Lateral ion implant straggle and mask proximity …

1 1946 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 50, NO. 9, SEPTEMBER 2003 Lateral Ion implant straggle andMask proximity EffectTerence B. Hook, J. Brown, Peter Cottrell, Fellow, IEEE, Eric Adler, Dennis Hoyniak, Jim Johnson, and Randy MannAbstract Lateral scattering of retrograde well implants isshown to have an effect on the threshold voltage of nearby threshold voltage of both NMOSFETs and PMOSFET sincreases in magnitude for conventional retrograde wells, but fortriple-well isolated NMOSFETs the threshold voltage decreasesfor narrow devices near the edge of the well.

2 Electrical data,SIMS, and SUPREM4 simulations are shown that elucidate Terms CMOS, high energy implanters, Lateral ion im-plant, Lateral scattering, NMOSFETs, nwell, parasitic bipolar gain,PMOSFETs, pwell, retrograde well implants , SIMS, INTRODUCTIONRETROGRADE well profiles have several key advantagesfor highly scaled bulk complementary metal oxide semi-conductor (CMOS) technology [1]. With the advent of high-en-ergy implanters and reduced thermal cycle processing, it has be-come possible to provide a relatively heavily doped deep nwelland pwell without affecting the critical device-related dopingat the surface.

3 The deep well implants provide a low resistancepath and suppress parasitic bipolar gain for latchup protection,and can also improve soft error rate and noise isolation. A deepburied layer is also key to forming triple-well structures for iso-lated-well NMOSFETs. However, deep buried layers can affectdevices located near the mask edge. Some of the ions scatteredout of the edge of the photoresist are implanted in the siliconsurface near the mask edge, altering the threshold voltage ofthose devices. In this paper, we show data of a deep boron ret-rograde pwell, a deep phosphorus retrograde nwell, and alsoa triple-well implementation with a deep phosphorus isolationlayer below the pwell.

4 Threshold voltage shifts of up to 100 mVcan be observed over a Lateral distance on the order of a mi-crometer. TSUPREM4 [2] was also used to illuminate some ofthe details of the phenomenon is shown schematically in Fig. 1. Ions scat-tered laterally just inside the photoresist edge will be able toemerge from the resist [3]. These may be implanted into thesilicon within the area that will become a transistor active-re-gion later in the process. The depth and concentration of theimplanted ions will depend on the angle and energy of the scat-tered ions. The details of the Lateral scattering depend on themass of the incoming ions and the mass of the species in theManuscript received January 27, 2003; revised May 13, 2003.

5 The review ofthis paper was arranged by Editor authors are with the Semiconductor Research and Development Center,IBM Microelectronics, Essex Junction, VT 05452 USA (e-mail: Object Identifier 1. Schematic representation of mask edge proximity from which they are scattered. Whether or not thereis a significant effect on the threshold voltage depends on theoverall width of the device, the location of the device relativeto the mask edge, the Lateral range of the effect , and the densityand depth of the scattered ions relative to those intentionallyimplanted in that region. The simulations and data described inthe following sections show that a quantitatively significant ef-fect can be observed for dimensions and implants which may befound in typical SIMULATIONRESULTSThe Monte Carlo implant models available in TSUPREM4are capable of modeling this effect , and some results of thismodeling are shown in Figs.)

6 2 4. Fig. 2 pictures a process witha deep boron implant . The pwell mask is shownon the left; the region to the right is where NMOSFETs are to bebuilt later in the process. A deep boron retrograde implant im-planted normal to the surface at 600 KeV has been performed(see region A in Fig. 2). For this simulation, default implantconditions were used with 500 000 implant trajectories. Min-imal heat was applied just to activate the dopant; this was notintended to be a simulation of the full technology dopants in region B are scattered out of the photoresist andare implanted into the silicon.

7 Had there been no mask edge,the entire profile would have been as in Region A. It is apparentthat this is a long-range effect : for a distance on the order of onemicrometer from the mask edge there is excess boron dopingnear the surface. That dopant near the surface (within the deple-tion region of the FET) will affect the threshold voltage. As the0018-9383/03$ 2003 IEEEHOOKet al.: Lateral ION implant straggle AND MASK proximity EFFECT1947 Fig. 2. Contour plot of simulated doping near resist mask edge. Only boronimplantation was performed in this simulation. In Region A only the normaldeep well profile appears.

8 Excess surface dopant near the resist edge is evidentin Region 3. Contour plot of simulated doping near resist mask edge. Both boron(intermediate and near-surface) and phosphorus (deep) implantations weresimulated. Region A is the deep phosphorus implant region. Region B is theisolated pwell area. In Region C there is a small pocket of scattered spacing of NMOSFETs from the mask edge is or less, the extra implant can cause a substantial increasein the threshold voltage of those devices, particularly when theyare one micrometer wide or 3 illustrates the effect when a deep phosphorus implant isused to form a pwell region isolated from the substrate.

9 In thiscase, Region A is the buried n-type layer and Region B is thepwell. (In the actual process sequence an additional n-type im-plant would be performed subsequently to complete the lateralisolation of the pwell region.) Region D is an area where thepwell implant has been scattered out of the resist edge; this in-creases the threshold voltage in this area. Region C is where thedeep phosphorus has been scattered out of the resist, and hasinverted the net doping at the surface; the net result is n-typedoping in this area. If this were to intrude in the FET channelregion, that portion of the channel would have a lower thresholdvoltage.

10 Fig. 4 shows the Lateral concentration profile of bothboron and phosphorus near the silicon surface. At andbeyond, the net doping is p-type; but, closer than to themask edge, the doping is n-type. The Lateral doping profile ofthe boron is as would be observed in the process of Fig. 2; inthis case the phophorus profile is superimposed on the scatteredFig. Lateral doping profiles of B and P immediately below thesilicon surface for the implants whose two-dimensional (2-D) contour plot isshown in Fig. dopant profiles of scattered ions using a novel stopping filmto eliminate the directly implanted ions from the silicon , and even inverts the sense of the doping at the surfacenear the mask process by which the implanted ions are stopped in theresist has two phases.