224 Nuclear Instruments and Methods in Physics Research B21 (1987) 224-228 North-Holland, Amsterdam

200 mm E N D STATION FOR ION BEAM IMPLANTERS

C. T A Y L O R , P. S P L I N T E R , A. W E E D , J. G R A N T and S. H O L D E N

Eaton Corporation, Ion Beam Systems Dioision, Beverly, MA 01915. USA

A new batch processing end station for ion implantation has been developed to automatically load and implant wafers up to 200 mm in diameter, to provide a wafer handling reliability of 50000 wafers between assists, to provide clampless wafer cooling to temperatures less than 100°C for beam powers of 4 kW, and to add less than 0.03 particles/cm 2. Preliminary test data shows reliability of 10000 200 mm wafers between assist or failure for the pick-and-place wafer handler, less than 0.12 1.0/.tm particles/cm 2 added to a 200 mm wafer during loading and implant, and a wafer handler cycle time of 9.0 s for 200 mm silicon wafers. This 200 mm end station is used on Eaton's NV-20-200 ion implantation system and NV-1000 series megavolt ion implanters. A prototype end station has operated in a 200 mm pilot process line for seven months.

1. Introduction

Eaton Corporation recently announced its NV-20-200 high current batch processing ion implantation system (fig. 1). The system is based on an end station designed for 200 mm wafers and having a reliability of 50000 wafers between failures or assists. Also provided are wafer cooling at full beam power of 4 kW (200 kV, 20 mA), low particulate contribution and high throughput. Additional features on the 200 mm end station are simple access and control for service, a touch sensitive CRT operator interface, remote monitoring with SECS II protocol, robotic communication protocol and low end station pressure during processing.

The primary design objectives of the NV-20-200 end station were safe wafer handling, hardware reliability and ease of service. The processing chamber was de- signed for atmospheric operation to allow backside vacuum chuck handling and a constant wash of laminar air flow. The chamber is inclined to reduce machine length and to secure wafers on the disk during all process events or system failures. Wafer safety is en- hanced by direct sensing of wafer position at all points of loading, including the disk. In the event of any hardware failure, the safety of the wafer is insured.

Pick and place action with vacuum chucks was selected as the most reliable handling technique avail- able. All motions are controlled by stepper motor drives with "pseudo"-encoder feedback, and all adjustments are "learned" in software providing drift-free operation with a minimum of setup effort. A large degree of wafer position tolerance was allowed in the design of the wafer handler and processing disk. Wafer cooling is provided by a clampless technique developed at Eaton for its NV-10 product line. Wafer orientation is pro- vided by disks with small angle variations, and fully selectable fiat or notch orientation.

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The desire for mechanical simplicity led to the selec- tion of a single disk and processing chamber configura- tion. All hardware is readily accessible and the disk can be easily removed for service. The handler and vacuum cycles are serial to the implant process as a result of this choice. Despite the intentional compromises in system design to enable simplicity and reliability, the 200 mm end station provides for throughputs of up to 180 100 mm wafers per hour and 125 200 mm wafers per hour.

2. System configuration

Eaton's 200 mm end station consists of a sealed vacuum chamber mounted to a frame approximately 45 ° from the horizontal as shown schematically in fig. 2. The lower portion of the chamber contains a hnear sliding vacuum seal with three differentially pumped stages and a 10.25 in. stroke capability. A drive spindle is mounted to the slide and provides rotary vacuum sealing and water cooling to the disk. The rotary spindle is driven by a high torque dc servomotor with the capabilities of high rotational speeds up to 1500 RPM and position accuracy of 1 part in 16 384 (0.008 in. at the outer edge of the wafers). An aluminum disk is mounted to the spindle and contains internal water passages which provide continuous cooling behind the wafer mounting sites.

The lower half of the vacuum chamber, which carries the disk and scanning mechanism is easily removed from the main portion of the end station on a roll-around cart. The exchange of a disk can be accomplished in less than half an hour. The main portion of the vacuum chamber contains a 10 in. cryopump with excellent pumping speed to the front surface of the disk.

C. Taylor et al. / 200 rnm end station 225

Fig. 1. Eaton Corporation's NV-20-200 ion implantation end station.

/ L ion

Source

- - Analyzer Magnet

End Station

Fig. 2, Schematic side view of NV-20-200 ion implantation system.

3. D i s k / w a f e r cooling

The wafer loading disk is aluminum and 44 in. in diameter. The outer edge of all size wafers is at a 19.5 in. radius, providing a consistent registration point for wafer handling. Five equally spaced constant width beam slits are located evenly around the disk cir- cumference. A 200 mm disk is shown in fig. 3. The beam current is measured in a Faraday cup that is stat ionary relative to the beam. A dc servomotor and a

~ig. 3. 200 mm wafer disk mounted to removable end station cart.

IV. NEW EQUIPMENT AND SYSTEMS

226 C. Taylor et al. / 200 mm end station

linear ball screw scan the disk and the moving portion of the sliding vacuum seal.

The total number of wafers in a batch is ten 200 mm wafers, fifteen 150 mm wafers, twenty 125 mm wafers and twenty five 100 mm wafers. Disks are available with an average implant angle of 0 ° and 7 °. Each mounting angle provides adequate centrifugal clamping pressure to facilitate high quality wafer cooling using Eaton's clampless cooling technique [1].

Clampless cooling provides intimate contact between the disk and wafers at low contact pressures. The con- tact interface provides heat sinking of more than 0.9 W / c m 2 with a temperature difference between wafer and disk of less than 70°C (calculated for Eaton's NV-20-200 from NV-10 measurements). There are no clamps to mask the wafer during implantation. In ad- dition, structural loading of the wafer is minimal and the stress concentrations associated with wafer clamps are avoided, thus reducing the risk of wafer breakage during processing.

The flat wafer support provides low implant angle variations, depending on the wafer diameter. 200 mm wafers experience an implant angle change of less than 1.5 °. The wafer mounting sites are made with a 0.20 in. placement tolerance. This increased window provides for more latitude in setup of the wafer handler and thus greater wafer loading reliability.

4. Wafer handler

The wafer handler for the 200 mm end station is a parallel action pick and place type. All handling of the wafers occurs with backside vacuum chucks. The wafer is completely supported on its backside during all mo- tions except the final transfer onto the disk, and pickup from the disk. Fig. 4 shows the schematic of the wafer handler configuration.

An input elevator delivers wafers to a shuttle mecha- nism. The shuttle acquires the wafer from the cassette and transfers it to a flat aligner/centering fence. Simul- taneously, the wafer previously processed at the flat aligner is delivered by the forward portion of the shuttle to the wafer loading device. A similar parallel action pick and place mechanism unloads the wafers and transfers them into cassettes. The wafer load and un- load transporters carry a dual fingered vacuum chuck, to allow for redundancy in securing and sensing the wafer.

The load/unload wafer transporters are mounted 72 ° apart directly to the process chamber. The eleva- tor /shutt le system is mounted to the chamber support- ing framework. The positioning of the arms allows simultaneous loading of one site while unloading occurs one fifth of the disk away. A dummy wafer elevator and transfer system is provided to deliver and acquire wafers needed to fill process batches.

' , Disk #

Input Transporter ~ ~ o )< Output

Inpoi IY 1 11 lit" I I I r a I I Output E'eve,o, [ I JIE'et" Dummy Elevator

Fig. 4. Schematic top view of NV-20-200 wafer handler.

5. Wafer handler design approach

The wafer handler of the Eaton NV-20-200 end station was designed to provide maximum reliability during operation. Backside vacuum chuck gripping of wafers was selected due to inherent simplicity, the abil- ity to positively sense the presence of the wafer, and the elimination of edge contact and sliding motion. Pick and place sequencing of wafers was selected to allow full parallel flow of wafers through the system and therefore minimum time for loading. Each of the wafer chucks, except the load/unload chucks fully supports the wafer on the backside. A dual vacuum chuck is used for the wafer load/unload transporters. Each side of the wafer load/unload chucks is separately pumped and sensed for vacuum.

The sensing of vacuum on each of the chucks in the wafer handler confirms the presence of a wafer in proper position. The disk sites each have a vacuum manifold used to draw vacuum for securing wafers to the clampless cooling sites, and for pressurizing the sites for wafer release after processing. This manifold is used on both clampless cooled and on uncooled disks to draw vacuum for wafer position sensing.

All motions in the handler are driven by stepper motors with fixed position optical sensors. These sensors provide repeatable position signals at critical points of the motion of the stepper motor drives. Final positions of mechanisms are "remembered" in software, and the position of stepper motors is confirmed after motions are complete to insure no step loss.

6. Reliability

The design objective, and specification for reliability of the 200 mm end station is 50000 wafers between assist or failure. Not only is the basic mechanism re- quired to provide this high reliability, but the entire system that keeps the machine operating at the "instal-

C. Taylor et a L / 200 mm end station 227

lation" is required to meet this specification. Eaton has begun a reliability development program for the end station with an objective of ensuring this high depend- ability.

The program recognizes all aspects of user experi- ence with reliability. First, the program has already validated the basic performance of the wafer handler system. In this step, design weaknesses are identified by statistical failure measurements. An example of design improvement was the increase in torque of the linear stepper motor used to raise and lower the input and output transporter vacuum chucks.

The second step of system reliability is the upgrade of the very complex control software which simulta- neously controls systems actions, checks sensor condi- tions, manages wafer inventory in the machine, and communicates with other control system in the ion implanter. The basic control logic sequence is generally a small portion of the control software. The error check- ing logic requires typically 70-80% of the total software. The system must be exercised for many thousands of wafers before subtle logic errors can be identified, ad- ditional intelligence developed to handle unexpected failure modes, or the many variations in operational logistics can be experienced. Continuous monitoring of failures in engineering testing, in-house production test- ing, and feedback from installations is required to iden- tify, diagnose and correct this type of system perfor- mance limit. This stage of reliability development is in process.

To date, more than 150000 wafers have been processed through three prototype end stations. Early reliability data indicated more than 1000 wafers be- tween failure or assist. This was improved to the 10000 wafer level by the techniques listed above. Much larger numbers of wafers will be processed to improve this level of performance. Recently, at SEMICON West, a prototype NV-20-200 end station ran 4300 200 mm wafers through a wafer handler cycle continuously without a single problem. In addition, a recent test of 20 000 wafers was completed with only one assist.

The third step of the reliability program is perhaps most critical in delivering a satisfactory system to customers. That requires typical Eaton service person- nel, trained by Eaton training techniques, to achieve the same basic performance as that experienced by factory development personnel. For that reason, the final stage of product testing consists of reliability testing of the end station, installed and maintained by service with instructions developed by training. This is the most important step in the reliability equation, and is the guarantee of product performance. This component of the reliability program is just under way, and will continue through the delivery of the first production NV-20-200s, and be monitored through the life of the product. The wafer handling reliability of 50000 wafers

between assists or failures will be achieved in an operat- ing fab line.

7. Wafer handler throughput

The throughput of the wafer handler is enabled by the use of pipelining of the wafers from cassette to flat aligner, and flat aligner to loader. Reliability is provided by the use of vacuum sensing of the wafer chucks to confirm wafer position, the smooth motions and the use of photo-optical sensors to confirm mechanism posi- tions.

The maximum operating time of the wafer handler is 12 s per wafer. The requirement of unloading 20% of the disk prior to initiating the simultaneous loading/ unloading cycle makes the effective specification of han- dling time per wafer on a disk to be 6/5 times 12 s, or 14,4 s per wafer. The mechanism speeds of a 200 mm wafer handler used in reliability testing were measured using video taping, and the logistics were reviewed for efficiency. Using an optimum logical sequence of ac- tions, the handler will load a 200 mm wafer every 9.0 s. This new sequencing is currently being programmed into the end station control software.

The total throughput of the NV-20-200 is dependent on the wafer handling time, implant time (minimum of 30 s) and an overhead time per batch of 106 s. This overhead accounts for chamber roughing, high vacuum and disk acceleration, disk deceleration, and chamber venting. Maximum throughput for the system is 125 200 mm wafers per hour and 180 100 mm wafers per hour.

8. Controls/user interlace

The computer control system for the 200 mm end station was designed for simple operator control, simple adjustment procedures, high level sensor and board diagnostics, and for remote monitoring of end station functions. The operator/maintenance interface is a touch sensitive CRT with three modes of operation (operational mode, configuration mode and mainte- nance mode). The computer control is capable of con- trolling many simultaneous actions of the end station and wafer handler, monitoring sensors, managing wafer inventory through the process, and providing the high level of intelligent error detection schemes required to ensure reliability required of the system.

The large number of simultaneous functions to be controlled and monitored led to the use of a multi- processing scheme on the Intel Multibus, and provisions for 160 isolated digital outputs, 128 isolated digital inputs and 16 analog inputs. A 16 Kbyte battery-backed memory is used for interprocessor communications and to store setup information. A total of six processors are used for controlling the simultaneous actions of the

IV. NEW EQUIPMENT AND SYSTEMS

228 C Taylor et al. / 200 mm end station

wafer handler. Two processors control the 12 stepper motors, two control the load/unload sequencing, one controls the CRT operator interface and one is used as a system supervisor.

The operator interfaces with the 200 mm end station by use of a touch sensitive CRT, with operational, configuration and maintenance modes of operation available. Configuration mode allows the operator to set system parameters, such as cassette type, flat aligner angle, flat or notch finding, wafer size, auto implant, disk implant angle and clampless cooling.

Operation mode is the only screen the normal oper- ator needs for running the end station. This consists of a set of soft keys and displays for status and diagnostic information. The status section supplies cycle informa- tion and operator prompts. The diagnostic display pro- vides operator feedback and error condition informa- tion. Diagnostics include wafer tracking errors and any hardware (sensor or board level) errors, which are con- stantly polled. The soft keys change depending on the state of the loading process. With an empty disk, LOAD keys are displayed, and with a full disk, IMPLANT keys are displayed. This feature prevents operator error in determining the next appropriate sequence. Ad- ditional soft keys are displayed if errors in wafer track- ing occur, and appropriate actions are enabled.

Maintenance mode is a password controlled set of screens which allow maintenance personnel to control and monitor each device in the end station. Function actions of LOAD and IMPLANT are also available out of normal sequence. A typical screen allows control of a particular device as well as monitoring of the status of all sensors associated with the device. With the aid of these maintenance screens and the diagnostic informa- tion indicating fault information, mean time to repair (MTTR) is greatly reduced.

Also in the maintenance mode is the ability to alter the setup position of a device. The maintenance person controls the device to the desired position and the computer is instructed to "learn" the new position. This eliminates the need for adjustments of sensors, allows the use of fixed position sensors and reduces the time and expertise needed to "tune" the system for peak reliabili ty.

Host communications is currently being imple- mented which allows end station status and control from the SECS II protocol through the Eaton Datalock system. In addition complete robotic communication protocol [2] for automatic cassette loading has been developed and will be installed on the end station.

9. Particulates

A second objective in the design of the 200 mm end station is the addition of fewer than 0.03 1.0 /Lm and larger particles per square centimeter to a wafer during loading, processing and unloading.

The vacuum chamber, with disk and wafers, remains closed during wafer loading. Only the small loading ports are opened. The mechanisms of the wafer handler are below the plane of the wafer, and no sliding contact to the wafer edges is allowed. Venting of the end station is performed with laminar, constant flow rate, allowing the elimination of turbulence without sacrificing throughput. Pumpdown is also designed to minimize turbulence. Venting and pumping ports are below the disk and wafer surface levels.

A laminar flow hood is mounted above the wafer handler system, further controlling particulates. Perhaps the most significant focus from a particulates point of view is the design of the wafer handler and disk to insure high reliability. On the disk, the wafer is not clamped and the locating hardware is made from materials that have been extensively tested for par- ticulate generation and minimum damage to the wafer.

A prototype of the NV-20 end station has been in operation for nearly seven months. The machine is used to process pilot runs of 200 mm silicon wafers. The clean room environment has not been characterized, but preliminary particulate measurements are encouraging. 200 mm wafers were processed through the end station in both a load/unload cycle and a full implant cycle. A 125 mm diameter section of the wafer was measured for particles before and after these two cycles. Particle counts were 0.18 0.3 ~m and larger, and 0.11 1.0 /~m and larger particles were added per square centimeter during the load/unload only cycle. For the complete load/unload and implant cycle, particle counts were 0.31 0.3 p,m and larger, and 0.12 1.0 ~m and larger particles added per square centimeter.

References

[1] V. Benveniste, these Proceedings (Ion Implantation, Tech- nology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 366.

[2] E. Garvey and L. Zimmerman, these Proceedings (Ion Implantation Technology, Berkeley, 1986) Nucl. Instr. and Meth. B21 (1987) 509.