Abstract:
Dynamic Random Access Memory (DRAM) cells are formed in a P well formed in a biased deep N well (DNW). PMOS transistors are formed in N wells. The NMOS channels stop implant mask is modified not to be a reverse of the N well mask in order to block the channels stop implant from an N+ contact region used for DNW biasing. In DRAMs and other integrated circuits, a minimal spacing requirement between a well of an integrated circuit on the one hand and adjacent circuitry on the other hand is eliminated by laying out the adjacent circuitry so that the well is located adjacent to a transistor having an electrode connected to the same voltage as the voltage that biases the well. For example, in DRAMs, the minimal spacing requirement between the DNW and the read/write circuitry is eliminated by locating the DNW next to a transistor precharging the bit lines before memory accesses. One electrode of the transistor is connected to a precharge voltage. This electrode overlaps the DNW which is biased to the same precharge voltage. This electrode provides the DNW N+ contact region.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to semiconductor circuits, and more particularly to wells used in semiconductor circuits.  
           [0002]    Some semiconductor circuits use wells biased at a predetermined voltage to obtain needed functionality or performance characteristics. For example, biased wells can be used to isolate transistors from each other. Thus, in some dynamic random access memories (DRAMs), NMOS transistors of DRAM cells are formed in a P-well formed in a biased deep N well (DNW) that isolates the P well from the P doped substrate. The P well itself is biased at a lower voltage than the substrate. Hence, the body regions of DRAM cell transistors in the P well are biased at a lower voltage than the body regions of NMOS transistors of read/write circuitry (for example, of sense amplifiers) that are fabricated in the substrate. The lower bias voltage in the P well reduces the leakage current through the DRAM cell transistors. The leakage current through these transistors is of concern because it could discharge the cells. At the same time, the lower bias voltage is not suitable for read/write NMOS transistors because the lower bias voltage would make these transistors slower. (Of note, the leakage current is not as big a concern for the read/write transistors as for the DRAM cell transistors.) The biased DNW isolates the DRAM cell transistors from the read/write transistors.  
           [0003]    In order to improve the electrical contact between a voltage source biasing the DNW and the DNW itself, the DNW is provided with a low-resistance, heavily-doped N+ contact region located at the substrate surface. The N+ contact region is formed in a separate N well which itself is formed in the DNW. The reason for the separate N well is as follows.  
           [0004]    One of the DRAM fabrications steps is a channel stop implant. The channel stop implant is a P-type implant performed into the NMOS transistor active areas and into field isolation regions. The purpose of the channel stop implant is to increase the punch-through voltages of NMOS transistors and the punch-through and threshold voltages of parasitic field transistors. The channel stop implant is blocked from N wells in which PMOS transistors are formed. To simplify mask generation, the mask for the channel stop implant is made to be a reverse of the mask used for the N-type implant that creates the N wells. Thus, the channel stop implant is implanted precisely into those areas which are blocked from the N-well implant.  
           [0005]    Besides the N wells containing the PMOS transistors, the channel stop implant is also blocked from the N+ contact region used to bias the DNW. This is done to prevent the channel stop P-type dopant from impeding electrical contact between the N+ contact region and the DNW. In order to enable the channel stop implant mask to be the reverse of the N well mask and still to block the channel stop implant from the N+ contact region, the N+ contact region is formed in the separate N well which is formed with the same N well mask as used for the N wells containing the PMOS transistors.  
           [0006]    It is desirable to reduce spacings associated with wells in the integrated circuit. Of note, a minimal spacing is typically required between a well and transistors outside the well. For example, in DRAMs a minimal spacing is required between the DNW and read/write circuitry transistors. It is desirable to reduce such spacings.  
         SUMMARY OF THE INVENTION  
         [0007]    According to the present invention, integrated circuit spacing requirements are reduced. In some embodiments, spacing requirements between wells and transistors outside the wells are eliminated. Therefore, the integrated circuit size can be reduced.  
           [0008]    More particularly, in some embodiments, the separate N wells containing the N+ contact regions in the DNWs are eliminated. This is made possible by modifying the channel stop mask not to be a reverse of the N well mask.  
           [0009]    Further, spacing requirements between wells and transistors outside the wells are eliminated as follows. When transistors outside the well (e.g., a DNW) are laid out, the transistor placed adjacent to the well is a transistor that can be used to bias the well. This transistor couples a predetermined voltage from one of its electrodes to the other. For example, in a DRAM, this transistor can be a precharge transistor that couples a predetermined voltage to a bit line to precharge the bit line before a memory access (e.g., a memory read operation). The predetermined voltage is also suitable to bias the well. The transistor electrode that receives the predetermined voltage is at least partially inside the well, biasing the well to the predetermined voltage. Therefore, the minimal spacing requirement between the well and the transistor is eliminated.  
           [0010]    In some DRAM embodiments, the channel stop implant mask blocks at least a portion of an area in which the DNW overlaps the precharge transistor drain region. Hence, the channel stop P dopant is prevented from impeding the electrical contact between the DNW and the drain region.  
           [0011]    In some embodiments, P and N conductivity types are reversed.  
           [0012]    Other features of the invention are described below. The invention is defined by the appended claims. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    [0013]FIG. 1 is a block diagram of a DRAM according to the present invention.  
         [0014]    [0014]FIG. 2 shows a cross-section of a portion of the DRAM of FIG. 1.  
         [0015]    [0015]FIGS. 3A, 3B are a circuit diagram of a portion of the DRAM of FIG. 1.  
         [0016]    [0016]FIGS. 4A, 4B are a top layout view of the portion of the DRAM of FIG. 1.  
         [0017]    [0017]FIGS. 5 and 6 are cross-section illustrations of the DRAM of FIG. 1 in the process of fabrication.  
         [0018]    [0018]FIG. 7 is a cross-sectional view showing drawn dimensions in some embodiments of FIG. 1. 
     
    
     DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0019]    [0019]FIG. 1 shows an integrated DRAM  110 . Memory arrays  120 . 1 ,  120 . 2  in DRAM  110  alternate with read/write (R/W) circuitry blocks  130 . 1 ,  130 . 2 ,  130 . 3  along the horizontal direction in FIG. 1 (the memory column direction). Each memory array  120  is surrounded on the left and right by R/W circuitry blocks  130 . Each memory array  120  is an array of memory cells. Each cell includes an NMOS transistor  210  (FIG. 2). Source regions  210 S and drain regions  210 D of transistors  210  of each memory array are formed in one or more P wells  216 . Each P well  216  is formed in a corresponding deep N well (DNW)  220 .  
         [0020]    Drain  210 D of each transistor  210  is connected to a bit line BL (or a complimentary bit line {overscore (BL)}, not shown in FIG. 2). Source  210 S is connected to a memory capacitor  226  which is also connected to a reference voltage VREF. P well  216  is biased to a negative voltage, −1.0V in some embodiments in which VCC=3.3V, to reduce leakage through transistors  210 . Deep N well  220  is biased to a positive voltage HVCC (half VCC) to isolate the P well  216  from P substrate  230 . VCC is 3.3V or 5.0V in some embodiments.  
         [0021]    In an adjacent read/write circuitry block  130  on the right of memory array  120  in FIG. 2, the transistor adjacent to DNW  220  is a bit line precharge transistor  236 . Drain  236 D of transistor  236  is connected to the same voltage source HVCC that biases the deep N well  220 . Source  236 S is connected to a bit line BL (or {overscore (BL)}). Gate  236 G receives an equalization signal EQ. Signal EQ is asserted high before a memory access operation to precharge the bit lines to HVCC.  
         [0022]    A portion of drain  236 D is inside DNW  220  so that the DNW overlaps the drain  236 D. Therefore, no minimal spacing is required between the R/W circuitry block  130  and the deep N well.  
         [0023]    Similarly, in R/W block  130  on the left of the memory array (not shown in FIG. 2), a bit line precharge transistor has a drain region overlapping DNW  220 . The drain region is connected to HVCC. Therefore, no minimal spacing is required.  
         [0024]    In some embodiments, the drains of the bit line precharge transistors  236  are the only N+ contact regions in the DNW that connect the DNW to HVCC.  
         [0025]    As seen in FIG. 1, DRAM  110  includes four boundaries between memory arrays  120  and R/W circuitry blocks  130 . Hence, four minimal spacings are eliminated. The number of spacings eliminated increases with the number of memory arrays. Some 4 Mb DRAMs include 16 memory arrays, and hence 32 boundaries between memory arrays and read/write circuitry blocks. Significant size reduction is therefore achieved.  
         [0026]    In FIG. 1, memory columns and their respective bit lines BL, {overscore (BL)}, extend horizontally. Memory rows and their respective word lines WL extend vertically. Each memory array  120 . i  is subdivided into a number of identical memory blocks M-i.j. (A circuit diagram of block M- 1 . 2  is shown in FIG. 3A.) Each memory block M-i.j is a single column of memory array  120 . i . Each word line WL of memory array  120 . i  runs through all blocks M-i.j of the array. Only one pair of bit lines BL, {overscore (BL)} runs through any given memory block M-i.j.  
         [0027]    The bit line pair BL, {overscore (BL)} of block M-i.j is connected to a read/write block RW-k.j (FIGS.  1 - 3 ) in an adjacent R/W circuitry block  130 . k  (where k=i or k=i+1) in a staggered fashion. Thus, the bit lines of memory block M- 1 . 1  are connected to read/write block (RW block) RW- 1 . 1  on the left of memory array  120 . 1 . The bit lines of memory block M- 1 . 2  are connected to RW block RW- 2 . 2  on the right of memory array  120 . 1 , and so on. Block RW- 2 . 2  is also connected to memory block M- 2 . 2 . Additional details of this staggered architecture are described in U.S. patent application “DRAM With Staggered Shared Bit Line Sense Amplifier Architecture”, attorney docket number M-3880 US, filed by Li-Chun Li et al. on Dec. 3, 1996 and incorporated herein by reference.  
         [0028]    In memory block M- 1 . 2  (FIG. 3A), and hence in every memory block M-i.j, a memory cell is located at the intersection of bit line BL with every other word line WL, and at the intersection of bit line {overscore (BL)} with each of the remaining word lines. The gate of each memory cell transistor  210  is connected to a respective word line WL, and the drain is connected to a respective bit line BL or {overscore (BL)}.  
         [0029]    All memory cell transistors  210  of a single memory array  120  are formed in the same P well  216  located in the same deep N well  220  (FIG. 2).  
         [0030]    As shown in FIGS. 3A, 3B, RW block RW- 1 . 2  includes: (1) precharge block  310  (FIG. 3A) for precharging the bit line segments running through memory block M- 1 . 2 ; (2) sensing block  314  (FIG. 3B) including a sense amplifier  320 ; and (3) precharge block  326  for precharging the bit line segments of memory block M- 2 . 2 . All the RW blocks RW-i.j are identical to each other, except that the leftmost blocks RW- 1 . j  omit the precharge block  310 , and the rightmost blocks RW- 3 . j  omit precharge block  326 .  
         [0031]    In precharge block  310 , precharge transistor  236  (FIGS. 2, 3A) has its drain connected to a metal-1 line HVCC-M 1  running vertically (in the plan view of FIGS. 1, 3A) through all the RW blocks in the R/W circuitry  130 . 2  (i.e. through blocks RW- 2 . 2 , RW- 2 . 4 ). This metal-1 line HVCC-M 1  carries the constant voltage HVCC. The source of transistor  236  is connected to bit line BL. The gate is connected to a metal-1 line EQ-M 1 . This line carries equalization signal EQ. Metal line EQ-M 1  runs vertically through all the RW blocks of R/W circuitry  130 . 2 .  
         [0032]    Metal line EQ-M 1  is connected also to the gate of NMOS equalization transistor  330  interconnecting the bit lines BL, {overscore (BL)}.  
         [0033]    NMOS transistors  334 ,  340  connect bit line segments BL, {overscore (BL)} of memory block M- 1 . 2  to respective bit line segments BL, {overscore (BL)} of sensing block  314  (FIG. 3B). Similar NMOS transistors  344 ,  350  connect the bit line segments of sensing block  314  to respective bit line segments of memory block M- 2 . 2 . The gates of transistors  334 ,  340  receive signal SS 1  distributed on a vertical metal-1 line SS 1 -M 1 . The gates of transistors  344 ,  350  receive signal SS 2  distributed on a vertical metal-1 line SS 2 -M 1 . Lines SS 1 -M 1 , SS 2 -M 1  run through all the RW blocks of R/W circuitry  130 . 2 . When signal SS 1  is asserted high, sense amplifier  320  amplifies the signals from memory block M- 1 . 2 . When signal SS 2  is asserted high, sense amplifier  320  amplifies signals from memory block M- 2 . 2 . At most one of signals SS 1 , SS 2  is high at any given time.  
         [0034]    In sensing block  314 , bit line BL is connected to a source/drain region of NMOS pass transistor  354 . The other source/drain region of pass transistor  354  is connected to data bit output line {overscore (DB)}. Bit line {overscore (BL)} is connected to a source/drain region of pass transistor  360  whose other source/drain region is connected to complimentary data bit output line DB. The gates of transistors  354 ,  360  receive a column select signal YS. Data lines {overscore (DB)}, DB are metal-1 lines running vertically through all the RW blocks of R/W circuitry  130 . 2 .  
         [0035]    PMOS transistors  364 ,  370  and NMOS transistors  374 ,  380  form two cross-coupled latches which form sense amplifier  320 . Bit line BL is connected to the gates of transistors  364 ,  374 , and the drains of transistors  370 ,  380 . Bit line {overscore (BL)} is connected to the gates of transistors  370 ,  380 , and the drains of transistors  364 ,  374 . The sources of PMOS transistors  364 ,  370  are connected to a vertical metal-2 line SLP. The sources of NMOS transistors  374 ,  380  are connected to a vertical metal-2 line SLN. Lines SLP, SLN run through all the RW blocks of R/W circuitry  130 . 2 . During amplification, line SLP is connected to a positive voltage, and line SLN is connected to ground. During precharge, both lines SLP, SLN are connected to the same precharge voltage HVCC. The sense amplifier operation and timing are described in U.S. patent application “Charging a Sense Amplifier”, Ser. No. 08/760,121, filed Dec. 3, 1996 by L. Liu et al. and incorporated herein by reference.  
         [0036]    Precharge block  326  is similar to block  310 . In particular, block  326  includes NMOS equalization transistor  381  connected between the bit lines BL, {overscore (BL)} and NMOS precharge transistor  383  connected to bit line BL. The drain of transistor  383  is connected to a vertical metal-1 line receiving the voltage HVCC and running through all the RW blocks of circuitry  130 . 2 . The gates of transistors  381 ,  383  receive equalization signals EQ provided on a vertical metal-1 line running through all the RW blocks of circuitry  130 . 2 . The drain of transistor  383  overlaps with the deep N well (not shown) of memory block M- 2 . 2 .  
         [0037]    [0037]FIGS. 4A, 4B are a layout view showing masks used to manufacture the blocks  310 ,  314 . DRAM  110  includes four polysilicon layers and two metal layers over the polysilicon layers. Bit lines BL, {overscore (BL)} are formed from the fourth polysilicon layer (“poly 4”). The bit line boundaries are shown by dashed lines.  
         [0038]    Stippled areas are mask openings through which N+ or P+ implants are performed into substrate  230 . Stippled region  210 S is the N+ source region of transistor  210  of the rightmost memory cell of memory block M- 1 . 2 . See also FIG. 2. In capacitors  226  (FIG. 2), the capacitor plates connected to memory cell transistors  210  are formed from the second polysilicon layer (not shown). The capacitor plate connected to voltage VREF is formed from the third polysilicon layer (not shown). This plate is shared by a number of memory blocks in a memory array. This poly-3 plate is interrupted between some memory blocks M-i.j of the array to allow metal-1 word lines WL to contact poly-1 word lines WL (each word line WL includes a metal-1 line running over a poly-1 line).  
         [0039]    Stippled region  410  (FIG. 4A) includes N+ drain  236 D of precharge transistor  236 .  
         [0040]    Metal-1 line HVCC-M 1  contacts drain region  236 D in contact region  418 . (In region  418 , line HVCC-M 1  contacts a doped poly-4 region. The poly-4 region contacts the drain region.)  
         [0041]    Metal-1 line EQ-M 1  contacts a poly-1 line EQ-P 1  in region  422 . Poly-1 line EQ-P 1  provides the gates for transistors  236 ,  330  (FIG. 3A).  
         [0042]    Poly-1 line SS 1 -P 1  provides the gates of transistors  334 ,  340 . Line SS 1 -P 1  contacts metal line SS 1 -M 1  in region  426 . Bit lines BL, {overscore (BL)} contact the source/drain regions of transistors  334 ,  340  in contact regions  430 .  
         [0043]    Poly-1 line YS-P 1  provides the gates of pass transistors  354 ,  360 . Poly-1 line YS-P 1  contacts a metal-1 region in contact region  434 . The metal-1 region contacts a metal-2 region which provides Y-select signal YS. Stippled region  440  includes source and drain regions of transistor  360 . (Region  440  is a mask opening through which the dopant is implanted. This implant is also masked by poly-1 line YS-P 1 , causing the source and drain regions to be spaced from each other.) A source/drain region of transistor  360  contacts the metal-1 line DB in region  444 .  
         [0044]    Similarly, stippled region  450  includes the source and drain regions of transistor  354  of RW block RW- 2 . 2  and of transistor  354  of the next RW block RW- 2 . 4 . Poly-1 line YS 2 -P 1  provides the gates of the pass transistors of RW block RW- 2 . 4 . The common source/drain region of transistors  354  of the two RW blocks contacts the data line DB in contact region  454 .  
         [0045]    Bit lines BL, {overscore (BL)} contact the source/drain regions of transistors  360 ,  354  in contact regions  455 .  
         [0046]    In FIG. 4B, poly-1 line  364 -P 1  extending essentially directly below the bit line {overscore (BL)} provides the gate of transistor  364 . Poly-1 line  370 -P 1  extending essentially directly below the bit line BL provides the gate of transistor  370 . Stippled region  460  includes the P+ sources and drains of the two PMOS transistors. Line  370 -P 1  contacts bit line {overscore (BL)} in contact region  464 . Poly-1 line  364 -P 1  contacts bit line BL and the drain of transistor  370  in contact region  468 . Bit line {overscore (BL)} contacts the drain of transistor  364  in contact region  472 . Metal-2 line SLP (not shown in FIG. 4B) contacts the common source of transistors  364 ,  370  in contact region  476 .  
         [0047]    Poly-1 line  374 -P 1  provides the gate of transistor  374 . Poly-1 line  380 -P 1  provides the gate of transistor  380 . The two poly-1 lines extend between the bit lines essentially in parallel with the bit lines. Poly-1 line  380 -P 1  contacts bit line {overscore (BL)} in contact region  480 . Poly-1 line  374 -P 1  contacts bit line BL in contact region  482 .  
         [0048]    Stippled region  484  includes the sources and drains of transistors  374 ,  380 . Bit line BL contacts the drain of transistor  380  in contact regions  486 . Bit line {overscore (BL)} contacts the drain of transistor  374  in contact regions  490 . Metal-2 line SLN (not shown) contacts the common source of transistors  374 ,  380  in contact region  494 .  
         [0049]    Bit lines BL, {overscore (BL)} contact the source/drain regions of transistors  344 ,  350  (FIG. 3B) in contact regions  430  (FIG. 4B). Transistors  344 ,  350  are not shown in FIG. 4B.  
         [0050]    [0050]FIG. 5 shows the beginning stages of fabrication of DRAM  110 . Wafer  230  doped with boron has a doping concentration of 3×10 15  cm −3 . Initial silicon dioxide layer  510  is grown by thermal oxidation to a thickness of 300 to 1000 nm. Oxide  510  is patterned by standard photolithographic techniques to expose a region  520  into which dopants will be implanted for P well  216  and DNW  220 .  
         [0051]    A protective silicon dioxide layer  530  is grown by thermal oxidation to a thickness of 30 to 300 nm (100 nm in some embodiments). Phosphorous is implanted into region  520  at the energy 180 keV to create DNW  220 . The ion dose is 1 to 9 times 10 13  atoms/cm 2  (1.5×10 13  atoms/cm 2  in some embodiments.)  
         [0052]    Phosphorous is driven in by heating the wafer in nitrogen atmosphere at a temperature of 1150° C. for 510 to 1000 minutes (950 minutes in some embodiments). The deep N well diffuses laterally and downward as shown in FIG. 6.  
         [0053]    A blanket etch removes protective oxide  530  and a small portion of oxide  510 . Protective silicon dioxide  610  is grown thermally to a thickness of 30 to 300 nm (100 nm in some embodiments) by wet oxidation performed at 950° C. for 10 to 60 minutes. Boron is implanted at an energy of 30 to 180 keV (60 keV in some embodiments) to form P well  216 . The ion dose is 1×10 13  to 9×10 13  cm −2  (2×10 13  cm −2  in some embodiments).  
         [0054]    Then oxide layers  510 ,  610  are removed. A protective 100 nm layer of silicon dioxide (“third protective oxide”, not shown) is grown by wet oxidation performed at 950° C. for 10 to 60 minutes. Photoresist (not shown) is deposited and patterned to expose N well regions  620  (FIG. 2) in which the PMOS transistor  370  and other PMOS transistors will be formed. Phosphorous is implanted at an energy of 30 to 180 keV to form the N wells. The ion dose is 1×10 13  to 9×10 13  cm −2  (1.2×10 13  cm −2  in some embodiments). Then a well drive-in step is performed at a temperature of 1150° C. for 200 to 800 minutes (250 minutes in some embodiments). The resulting depth of DNWs  220  is about 5 μm. The depth of P wells  216  is 2 μm. The depth of N wells  620  is 3 μm. The distance dw between the right edge of P well  216  and the right edge of respective DNW  220  is 2 μm.  
         [0055]    The third protective oxide is removed. 470 nm thick field oxide regions  630  are grown between transistor active areas by LOCOS oxidation performed at 1000° C. for 90 minutes using methods known in the art.  
         [0056]    A 30 nm sacrificial layer of silicone dioxide (not shown) is grown by wet oxidation performed at 850° C. for 40 minutes. Blanket ion implantation of BF2 is performed through this sacrificial oxide at an energy of 70 keV to adjust transistor threshold voltages. The ion dose is 3.2×10 12  cm −2 .  
         [0057]    Then a deep P type implant (channel stop implant) is performed into NMOS transistor active regions to enhance the P type dopant concentration under the NMOS transistors and the field oxide regions. The resulting P− channel stop regions are shown at  640 . Regions  640  increase the punch-through voltages of NMOS transistors and parasitic transistors formed under field oxide  630 . Regions  640  also increase the parasitic transistor threshold voltages. Regions  640  are formed by implanting boron at the energy 120 keV. The ion dose is 8×10 12  atoms/cm 2 . The implant mask protects N wells  620  during this implant. The implant mask also protects regions CN of equalization transistor drains  236 D. Each region CN overlaps an area in which the respective drain  236 D meets the respective DNW  220 . Protecting the regions CN from the channel stop implant serves to improve the electrical contact between drains  236 D and DNWs  220 .  
         [0058]    In some embodiments, the channel stop implant mask is the reverse of the N well  620  mask except that the channel stop implant mask also covers regions CN.  
         [0059]    Each region CN is spaced from the respective gate  236 G. This spacing allows a portion  640 . 1  of channel stop region  640  to extend under the drain  236 D, thus increasing the punch-through voltage of transistor  236 . Each region CN is also spaced from the edge of the respective drain  236 D where the drain meets field oxide  630  (the left edge of drain  236 D in FIG. 2). This spacing allows a portion  640 . 2  of channel stop region  640  to extend from under the field oxide to a region under the drain  236 D. This helps to improve the punch-through voltage and the threshold voltage of the field transistor formed at the location of oxide  630 .  
         [0060]    In some embodiments, after formation of the implant mask used to form channel stop regions  640 , but before the boron implantation forming the channel stop regions, another boron implant is performed through the sacrificial oxide at an energy of 30 keV to adjust the threshold voltages of NMOS transistors. The ion dose of this implant is 2×10 12  cm −2 .  
         [0061]    After the channel stop implant, the sacrificial oxide is removed.  
         [0062]    Gate oxide (not shown) is grown to a thickness of 5 to 18 nm (8 nm in some embodiments) by oxidizing the structure at 700 to 1000° C. (850° C. in some embodiments) for 10 to 60 minutes. Polysilicon or polycide gates of transistors  210 ,  236 ,  370 , and other transistors, are formed by known techniques.  
         [0063]    Phosphorous is ion-implanted at 25 keV to form LDD regions of NMOS transistors. The ion dose is 2×10 13  cm −2 . Then pocket ion implantation of boron is performed at an angle of 25° and an energy of 60 keV into regions underlying NMOS sources and drains, to increase the NMOS punch-through voltages. The pocket implant ion dose is 1.2×10 13  cm −2 . These two implants—NMOS LDD and P-type pocket—are performed using the same photoresist mask (“NMOS LDD mask”, not shown) patterned by standard photolithographic techniques. The mask covers regions LN one of which is shown in FIG. 2. Each region LN covers the region CN and extends to the adjacent field oxide region  630  separating the respective drain  236 D from the respective P well  216 . By covering the regions LN, the mask blocks the pocket-implant boron from areas in which the drain regions  236 D meet the respective DNWs  220 . Thus, the mask helps to improve the contact between drain regions  236 D and DNWs. At the same, the mask exposes a portion of the drains  236 D adjacent the respective gates  236 G and also exposes the source regions  236 S.  
         [0064]    Pocket implants are described in U.S. Pat. No. 5,618,740 entitled “Method of Making CMOS Output Buffer with Enhanced ESD Resistance”, issued Apr. 8, 1997 to T. Huang and incorporated herein by reference.  
         [0065]    BF2 is implanted to form LDD regions of PMOS transistors such as transistor  370 . A pocket implant of phosphorous is performed into the PMOS transistor regions to increase their punch-through voltages. A 100 nm layer of silicone nitride (not shown) is deposited by LPCVD and etched to form spacers on transistor gate sidewalls. The silicon nitride deposition temperature is 780° C., and the deposition time is 40 mins. A 20 nm layer of silicone dioxide (not shown) is grown at 875° C. on exposed silicone surfaces. Arsenic and BF2 are implanted in successive ion implantation steps. The arsenic implant forms heavily doped portions of NMOS source/drain regions. The BF2 implant forms heavily doped portions of PMOS source/drain regions and also forms one or more P+ contact regions (not shown) in each P well  216 . The P+ contact regions will contact a voltage source that will bias the P-wells. In some embodiments, the mask used in the BF2 implant is the reverse tone of the NMOS LDD mask, except that both the NMOS LDD mask and the BF2 mask block the regions LN.  
         [0066]    Remaining fabrication steps are known in the art.  
         [0067]    The dopant concentration in DNWs  220  is 1×10 16  atoms/cm 3 . The dopant concentration in drains  236 D is 1×10 20 atoms/cm 3 . The higher dopant concentration in the drain regions improves the electrical contact between DNWs  220  and doped poly-4 regions (not shown) contacting the drains  236 D and also contacting the metal-1 lines HVCC-M 1 .  
         [0068]    [0068]FIG. 7 illustrates some drawn mask dimensions in the cross section of FIG. 2. The lateral distance d1 between the right edge of the rightmost source region  210 S in a P well  216  and the right edge of the respective DNW  220 /P well  216  mask opening (corresponding to the left edge of oxide  510  in FIG. 5) is only 0.45 μm. This spacing is small due to elimination of a separate N+ contact region in the DNW and of a separate N well containing the N+ contact region. The distance d2 between the right edge of the DNW mask opening and the left edge of the mask opening for drain region  236 D is 1.5 μm. The drawn distance d3 between the left edge of the region  236 D and the left edge of the region CN is 0.4 μm. The width d4 of each region CN is 3.1 μm. The distance d5 between the right edge of the region CN and the left edge of gate  236 G is 1.0 μm.  
         [0069]    The distance d6 between the left edges of region LN and drain  236 D is 1.0 μm. The rights edges of regions LN, CN coincide.  
         [0070]    In this embodiment, DNW  220  diffuses laterally after implantation by 75-80% of the DNW depth. Thus, if the DNW depth is 5 μm, the DNW diffuses laterally by 3.75 μm to 4 μm. The minimal photolithographic line width is 0.5 μm, and the maximum alignment error is 0.6 μm. The minimal spacing between the DNW mask and the gate of the nearest transistor is only d2+d3+d4+d5=6 μm.  
         [0071]    The above embodiments illustrate but do not limit the invention. In particular, the invention is not limited by any particular dimensions, fabrication techniques, temperatures, energies, or other process parameters, or by layer compositions or layout. The invention is not limited to DRAMs or any other particular circuitry. Other embodiments and variations are within the scope of the invention as defined by the following claims.