Abstract:
A high voltage radiation hardened power integrated circuit (PIC) with resistance to TID and SEE radiation effects for application in high radiation environments, such as outer space. TID hardness modification include forming gate oxide layers after high temperature junction processes, adding implant layers to raise the parasitic MOSFET thresholds with respect to native thresholds, and suppressing CMOS drain-to-source and intrawell transistor-to-transistor leakage. In addition, radhard field oxide is utilized. SEE ruggedness is improved by reducing the epi thickness over that of non-radhard devices, and increasing the epi concentration near the substrate junction. A radhard PIC rated to 400 V and capable of operating at 600 V or more is provided. The inventive PIC can withstand 100 krads of TID and a heavy ion Linear Energy Transfer of 37 MeV/(mg/cm 2 ) at full rated voltage.

Description:
This application claims the benefit of U.S. Provisional application Ser. No. 60/179,843, filed Feb. 2, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to radiation hardened (“radhard”) power integrated circuits which have improved resistance to damage by large (megarad) doses of ionizing radiation, or by single or plural event high energy charged particles. 
     2. Brief Description of the Related Art 
     Power integrated circuits (PICs) are well known in the prior art. PICs typically include circuitry for controlling MOS-gated power transistors. In some PICs, the MOS-gated power transistors are integrated into the same silicon that carries the control circuitry. A typical PIC of this type is the IR2112 PIC made by the International Rectifier Corporation. 
     The PICs like most electronic devices, do not function well in a high radiation environment, such as in outer space. The effects of ionizing radiation can accumulate over time, resulting in device degradation. Also, heavy ion strikes can lead to catastrophic failure. When power devices are employed in such environments, the devices are typically more susceptible to these problems because of their large depletion volumes and large device areas. Power MOS-gated devices specifically designed for use in radiation-rich environments, commonly termed radhard devices, are well known. Such devices are described in U.S. Pat. Nos. 5,338,693; 5,831,318;and 6,165,821 which are assigned to the assignee of the present application. The design rules for such radhard devices are quite different from those of conventional MOS-gated devices, such as power MOSFETs, IGBTs MOSgated thyristors and the like. These different design rules arise to ensure the continued operation of the MOS-gated device in an ionizing radiation environment. The design rules generally call for the use of the thinnest possible gate oxide (for example, about 700 to 900 Å for a 400 volt reverse breakdown device) to minimize threshold voltage shift in the presence of a high radiation background. A “late gate” process sequence is also employed which reduces the exposure of the gate oxide to high temperature process steps. It would be desirable to provide a radhard PIC that utilizes the above-noted design features and can function in a high radiation environment. 
     SUMMARY OF THE INVENTION 
     The present invention overcomes the deficiencies in the prior art, such as those described above, by providing a “radhard” PIC (that is, a PIC that can operate in a high radiation environment). A discussion of radiation resistance in electronic devices as it relates to total dose and single event considerations is provided in U.S. Pat. No. 6,165,821 issued Dec. 26, 2000, the entire disclosure of which is incorporated herein by reference. 
     The PIC of the present invention includes a latch-up immune 20V CMOS and a 600V LDMOS. The novel device provides high voltage operation that can control loads of, for example, about 400 volts to at least 600 volts. Using the novel production process in which gate oxides are formed late in the process, the gate oxides are not exposed to high temperature processing steps, making the devices less susceptible to damage in high ionizing radiation environments. A high voltage, junction isolation (HVJI) process is used. The HVJI process is disclosed in U.S. Pat. No. 5,023,678 issued Jun. 11, 1991, the entire disclosure of which is incorporated herein by reference. 
     A preferred process for manufacturing a radiation hardened power integrated circuit according to the present invention includes steps for enhancing TID and SEE capabilities. 
     Enhanced TID hardness is provided by forming gate oxides late in a high voltage, junction isolation process so as to avoid exposing the gate oxides to high temperature processing steps. In addition, two implant layers are included in the CMOS to raise the parasitic MOSFET thresholds with respect to native thresholds. Preferably, a radhard field oxide is provided as well. The layout of the device accounts for these processes and suppresses CMOS drain to source and intra-well transistor-to-transistor leakage, in particular with respect to polygate to field oxide overlap and individual ringing of CMOS devices with channel adjust layers. 
     SEE ruggedness is provided by a reduced epi thickness and increased epitaxial layer (epi) concentration near the substrate junction. Accordingly, the device has reduced charge collection volume and reduced parasitic bipolar gain, which in turn provides a reduced chance of a single event latch-up condition after a heavy ion strike. The increase epi concentration near the substrate junction also has the added benefit of truncating charge funneling effects in the silicon. 
     The PIC thus produced demonstrates total ionizing dose (TID) capabilities of at least 100 krads, and single event effects (SEE) capabilities in terms of linear energy transfer coefficient (LET) of at least 37 MeV/(mg/cm 2 ) at full rated voltage. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-section through the final HVJI PIC chip of the present invention, showing a high voltage lateral DMOS section and a CMOS control section integrated into the same chip. 
     FIG. 2 is a cross-section showing a starting wafer at the beginning step of the process according to the present invention. 
     FIG. 3 illustrates the isolation block steps performed on the chip of FIG.  2 . 
     FIG. 4 illustrates the P well and resurf block formation steps performed on the chip of FIG.  3 . 
     FIG. 5 illustrates the chip of FIG. 4 undergoing the steps of deep P + /body block formation according to the present invention. 
     FIGS. 6 and 7 show the steps of DMOS NPLUS, PCH and NCH field adjust block formation having been performed on the chip of FIG.  5 . 
     FIG. 8 illustrates formation of the Poly 1 block on the chip of FIG.  7 . 
     FIG. 9 illustrates formation of the active area and the gate polysilicon block on the chip of FIG.  8 . 
     FIG. 10 shows the chip of FIG. 9 after the next block of steps used to define the NMOS N +  regions, the N-L DD  region, and the SHP + . 
     FIG. 11 shows the block of FIG. 10 after formation of the interlayer oxide and contact openings. 
     FIG. 12 shows formation of metal features on the block of FIG.  11 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to FIG. 1, a preferred embodiment of the device made by the novel process is shown in cross-section. The illustrated device is a 600 V capable version of the industry standard IR2110 power MOSFET gate driver, which is a high voltage driver with independent high and low side output channels. 
     In FIG. 1, only a small fraction of the silicon is shown, but it will be seen that a common P −  substrate  20  has an epitaxially formed layer  21  on top. Layer  21  is divided into a CMOS control segment  23  and a lateral power DMOS segment  24  by P +  isolation sinkers  25 ,  26 ,  27 . As will be later described, epitaxial layer  21  is divided into top and bottom segments. 
     The lateral DMOS structure  24  includes a P −  resurf region  30 , spaced P body regions  31  and  32 , and a deep P +  body  33 . An N +  DMOS source region  34  formed in P body  32  is spaced inwardly from the edge to create an N channel region  38 . A gate oxide  35 , a polysilicon gate  36  and interlayer oxide  37  overlies N channel region  38  and the space between P body regions  31  and  32  defining P channel region  39 . 
     The top surface of the silicon chip is covered by field oxide segments  40 - 45 . Aluminum contacts  50 - 57  are connected to the silicon as shown, and spaced polysilicon strips  60 ,  61  and  62  (parts of the poly 1 layer) are deposited on field oxide layer  43  to form field plate capacitors. Strips  60 ,  61  and  62  are covered by interlayer oxide and a second poly layer, poly 2 covers the insulated poly 1 strips  60 ,  61  and  62  to form a high voltage termination. An N +  diffusion  63  provides the DMOS drain contact  54 . 
     The CMOS structure  23  includes a P well  70  which contains N +  diffusions  71 ,  72 , with an overlying polysilicon gate  73 , forming an N channel MOSFET. P +  diffusions  74  and  75 , with overlying polysilicon gate  76 , form a P channel MOSFET. 
     The output of the CMOS section  23  is internally connected to the lateral DMOS device  24  to control its operation in any desired manner. Obviously, any number of CMOS segments  23  may be provided and interconnected to form any desired control, and segment  23  is only illustrative of one typical CMOS element. 
     The novel manufacturing process is next described by way of a specific example. Variations and other process flows that can be used will be obvious to those of skill in the art based on their understanding of the present invention. Also, those of skill in the art will know that although only a few single devices are shown and discussed below as illustrative, a large number of devices will be formed on a common wafer that subsequently is diced to provide individual devices that are housed appropriately. Process parameters are approximate, and may be varied within a range of the value given based on process and material conditions without departing from the scope of the invention, as will be known to those of skill in the art. In the example below, routine steps such as pre-diffusion cleaning, baking, and washing, for example, generally have been left out of the description for the sake of brevity. 
     FIG. 2 shows a representative portion of the starting wafer consisting of the P substrate  20  which may be 40 ohm-cm., for example. The N epitaxial layer  21  consists of a bottom As-doped epi layer  21   a  which is 2 μm thick and 0.72 ohm cm., covered by a P-doped epi layer  21   b  which is about 8 μm thick, 3 ohm cm. Standard oxide growing processes may be utilized. The thickness of epi layer  21  is less than that of the typical non-radhard PIC of this type, such as the IR2112 PIC made by the International Rectifier Corporation, and the higher epi concentration near the substrate, provide a reduced charge collection volume and reduced parasitic bipolar gain. 
     Referring to FIG. 3, a block of steps is undertaken for creating P + -type isolation regions  25 ,  26  and  27 , as illustrated for region  26  in particular, using a two-part diffusion process. Initially, isolation oxide  42  is grown to about 10,000 Å using any standard oxide growing process, for example. Then, an isolation mask is utilized to oxide etch openings above the areas where the P +  isolation regions are to be formed. A typical 6:1 BOE solution is utilized (Buffered oxide etch: 6:1 BOE=6 parts 40% NH 4 F and 1 part 49% HF ). 
     Once the opening is provided, isolation capping oxidation takes place at about 900 ° C. for 20 minutes. The oxidation is performed wet to produce an approximately 600 Å capping layer, after which isolation regions, such as region  26  shown in FIG. 3, are pre-formed using boron ion implantation. The ion implantation step is carried out at an energy of approximately 50 KeV and a dose about 1.0E16 cm −2 , for example. The isolation region is diffusion driven for 120 minutes using N 2  at 1200° C. for 12 minutes, and wet at 1050° C. (2,500Å). The chip then is etched to clear all oxides (approx. 13 minutes). 
     FIG. 4 schematically shows the formation of P well  70  and P resurf  30 . Preferably, P well base oxidation takes place at 900° C. for 30 minutes, wet, to form an 800 Å layer, and a P well mask is applied. Implantation proceeds using boron ions at an energy level of about 80 keV and a dose of about 1.1E13 cm −2 . After the photoresist is stripped, a resurf mask is applied, and implantation of resurf region  30  takes place using boron ion implantation at an energy level of about 80 keV and a dose of 3.33E12 cm −2 . After another photoresist strip, the resurf and P well regions are diffusion driven at 1,200° C. for 280 minutes with N 2 , and 40 minutes wet, preferably simultaneously growing an approximately 6,500 Å layer of oxide. 
     The development of lateral DMOS  24  continues with the formation of P body  32  and deep P +  region  33  as illustrated in FIG. 5. A master mask is applied for the deep P + /P body, and a plasma descum process is performed to open both areas to oxide etching. The photoresist is stripped, etching 6500 Å oxide down to the silicon surface. Screening oxide is grown at 1050° C. for about 18 minutes, dry, to grow about a 300 Å layer. P body formation begins with boron ion implantation at an energy level of about 80 KeV and a dose of 7.8E13 cm 2 . Then a P body protecting mask is applied, and deep P +  boron ion implantation takes place, again at an energy of about 80 KeV, and with a dose of 2.5E14 cm −2 . After resist ash treatment and hot pot stripping, P body  32  and deep P +  region  33  are driven at 1175° C. for 90 minutes using N 2 . 
     The next block of steps in formation of the lateral DMOS is illustrated in FIGS. 6 and 7, in which source  34 , and the P channel and N channel field adjust layers of the CMOS, are provided. The CMOS field adjust layers raise the parasitic MOSFET thresholds with respect to the native thresholds. 
     The process proceeds by applying a DMOS N +  mask to open the N +  region in the DMOS area only. Then, DMOS N +  region  34  is formed using arsenic ion implantation at an energy of about 120 KeV and a dose of 5.0E15 cm 2 . The newly-formed DMOS N +  region is annealed at 975° C. for 120 minutes (N 2 ). Then, buffered oxide etching clears all oxides (clears 6,500 Å oxide). A screening oxide layer preferably is formed, for example, as pre-grown field oxide produced at 900° C. for 30 minutes, under wet conditions to a thickness of about 800 Å. 
     Referring to FIG. 7, initial formation of CMOS  23  proceeds by applying a p-channel field adjust mask, and p-channel field adjust phosphorus implantation takes place at an energy level of about 120 KeV, and a dose of about 4E13 cm −2 , to form parasitic transistor channels  74 ,  75 . Following hot pot stripping, the n-channels  71 ,  72  are formed using an adjust mask and boron ion implantation at an energy level of about 50 KeV and a dose of about 8E13 cm 2 . Stripping and pre-diffusion cleaning (no HF) precede field oxide (F ox ) deposition using undoped TEOS (Tetraethoxysilan, Tetraethylorthosilicate) to deposit approximately a 7,200 Å layer. This results in a field oxide having a total thickness of 8000 Å. 
     FIG. 8 illustrates the block of steps used to form the resistor strip  61 , for example, of the poly 1 layer of FIG. 1 in lateral DMOS  24 . Directly following the previous deposition of F ox  layer  43 , the process proceeds with formation of the poly 1 layer using LPCVD (low pressure chemical vapor deposition) to deposit an undoped layer about 7,500 Å in thickness. Resistor strip  61  is implanted using boron ion implantation at an energy level of about 80 KeV and a dose of about 3.6E14 cm 2 . Following pre-diffusion cleaning, poly 1 oxidation takes place at 900° C., for 15 minutes, wet, to form a layer of about 450 Å in thickness. Subsequently, a mask is applied and oxide etching (6:1 BOE, photoresist strip, 75 seconds) is utilized to form a protective poly oxide layer over resistor strip  61 . This is followed by a wet sink etch of the exposed poly 1 layer and oxide degloss in 100:1 HF for 600 seconds, which removes the oxide ledge. The Poly 1 layer is driven and interpoly oxide is grown at about 975° C. for 60 minutes using N 2 , and for 80 min. wet to grow about 6,000 to 6,500 Å on the poly 1 layer. 
     The active area and gate polysilicon  36  of lateral DMOS  24  are produced using process steps illustrated in FIG.  9 . Initially, the active area is opened using a mask, and oxide etching is performed (6:1 BOE; 11 min., 45 secs). Following the photoresist strip and pre-diffusion cleaning (etching F ox  and interpoly oxide to Si), sacrificial gate oxide is grown at 900° C., and pushed in 750° C., 15 min., O 2  (approx. 100 Å). A sacrificial gate oxide blanket etch is performed, and gate oxide is grown at 900° C., and pushed in at 750° C., 28 min. wet (800 Å). In-situ doped gate poly 2 is provided by LPCVD deposition to a thickness of about 5,000 Å. The poly 2 implant is generated next using phosphorus at an energy level of about 80 KeV and a dosage of about 1E16 cm −2 . A gate poly 2 mask is applied, and poly 2 is dry etched. A photoresist strip completes this processing block of steps. 
     FIG. 10 shows the next block of steps used to define the NMOS N +  regions  71 ,  72 ; the N-L DD  region  80 , and the SHP +  of CMOS  23 . These steps begin with application of a mask and implantation of the NMOS N +  implant: Phosphorous ion (P + ), at an energy level of about 80 KeV and a dose of about 5.0E15 cm −2 . After ash and wet photoresist strip, blanket N-LDD implantation is performed at an energy level of about 120 KeV, with a dose of about 5.0E12 cm −2 . Then, the SHP +  mask is applied, and SHP +  implantation takes place using boron ion (B + ) implantation at an energy level of about 50 KeV and a dose of about 5.0E14 cm −2 . Photoresist strip completes this process block. 
     FIG. 11 illustrates the next block of steps used to form the interlayer oxide and contact openings. Initially, the wafer is pre-diffusion cleaned using no HF. Then, undoped TEOS is deposited to a thickness of about 15,000 Å, and the poly 2 layer is driven in at about 900° C. for about 60 minutes under nitrogen. A contact mask is applied, and the oxide is wet etched (6:1 BOE, 2 mins., 30 secs) to approximately 8,000 Å and dry etched to silicon. 
     The block for formation of metal contacts is illustrated in FIG.  12 . Following a pre-metal cleaning, aluminum sputtering is used to coat the top surface of the PIC with a layer about 20,000 Å thick of aluminum with 1% silicon. A metal mask is applied, and a wet aluminum sink etch at about 50° C. is used to provide metal contacts  50 - 57 . A photoresist strip and wet defreckle complete this block. 
     To complete the PIC device, a passivation block of steps is carried out, consisting of the following steps (not shown): First, a post aluminum clean takes place, followed by LPCVD oxide deposited at about 405° C., 2-4% PHOS doped to a thickness of about 15,000 Å. A pad mask is applied, which is followed by a sink pad etch. Photoresist strip completes the device. 
     Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.