design considerations of a vlsi compatible production mev ion implantation system

Nuclear Instruments and Methods in Physics Research B21 (1987) 285-295 285 North-Holland, Amsterdam

D E S I G N C O N S I D E R A T I O N S O F A VLSI C O M P A T I B L E P R O D U C T I O N MeV I O N I M P L A N T A T I O N S Y S T E M

N o r m a n T U R N E R , Kenneth H. P U R S E R and Manny S I E R A D Z K I

lonex / HEI Corporation, 4 Mulliken Way Newbur3,port Massachusetts 01950, USA

An MeV Ion Implantation System has been developed to address the requirements for VLSI wafer fabrication. These requirements are discussed with respect to considerations for machine design. The system is reviewed and its performance evaluated.

1. Introduction

In recent years there has been an increasing number of requirements for production ion implantation systems that operate from the hundreds of kilovolt to the several megavolt energy range. Traditional production systems, those labeled "Medium Current" and "High Current," have maximum accelerating potentials of 200 kV. There are some systems offered by manufacturers of the 300-500 kV range and higher, but to date these have been research systems that have been modified. These upgraded systems have failed to offer the features and resultant productivity that are required in today's front-end wafer fabrication facilities.

Fig. 1 shows potentially many applications for such systems that cover a very broad range of energies and dose; [1,2] however, the principal applications driving the incorporation of high energy implantation into the production line in order of present iinportance are: - Formation of retrograde and conventional well struc-

tures in submicron CMOS technology [3,4]. - Formation of buried grids and other structures in

both MOS and Bipolar [2]. - Customization of memory and logic devices late in

the manufacturing process [2]. A review of these applications will define practical limits of the key design parameters and the architecture of a production VLSI compatible ion implanter.

2. Machine requirements

2.1. Energy

Typical ion energies required for the above applications are: Retrograde well 300-1500 keV Buried grids, layers,

interconnect 1500-5000 keV Device customization 750-2500 keV

0168-583X/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

1016

1015 oJ

~ 1014

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1012

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I OEEP AMORPHOUS [ METAL LAYERS INTERFACE MIXING

I OXYGEN PRECIPITATE LAYERS ~ ' ~

BURIED INTERCONNECTS

BURIED GRIDS/PLANES

CONVENTIONAL & RETROGRADE WELLS

! I I 3 4 5

ENERGY ( MeV )

Fig. 1. Applications for high energy ion implantations showing typical dose and energy ranges.

The single fastest growing application for high energy implantation is the retrograde well. Presently, the fabrication of a conventional well structure requires a predeposition and drive. This drive can represent as much as 50% of the thermal budget of the fabrication process [1]. Phosphorus has been employed as the N- type dopant in a conventional well in a effort to reduce drive time due to its diffusion properties. The use of phosphorous in the implanted well has resulted from the relative availability of in-house 400-500 kV systems that accelerate double-charged ions to the desired energy [3,5]. In the future, arsenic may offer some advantages over phosphorous for this application, suggest- ing that ion energies in the 3 MeV range may be required. The low energy operating range of the machine should overlap existing production equipment for maximum versatility.

IV. NEW EQUIPMENT AND SYSTEMS

a.)

n n IGIG N

b.)

F] F]F]

286 N. Turner et a L I A n Me V ion implantation system

Fig. 2. (a) Implant shadowing due to high aspect mask and 0 ° implant angle. (b) Variation of implant zone shape at wafer edges vs wafer center due to varying implant angle across wafer.

2.2. Dose and beam current

The dose requirements for the applications discussed above generally fall into two distinct regimes and are shown in fig. 1:

1 × 1012-2 X 1013 ions/cm 2

(implanted wells, buried grids, late customization) and

1 × 1015-5 × 1015 ions/cm 2

(some buried structures and interconnects).

These two dose regimes define beam current requirements that are nearly independent of end station type (batch or serial), scan approach, and wafer handling techniques. The maximum useable beam current Imax(gA) for a specific dose is simply;

I ~ = D A K / T ~ i n,

where D = dose (ions/cm 2), A = implanted area (cm'-), K-- ionic charge constant (1.6023 × 10- t3 gC/ion), Tmi n = minimum required time for uniform implantation.

At a dose of 2 × 1013 ions/cm 2 a total current of 100-200 g A is typically necessary for an implanter to operate at maximum throughput. However, at the higher doses of 1 × 1015-5 x 1015 ions/cm 2 beam currents of 10-20 mA are required. Currents of 200 gA and 10 mA on target result in power densities of 0.05 and 2.5 cm 2 respectively at energies of 1 MeV for implant areas of approximately 4000 cm 2. These power densities require different cooling technologies [6] which have significant implications for overall machine cost. Most practical applications of high energy implantation today are ad- dressed by total currents of -- 200 gA.

2.3. Scanning

Requirements for beam scanning are similar to other -types of production ion implantation equipment suit- able for VLSI device fabrication. Today's trend toward larger wafer sizes of 150-200 mm diameter wafers are setting new demands on scanning requirements. Fig. 2a. shows a high aspect ratio mask and the normal shadowing which occurs whenever implant angle is 0 ° (normal to wafer). Fig. 2b shows a varying implant angle across the wafer causing preferential die locations (center of wafer vs edge of wafer). Such problems are not associated with traditional aspects of uniformity, but may effect die-to-die operating and reliability characteristics. Furthermore, control of the azimuthal or wafer fiat angle is important in order to minimize planar channel- ing phenomena [7].

2.4. End station and wafer handling

Superior contamination control is required for the manufacture of VLSI devices with submicron feature sizes [8]. A VLSI wafer handler should handle wafers gently with a minimum number of transfers, while controlling completely all possible wafer motion. Clear- ly, edge or surface clamping, sliding and bumping should be avoided, as it can be a major source of particle generation. It is important to control the environment of the staging or loading area, as well as the implant target chamber. A major contributor of particulate contamination has been the speed and frequency of the vent and rough cycles in the wafer handling process [9,10]. Critical areas are the load locks and the implantation chamber. One way of solving this major problem is eliminating the venting of the implant chamber in routine cycling. The wafer handling system should be designed so that a slow rough and vent cycle can be used without reducing throughput.

N. Turner et al. / A n Me V ion implantation system 287

2.5. Equipment automation

The following are requirements for the automation level of the control system: - Simplicity of operation. - Operator prompting with an interactive setup screen. - Automatic reproducibility of setup. - On board "expert system" diagnostic capability. - Remote diagnostic capability. - Host computer capability.

3. Most common high energy accelerator techniques

At least three quite different acceleration techniques have been used to impart MeV energies to charged particles. The principles of each are shown in fig. 3.

a) Tandem acceleration: A two-stage dc acceleration process utilizing negative ions, injected at ground potential, and accelerated to a high voltage terminal maintained at a positive potential as high as several mega-electronvolts. At the terminal, the negative ions are converted into positive ions and are again accelerated to ground potential. This technique of using the same acceleration potential twice for accelerating each ion leads to the name "tandem accelerator".

b) Single ended dc acceleration: A single stage dc acceleration system where the ion source is located within a high-voltage terminal of a pressurized electro- static accelerator.

c) Rf acceleration: A multiple-gap rf acceleration system operating in the frequency range 5-10 MHz using a linac geometry. To make possible changes in the ion energy and its mass it is necessary to arrange that the individual accelerating electrodes have both ampli- tude and phase of the ac accelerating voltage indepen- dently adjusted for each separate acceleration electrode.

When comparison is made between all aspects of these three different acceleration techniques, the tandem accelerator has several advantages:

~(-)~

a.

b.

c.

~ , (+) II-

Fig. 3. Simplified schematic of common high energy accelerator techniques: (a) Tandem accelerator. (b) Single ended dc

accelerator. (c) rf accelerator.

beam current limitations of conventional tandems have disappeared due to a major effort by Ionex to develop a new method of negative ion generation. This new negative ion generation process allows the tandem implanter to produce significant beam currents which are compatible with production requirements for device fabrication. The basic physics which underlies this new technique is described in a companion paper by O'Connor and Joyce [11].

4. System description

Advantages of tandem compared to rf acceleration

- Lower power consumption

- Low cost - Wide energy range - Simplicity of control

Advantages of tandem compared to single ended acceleration

- Ion source accessibility

- Low stored energy

Until recently, the single greatest drawback of tandem implanters has been low beam currents when compared to that available from conventional implanters. During the past few months, however, the

The overall layout of the lonex Model IX-1500 Im- planter can be seen from fig. 4. The system can be divided into four major sections:

A n injector section: where positive ions are extracted from the source and charge exchanged to negative, and magnetically mass-analyzed with M / A M > 100. After analysis, the ions are accelerated to 200 keV, the minimum energy needed for efficient transmission through the tandem to the end station.

A tandem acceleration section: where the injected negative ions are accelerated towards a positive polarity high voltage terminal maintained at a potential between 0-750 kV. In the terminal, the ions pass through a windowless gas cell, where collisions with the gas mole-


288

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N. Turner et a L / An MeV ion implantation system

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Fig. 5. Overall optics section view of Ionex Model IX-1500 high energy implanter.

cules causes electrons to be knocked from the negative ions changing their charge from negative to positive. Because of their positive charge the ions are now re- pelled from the positive terminal and are accelerated a second time back to ground.

A beam filter section: where a deflection magnet filters the accelerated particles and allows only those of the wanted mass and charge state to enter the end station. Because a deflection is involved, this analyzer also minimizes any neutral particles from the beam.

The wafer handling end station: where any wafer diameter between 76 and 200 mm can be loaded in vacuum onto an implant disc.

4.1. The injector section

The cross-section of the injector section of the implanter can be seen in fig. 5. The ion source is an axial hot-cathode PIG source which uses either solid sources or gas for phosphorous and arsenic beam generation


/ / 1 / + / + 1 2

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CORONA SHIELOS

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f 1 I t I

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Fig. 6. Schematic diagram of high voltage multiplier for MeV Tandetron.

and gaseous BF 3 for boron. The source runs hot with its temperature being determined by equilibrium between radiation losses and a power input of 400-600 W. To ensure stability, the arc current is regulated by a closed loop system which operates on the filament temperature.

The computer codes used to determine the ion beam properties of the ion source are described in a companion paper by White [12]. The extraction system is designed to produce a stable beam of minimum emittance even in the presence of significant amounts of erosion. After being accelerated to an energy of 35 keV the ions pass through a charge exchange region and are converted from positive polarity to negative. The yields of negative ions to date for 35 keV, lIB+, 31p+, and 75As+ are 10, 21, and 15% respectively. In conjunction with the high current positive ion source several hundred microamperes of B- and more than 1 mA of P- and As- are available for acceleration by the tandem. The increase in angular divergence of the ion beam during charge exchange has been measured and found to be negligible, [11].

After formation, the negative ions are mass analyzed by passage through a 30 cm radius uniform-field magnetic dipole. After analysis the negative ions are accelerated to an energy as high as 200 keV before injection into the tandem accelerator section. This high injection

energy ensures efficient tandem transmission over a broad energy range.

4.2. The t andem accelerator section

The accelerator section of the tandem implanter consists of two in-line uniform-field acceleration re- gions, consisting of a series of equipotential planes each separated by about 2.5 cm. For mechanical stability this glass/titanium electrode structure is compressed with an end thrust of approximately 2000 kg. This design philosophy makes possible the shipment of complete accelerators without damage due to vibration and eliminates the need for disassembly.

A number of features have been incorporated into the acceleration tubes to guarantee their successful operation at gradients above 12 kV/cm. Secondary ion and secondary electron emissions axe reduced by mak- ing the equipotential electrodes from smooth, clean titanium metal plates. Equally important is that the acceleration region is electromagnetically suppressed by incorporating transverse electric and magnetic field components to sweep electrons quickly from the acceleration region and to keep the electron energies low.

The high voltage power supply which provides the acceleration voltage is high-current, constant-voltage source based on the Dynamitron principle invented by


290 N. Turner et aL / An MeV ion implantation srstem

Gas Impedance] (diameter 0.5")

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F . . . . . pp~A

Fig. 7. Schematic of high voltage charge exchange canal utilizing recirculation gas.

Cleland in 1956 [13]. A schematic of a parallel-fed voltage multiplier is shown in fig. 6. In this design the high voltage is produced by solid state rectification of a 50 kHz rf voltage generated by a solid state ac- r f converter. Because the power supply is a voltage source, rather than a current source, the device is easy to stabilize and does not need a constant corona load to accommodate variations in charge requirements. The current capability of Dynamitron type power supplies has been demonstrated up to 100 mA and is only limited by rectifier characteristics and the power rating of the oscillator. In the Ionex/IX-1500 implanter, the oscillator used makes available more than 4 mA during dc conditions at 0.75 MV. The charge exchange canal at terminal potential utilizes a stripper gas which escapes into the vacuum system from the ends of the charge exchange canal. For a given canal length and diameter, the flow of gas into the vacuum system varies as the cube of the stripper canal diameter and hence there are major advantages to keeping the canal diameter small. To minimize the effects of this diameter restriction the Ionex /HEI implanter includes a gas recirculating system shown schematically in fig. 7. Gas from the high- pressure makeup line adds to the recirculated output from the small turbomolecular pump which is fed again to the output of the center-fed charge exchange canal.

The effect of this pump is that the load of gas into the acceleration tube can be reduced by a factor of 20 below that of nonrecirculating designs.

4.3. Beam energy filter

After final acceleration of positive ions from the Tandetron, they are focused and pass through a 10 ° filter magnet which, with its resolving aperature, acts as a beam filter and neutral trap. Since during charge exchange in the Tandetron, both single, double and triple charged ions are created, the beam filter magnet may be used to select the desired beam.

4.4. Beam profile

Fig. 8 shows a computer generated beam profile through the complete implanter, for a 31p+ beam accelerated to a final energy of 1.5 MeV. The figure shows the beam waist formed in the stripper canal of the tandem accelerator, and the use of quadrupole lenses in controlling the beam profile. Because the beam axis has been represented as a straight line, the mass selection is hard to visualize in this figure. The two bending mag- nets have been labeled.

N. Turner et aL / An MeV ion implantation system 291

+: w J r Fu~lr -~ IT ~ F > LU Z W 111

C~,~

I"" ' • " 1 . . . . i

o $ O / ~ t . -

Fig. 8. Beam envelope for the Ionex model IX-1500 ion implanter atbeamenergyofl.5 MeV, P+.

The computer modeling is used to design a system which operates in a stable fashion over a wide range of different acceleration voltages with a minimum of ad- justment. The tuning of the beam in the implanter will be easily accomplished under either manual or computer control.

4.5. Scanning

Wafers are mechanically scanned through the ion beam, while mounted on a 36-inch diameter disc having both a rotational axis and transverse axis which also may be rotated through 100 ° (tilt axis). A disk rotational speed of 1000 rpm and an average transverse scan speed of 2 ips is provided. Total transverse scan dis- tance is adequate to properly overscan the diameter of a 200 mm wafer.

Wafers are mounted on the inside of a 10 ° conical shaped disc. Wafers are held on the disc, using only centrifugal force during disc rotation. After wafer un- loading and loading, the disc is spun while in the horizontal position. After achieving several hundred revolutions per minute, the disk is rotated about its tilt axis to a vertical position (0 ° implant angle) or near vertical (n ° implant angle) position.

4.6. Dose control

After the selection ion beam is deflected through the beam filter magnet 10 o, it converges through an aperture which separates other charge state beams, as well as the neutral beam. After emerging from the cooled aperature, the ion beam may be allowed to be on the disc, or interrupted by a setup Faraday cup which also serves as a beam interrupter or flag. The resolving aperature also defines the shape of the beam at the disc which ensures proper and adequate overscan.

P R O M E T R I X O m n i M a p K S 2 0

THURSDAY JUN 19 1986 11 18

DISK ID: D E F A U L T C O N F t G .

SCALE: 1.000 PROBE ID: 8797

TITLE: F3-02,B+. 2E13.400KEV. HEI FILE: 3 3

TOTAL SITES 121 MEAN 1421 Ohrlns/scl GOOD SITES 119 STD DEV 0 247 percent WAFER DIA 12500MM:492 IN MINIMUM 1412 ohms'sq TESTDIA 11430 MM : 450 IN MAXIMUM 1430 ohms:sq

WAFER ID F3,02 INTERVAL 1 00 percent LOT ID CURRENT AUT 7 48MV 0 0236MA PROC DATE - - SORTING 3 0 SIGMA

Fig. 9. Sheet resistivity map of 400 keV, B +, 2E13 implant after 900 ° C, 30 rain anneal, showing typical uniformity of

0.25%, lo.

After evaluating wafers implanted on the prototype system, implant uniformity of 0.25%, were achieved. See fig. 9.

4. 7. Wafer handling

The design goals for the wafer handler are shown in table 1. An overview of the design is shown in fig. 10.


292

Table 1 Wafer handling performance goals

N. Turner et al. / An MeV ion implantation system

Wafer size

Wafer throughput

Cassette types

Load capacity

76-200 mm wafer types SOS, Si, GaAs

110 W/h 200 mm wafer orientation + 2 ° flats/notches 135 W/h 150mm 150 W/h 125 mm particulates < 0.05 P/cm 2 > 0.5 #m 170W/h 100ram 185 W/h 75 mm wafer inventory slot to slot integrity

semi-standard wafer handling gentle backside only, no sliding, no

50 wafers edge clamping (2 cassettes)

S ~ "IF'ml

Fig. 10. Overall view of a vacuum wafer handling and scanning system showing cassette locks, wafer transfer arm, wafer flat orienter, with disk tilted near horizontal in load/unload.

In order to meet or exceed the stringent particle specifications for future VLSI device needs, the basic system concept of placing the entire handling system in vacuum was selected. A comparison of the advantages of a vacuum exchange end station is summarized in table 2.

4.8. System operation

A cassette of wafers is placed in each of the cassette locks. After lock pump down, a narrow swing valve

opens, allowing wafers to pass through from the cassette to the process chamber. Wafers are transported in the horizontal posit ion via a three axis transport arm from the cassette to flat orienter, and then to disk. Wafers in the cassette are placed on the transfer arm via vertical mot ion of the cassette elevator. During lock pumpdown, wafer positions in cassette slots are mapped by a col- l imated light beam.

The wafer transfer arm is driven by two dc servo motors and encoders. Linear motion of the wafer is obtained, by driving both motors synchronously in op-

N. Turner et aL / A n Me V ion implantation system 293

Table 2 Advantages of vacuum wafer exchange over atmosphere exchange for a batch process end station

Smaller vacuum pumps may be used, since chamber pumpdown is not serial to throughput

Implant may start as soon as disc is loaded since vacuum pressure is already at a low base value

Dose accuracy is improved since pressure dependant Faraday effects are minimized

Any particulates in the chamber are not 'transported' due to turbulent chamber venting, each time wafers are exchanged

Using multiple cassettes allows slower pump down and venting of lock chamber thus reducing particle transport via gas turl)ulence

Cassettes placed in vacuum locks, rather than a "clean air stream", tend not to gather particles as a function of time

Table 3 Disc parameters for model Ionex-1500 ion implanter

Wafer Batch Max. throughput Total diameter size (120 s scan (mm) implant time) area

(wafers/h) (cm 2)

Wafer area scan area (~)

200 9 110 4394 64.3 150 13 135 3680 62.4 125 15 150 3045 60.5 100 20 170 2720 57.7 76 25 185 2101 54.0

4.10. System throughput

posite directions. Rotary motion of wafer is obtained by driving one motor while holding the other. All motors are controlled via motor control card interfaced to a microprocessor controller.

Overall throughput varies according to wafer size. A timing diagram of 150 mm wafer throughput is shown in fig. 11. Wafer exchange time is 12 s. Wafer travel length has been reduced to a minimum to ensure a simple and rapid exchange time. Batch sizes for one disc are shown in table 3.

4.9. Wafer orientation

Wafers to be implanted are moved to the orient station via the transport arm. The wafer is lifted and rotated by the orienter. The wafer orienter, using an optical edge detector and associated electronics, locates the center of the wafer flat or notch and also measures the amount of nonconcentricity. The wafer flat is rotated to the selected orientation angle and placed back on the arm, on center.

4.11. Control system

A distributed microprocessor system employing the I B M - P C - A T is used. A system block diagram is shown in fig. 12. Dedicated microprocessors are used for both the end station wafer contro l / scan control, and for vacuum control. Implanter ion source, injector, and Tandet ron are interfaced using A / D , D / A and I / 0 to " R e m o t e Control Units." Both dedicated micros and the Remote Control Units are connected to the Main Controller via serial light pipe links. The main con-

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C A S S E T T E L i P U M P DOWN = ~ V E N T ~ , - -- - "- EXCHANGE A C T I V I T Y ~ ,. : _--_: ~ j ~ J

Fig. 11. Timing diagram for maximum throughput of 135, 150 mm wafers/h utilizing a minimum implant time of 120 s.


294 N. Turner et al. / A n Me V ion implantat~n system

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Fig. 12. Overall block diagram of distributed microprocessor based control system utilizing two touch-sensitive control CRTs; one for implanter control and one for process control, diagnostics and maintenance.

troller is operated by either of two touch-sensitive CRTs. The wafer load station has a menu driven touch sensitive screen and the engineering terminal is also touch sensitive, and equipped with a keyboard.

An operator may select recipes, initiate setup, adjust machine setup parameters, and start and stop the implant from the wafer load area. The engineering terminal, in addition, provides password access to create or edit implant recipes, select implant setup parameters to be interlocked (implant monitor) and implant parameters to be logged. Data logging may be done on floppy disk, printer or transported via host link to off line data storage.

5. Schedule

The first systems are under construction and delivery is being planned for spring 1987.

6. Gonclusion

The IX-1500 high energy implanter is being designed as a production ion implanter providing the capability for the most modem fab line. Providing a simple ion source concept, in a conventional air insulated terminal and using Tandetron dc acceleration with solid state driver, provides a reliable and compact beam line. The mechanical scanning end station is simple and provides excellent uniformity and dose accuracy. In addition, the ability to quickly vent and pull out the entire disk and

scan system provides easier routine maintenance. Vacuum handling of wafers should provide superior particle performance via allowing slow venting and pumpdown of cassette locks without reducing throughput.

The authors would like to acknowledge the significant contributions of the many talented people involved in the development of the high energy ion implanter.

References

[1] D. Pramanik and M. Current, Solid State Technology (May 1984).

[2] J.F. Ziegler, Proc. 5th Ion Implantation Conf. Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 270.

[3] J. DePontcharra, P. Spinelli and M. Bruel, IEEE Trans. Electron Devices (1986), to be published.

[4] A. Stolmeizer, IEEE Trans. Electron Devices ED33 (4) (1986) 450.

[5] P. Spinelli, Proc. 5th Int. Conf. on Ion Implantation Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 283.

[6] M. Sieradzki, Proc. 5th Int. Conf. on Ion Implantation Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 237.

[7] M. Current, N. Turner, T.C. Smith and D. Crane, Proc. 5th Int. Ion Implantation Conf., Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 336.

[8] M. Current, Proc. 5th Int. Ion Implantation Conf., Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 9.

[9] D.H. Douglas-Hamilton and C. Taylor, Proc. 5th Int. Ion Implantation Conf., Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 196.

[10] J. Pollack, N. Turner, R. Milgate, R. Resnek, and R.


Hertel, Proc. 5th Int. Ion Implantation Conf., Equip. and Tech. (1984) Nucl. Instr. and Meth. B6 (1985) 202.

[11] J. O'Connor and L. Joyce, these Proceedings (Ion Implan- tation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 334.

[12] N. White, these Proceedings (Ion Implantation Technol- ogy, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 339.

[13] U.S. Patent, M. Cleland, Aperture for high energy acceleration.


design considerations of a vlsi compatible production mev ion implantation system

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