FIG. 2, the N-type extension 240 can also form the N-type extension 540. The performance of this complementary structure can be influenced by the N-type extension 540 being designed to totally deplete in the NMOS. To alleviate a P substrate 502/P+ source 524 punch through voltage limitation on the breakdown voltage and a high pinched resistance in the N body under the source 524, the source 524 can be formed in the high voltage N-type well 504 with the P-type drain extension 510 formed over the N-type extension 540 in the extension region. According to various embodiments, the N-type extension 540 can overlap the N-type well 504 in the region where the source 524 and/or channel under gate 512 is formed. In some cases, the N-type extension 540 can be extended across the entire N-type well 504 to achieve minimum pinched resistance under the source 524. The deep N-type well 504 can have its perimeter surrounded by the P-type drain extension in the N extension layers that will act as a depletable junction termination extension region to reduce junction curvature limited breakdown.

According to various embodiments, NDMOS devices, such as those described herein, can have breakdown voltages (BVDSS) ranging from about 50V to about 1200V. According to some embodiments, NDMOS devices, such as those described herein, can have breakdown voltages ranging from about 150V to greater than about 800V. Complementary devices, such as the PMOS devices described herein, can be made to operate over the same range of voltages. Still further, devices, such as those described herein, can have improved specific ON resistance, which can vary with breakdown voltage. For example, NDMOS devices disclosed herein having a breakdown voltage of about 250V can have a specific on resistance of about 3.2 Ωmm². Complementary PMOS devices, made with a breakdown voltage of about 250V can have a specific ON resistance of about 9.6 Ωmm².

Some conventional MOS devices that include dual drain extensions have drain contact diffusions that can be formed in the body layer of the device. The drain body breakdown voltage (BVDSS) in such a device, however, can be influenced by the plane breakdown of the drain to body (or well) junction. This can also be the case where a feature that completely eliminates junction curvature is included as part of the device. Plane breakdown is set primarily by the doping of the lightly doped side (the drain) of the junction. Breakdown increases as doping decreases.

Body doping can be constrained to a relatively high value by the need to set threshold voltage. Typical body doping for a PMOS device is about 1E15 cm⁻³ for a gate oxide ˜/>1000 Å. The plane junction breakdown with this doping is about 250V. Thinner gate oxides, such as, for example, about 400 Å, can require body doping ˜/>1E16 cm⁻³, for which the plane junction breakdown is about 50V. NMOS devices require higher body doping for a given gate oxide thickness and threshold voltage than do PMOS devices, so their breakdown voltage can be more limited than in a PMOS device.

The drain to body junction of devices described herein, however, can be the junction between the N-type extension, such as N-type extension 340 or 440 that form both body and depletable extension regions, as shown in FIGS. 3 and 4, respectively, or the N body 504 and the depletable N-type extension 540 to the P-type substrate, such as 302, 402, or 502 that is the drain. According to various embodiments, the substrate can include an island having doping this is made as low as desired to set the junction breakdown to the desired value that can exceed 1000V. The planar breakdown limit of the junction can be removed by surrounding the perimeter of the junction with a region of P-type drain extension over N-type extension. In this case, both totally deplete at a drain to body voltage below that at which breakdown in the planar edge of the junction is reached. For example, the P-type drain extension, such as 310, 410, or 510, shown in FIGS. 3-5, respectively, to the N-type extension and/or N well can also be part of the drain to body junction. In this case, the breakdown voltage can be made high by designing both the N extension and the P extension to totally deplete at a voltage lower than that at which the junctions of which they form one side would breakdown.

FIG. 6 shows another PMOS device 600 according to various embodiments of the invention. PMOS device 600 can comprise a P-type substrate 602, an N-type well 604, a P-type drain extension 610, a gate 612 having sidewall spacers 614, a P+ drain contact 620, an N+ body contact 622, a P+ source 624, a gate oxide 630, a P+ link layer 650 that links the P-type drain extension 610 to the channel under the sidewall spacer 614, and field regions 660, such as LOCOS, STI, and/or other structures as will be known in the art adjacent to the P-type drain extension.

According to various embodiments, in the PMOS device 600, source and drain extension 610 can be formed in the N-type well 604 without the need for an N-type extension. The N-type well 604 doping profile can be adjusted such that it totally depletes the P-type extension 610.

It is contemplated to invert all conductivity types explicitly described herein to produce a dual drain extension PMOS and a single drain extension NMOS using the structures described above.

FIGS. 7a and 7b show an MOS device where P⁺ drain contact 712a is formed in P⁻ type drain 712, P⁺ source 714 is formed in the N⁻ body 711 and N⁺ body contact 711c is provided in the N⁻ body 711. The MOS channel region 711b is in the N⁻ body 711 below the MOS gate 716 and Gate Oxide 713. The N type top gate 721 is provided along the surface 711s of the body 711 above the P type drift region 717 which acts as a JFET channel. The lateral edge or peripheral edge of both the top gate 721 and drift region 717 extend to the drain-to-body junction 715 and preferably terminate at the junction 715. It is noted that situations may exist where the doping level in the top gate may be sufficiently high so as to render it desirable to provide a shorter top gate having a lateral extension which stops short of contacting the junction 715. In this case care should be taken to insure that any nondepleted portion of the top gate does not result in a breakdown of the top gate-to-drift region junction 717a. Proper doping of the top gate 721 will generally be a sufficient preventative step. Dashed line 721p designates the peripheral edge of top gate 721 in an embodiment where the top gate does not extend all the way to the junction 715.

The structure of FIG. 7a provides reduced ON resistance in the JFET channel 717. The reduction in ON resistance is accomplished by providing a structure which can accommodate increased drift region doping without suffering from reduced body-to-drain breakdown. This is possible because of the provision of the top gate 721. The top gate-to-channel depletion layer which holds some channel charge when reverse biased, is in addition to the channel charge held by the bottom gate to channel depletion layer of the prior art. This additional channel charge, in the form of ionized channel impurity atoms, causes the reduction in channel resistance. It is possible to provide more than twice the doping level previously acceptable due to the additional ability to hold channel change. Thus, for a drift region 717 having a doping of 1×10¹² boron atoms per square centimeter in a bottom gate arrangement, the present invention will permit 2×10¹² boron atoms per square centimeter. Thus, the ON resistance will be only half the ON resistance of the prior arrangement.

In order to optimize performance of the structure of the invention, the top gate 721 must be designed differently than an ordinary JFET gate. Top gate 721 should become totally depleted at a body-to-drain voltage of less than the breakdown voltage of the top gate-to-drain junction 715a. Since top gate 721 is connected to body 711, the voltage at the top gate-to-drain junction 715a will equal the voltage of the body-to-drain junction 715 voltage and the top gate-to-drain breakdown voltage should be greater than the voltage at which top gate 721 becomes totally depleted. Additionally, the top gate 721 must totally deplete before the body 711 to channel 717 depletion layer reaches the top gate 721 to channel 717 depletion layer to thereby assure that a large top gate 721 to drain 712 voltage is not developed by punch-through action from the body 711. An ordinary JFET gate never totally depletes regardless of operating conditions.

In addition to the above described characteristics of the device of the invention, it is also necessary to insure that the channel of the JFET drift region 717 contacts the inversion layer MOS surface channel. This can be accomplished as shown in FIG. 7b where an implant mask 750 having a tapered edge 751 is provided over the body 711. An implant aperture 752 is provided in mask 750 at the location where the P drift region 717 and N top gate 721 are to be formed. The aperture 752 is shown as exposing the protective oxide 753. Ion implantation is not substantially affected by the oxide 753 due to the oxide thickness of only about 0.1-0.2 micrometers, yet the oxide provides surface passivation for the underlying body 711.

The drift region 717 is ion implanted and, because of the graduated thickness of the implant mask 750 (along the edge 751), the depth of the implanted drift region 717 is graduated or tapered. In the illustration, a fairly good rounding of the drift region 717 occurs at the peripheral edges or extremities 717a, 717b of the region 717. The curved extremity 717a is of interest because at this location the channel of the JFET drift region 717 contacts the surface 711s of body 711 beyond the end 721a of top gate 721 and is desirably beneath the gate 716 of the MOS device. The top gate 721 may be ion-implanted using the implant mask 750 but at an energy level which results in a shallower implantation. This tapered profile, particularly if curved, provides improved performance.

A further extension of the invention is illustrated in FIG. 8 which shows an LDMOS device where N⁺ drain contact 712a is formed in an N⁻ type substrate and an N⁺ source 714 and P⁺ body contact 711c are formed in a P⁻ type body region 8240. The DMOS channel region 711b is in the P⁻ body 8240 below the DMOS gate 716. The N type first drift region 8217 is provided along the surface 711s of the substrate 711 above a P⁻ type separation region 8250. A second drift region 217a exists in the substrate 711 underneath the P⁻ type separation region. The lateral edge of both the first drift region 8217 and the separation region 8250 extend from the gate 716 to the N⁺ drain contact 712a.

The structure in FIG. 8 provides reduced ON resistance by way of the second (surface) drift region 217a. To illustrate this, consider an example in which the N⁻ region 711 has a doping of 1×10¹⁴ ions cm⁻³. The top gate layer 8217 has an integrated doping of about 1×10¹² ions cm⁻² and is preferably not more than two microns thick while maintaining full breakdown. The thickness of the N and P layers 8217, 8250 together is preferably less than ten microns and can be less than one micron. The same integrated doping in the N⁻ body 711 requires a thickness of 100 microns. Thus, the N and P layers 8217, 8250 respectively consume only a small fraction of the N⁻ thickness required to provide doping equal to that portion of the N layer of the prior art device.

The lateral spacing between the drain contact 712a and the channel 711b in the device described above would be approximately 30 microns. In such a device, even if a full 100 micron thick N⁻ body 711 were provided, it would have a higher resistance than the N⁻ first drift region 8217 provided according to the invention. This is because the average path length of current flowing from the drain contact 712a down through the thick N⁻ body 711 and back up to the surface edge of the channel at the drain-to-body junction would be greater than the direct path through the N⁻ first drift region.

Maximum breakdown is achieved in the invention by providing doping densities of the N and P layers 8217, 8250 such that they become totally depleted before breakdown is reached at any point along the junctions which they form with adjoining regions and before breakdown is reached at the junction between them. To insure that this occurs, the N region 8217 should have an integrated doping not exceeding approximately 1×10¹² ions cm⁻² and the P region 8250 should have a higher integrated doping not exceeding about 1.5 to 2×10¹² ions cm⁻².

To insure proper depletion of the P and N regions 8250, 8217, they must have the proper voltages applied. The N layer bias is achieved by connecting the N first drift region 8217 to the higher concentration N⁺ drain contact 712a by overlapping the N first drift region 8217 and drain contact 712a. The P region 8250 bias is achieved by overlapping the P region 8250 with the P⁻ body 8240 at least at one end of the channel, thereby applying the body voltage to the P layer 8250. This is illustrated in FIG. 9.

With this structure and choice of doping levels, the desired results are achieved. When a reverse bias voltage is applied to the drain-to-body junction between P⁻ body 8240 and N⁻ body 711, the same reverse bias appears on both the PN⁻ junction 8260 and the PN junction 8270. Depletion layers spread up into the N first drift region 8217 and down into the N⁻ body 711 from the P layer 8250. In a preferred embodiment, the P and N first drift region dopings are chosen such that the N layer 8217 becomes totally depleted at a lower voltage than that at which the P layer 8250 becomes totally depleted. This insures that no residual undepleted portion of the N layer 8217 is present which could reduce breakdown voltage.

As a result of the invention, the improved DMOS device provides a reduced resistance current path in the drain which does not depend on the N⁻ doping. This allows the N⁻ doping to be reduced to achieve a desired breakdown voltage with good manufacturing margin, while maintaining desirable low drift region resistance. In a multi-device process which includes LDMOS devices, the N⁻ region can be adjusted to achieve the desired characteristics of one or more of the other device types, while the N first drift region 8217 sets the drift region 8217 resistance of the LDMOS.

Still another embodiment, as illustrated in FIG. 10, provides no gap between the P− body 8240 and the P region 8221 adjacent to the channel edge. The absence of the gap prevents current from flowing in the N− body 711; so the entire drift region current path is in the N first drift region 8217. Elimination of the gap also allows the device structure to be made smaller. As with the other structure, the N and P regions may be self-aligned to the gate edge, as illustrated in FIG. 10, or not self-aligned. They may also be covered by thick or thin oxide as a design option.

While the invention has been illustrated with respect to one or more implementations, alterations and/or modifications can be made to the illustrated examples without departing from the spirit and scope of the appended claims. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular function. Furthermore, to the extent that the terms “including”, “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description and the claims, such terms are intended to be inclusive in a manner similar to the term “comprising”.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.

Claims

1. A complementary integrated circuit structure comprising:

a first mos device comprising,

a first gate;

a source doped to a first conductivity type;

a drain extension doped to the first conductivity type separated from the source by the first gate;

a drain contact doped to the first conductivity type; and

an extension region doped to a second conductivity type underlying at least a portion of the drain extension adjacent to the first gate;

a second complementary mos device comprising,

a second gate;

a dual drain extension structure; and

an island doped to the first conductivity type underlying the first gate and the second gate,

wherein,

the drain contact of the first mos device contacts the island; and

the first conductivity type is one of N-type and P-type, and the second conductivity type is the other one of N-type and P-type.

2. The complementary integrated circuit structure according to claim 1, wherein the drain extension in the first mos device is formed from a layer that forms a drain extension in the second complementary mos device.

3. The complementary integrated circuit structure according to claim 1, wherein the extension region in the first mos device is formed from a layer that forms an extension region in the second complementary mos device.

4. The complementary integrated circuit structure according to claim 1 wherein first mos device further comprises:

a body contact contacting the extension region and doped to the second conductivity type; and

the drain contact further contacts the drain extension.

5. The complementary integrated circuit structure according to claim 1 further comprising:

a field region disposed adjacent to the drain extension.

6. The complementary integrated circuit structure according to claim 1, wherein the drain extension has a net doping concentration of about 5E11 ions/cm² to about 1.5E12 ions/cm².

7. The complementary integrated circuit structure according to claim 1, wherein the extension region has a net doping concentration of about 1E12 ions/cm² to about 3E12 ions/cm².

8. The complementary integrated circuit structure according to claim 1 further comprising:

a link layer doped to the first conductivity type contacting the drain extension and a channel disposed under the gate, wherein the link layer has a higher dopant concentration than the drain extension.

9. The complementary integrated circuit structure according to claim 1 further comprising:

a well doped to the second conductivity type surrounding the source.

10. The complementary integrated circuit structure according to claim 8, wherein the link layer has a length of about 0.5 μm to about 5.0 μm.

11. The complementary integrated circuit structure according to claim 1 wherein the extension region surrounds the drain extension.

12. A complementary integrated circuit structure comprising:

a first mos device comprising,

a first gate;

a first source doped to a first conductivity type;

a drain contact doped to the first conductivity type; and

a single drain extension separated from the first source by a first gate; and

a second complementary mos device comprising;

a second gate;

a second source doped to a second conductivity type;

a dual drain extension separated from the second source by a second gate,

an island doped to the first conductivity type underlying the first gate and the second gate,

wherein the first conductivity type is one of N-type and P-type, and the second conductivity type is the other one of N-type and P-type.

13. The complementary integrated circuit structure according to claim 12, wherein the single drain extension comprises a layer that is also used in the dual drain extension.

14. The complementary integrated circuit structure according to claim 12, wherein the single drain extension totally depletes at a reverse drain to body bias before breakdown occurs at a drain extension to body junction under an edge of the first gate.

15. The complementary integrated circuit structure according to claim 12, further comprising:

a link layer doped to the first conductivity type contacting the single drain extension and a first channel disposed under the first gate, wherein the link layer has a higher dopant concentration than the drain extension.

16. The complementary integrated circuit structure according to claim 15, wherein the single drain extension totally depletes at a drain to body reverse bias below that at which a link layer to body junction under an edge of the first gate breaks down.

17. The complementary integrated circuit structure according to claim 12, wherein the first source is disposed in a first well doped to the second conductivity type.

18. The complementary integrated circuit structure according to claim 12, wherein the first mos device and the second complementary mos device have a breakdown voltage greater than about 50V.

19. The complementary integrated circuit structure according to claim 17, wherein a portion of the single drain extension is disposed in the first well.

20. An integrated circuit structure comprising:

a first mos device comprising,

a first gate;

a first source doped to a first conductivity type;

a first drain extension formed in a first layer, the first drain extension doped to the first conductivity type and separated from the first source by at least a portion of the first gate; and

a first well region formed in a second layer underlying the first source and at least a portion of the first drain extension, the first well region doped to a second conductivity type; and

the first mos device further comprising a drain contact doped to the first conductivity type and formed at least partially in the first well region and overlapping at least a portion of the first drain extension; and

at least a second mos device comprising either a first region made from the first layer or a second region made from the second layer.

21. The integrated circuit structure of claim 20, wherein the drain contact is formed entirely within the first well region.

22. The integrated circuit structure of claim 20, wherein the first drain extension is formed entirely within the first well region.

23. The integrated circuit structure of claim 20, wherein a portion of the drain contact is formed outside the first drain extension.

24. The integrated circuit structure of claim 20, wherein the first well region is formed within a third layer doped to the first conductivity type.

25. The integrated circuit structure of claim 24, wherein at least a portion of the first well region between the first drain extension and the third layer depletes at a first drain to a first well voltage below a breakdown voltage of the first mos device.

26. The integrated circuit structure of claim 25, wherein a portion of the first well region doped to the second conductivity type between the first drain extension doped to the first conductivity type and the third layer doped to the first conductivity type depletes due to a combined extension of depletion layers into opposite sides of the first well region from at least one PN junction between the first drain extension and first well region and between the third region and first well region.

27. The integrated circuit structure of claim 26, wherein the first drain extension depletes at the first drain to a first well bias voltage less than a breakdown voltage of the first mos device.

28. The integrated circuit structure of claim 20, further comprising a first link region doped to the first conductivity type adjacent to the first gate at a first edge and abutting the first drain extension at an opposite edge.

29. The integrated circuit structure of claim 20, further comprising a second well region doped to the second conductivity type below at least the first source and the first gate.

30. The integrated circuit structure of claim 20, wherein the at least a second mos device comprises a first region made from the first layer and a second region made from the second layer.