Simulation Decreases Design Cycle for Next Generation of Ion Im planters

--by Richard Mandel

Varian Semiconductor Equipment Associates, Inc. based in Gloucester, MA, designs, manufactures, markets and services ion implantation systems--semiconductor processing equipment used in the fabrication of integrated circuits. Ion beam implanters are used to dope silicon wafers with impurities to make a semiconductor. An earlier approach to doing this was to deposit a material such as boron on the wafer surface and then diffuse or drive it into the silicon by exposing it to controlled periods of high temperature. As device geometries became smaller, the sideways diffusion caused by this method became more of a problem, so the industry converted to the ion implantation process. In the ion implantation process, the dopant molecules are implanted vertically into the surface of the silicon wafer by a high-energy ion beam, eliminating any appreciable side-ways diffusion.

Simulation of ion beam path (black) through corrector magnet (blue). The coils are red.

VSEA currently offers three types of systems: high current ion implanters, exemplified by the company's VIISta 80, which are used for high throughput, serial wafer processing; VIISta 810 medium current implanters, which offer greater precision and supports feature sizes down to 0.10 micron and wafer sizes up to 300mm; and high energy ion implanters--such as the Kestrel system--which combine advantages in cost of ownership with a compact design.

An important requirement of the next generation of ion implanters is shorter beam lines, because the beam is continually spreading out over its length as the ions repel each other in a phenomenon known as space charge. Designers of ion implanters have always had a difficult time shortening the beam, however, because it requires placing the magnets that control the beam in close proximity to each other where they interact in a manner which is difficult to predict. The software is helping to predict the interaction of multiple magnets while simultaneously modeling the space charge of the ions, the trajectory of the ion beams, and the electrical fields within the implanter.

Controlled by magnetic fields

Varian's ion implanters work in a similar manner. Ions are pulled from a hot plasma source by means of extraction electrodes. The ions pass through an acceleration/deceleration device which changes the energy of the ion beam. The beam then passes through a number of magnetic fields which alter the beam in a specified way. For example, an analyzer magnet purifies the beam by filtering out unwanted species. If the source material is boron fluoride, the analyzer magnet will filter out the fluorine so that only the desired dopant, the boron ions, remains in the beam. Another magnet, called the corrector magnet, shapes the ion beam into the correct form for deposition onto the wafer. The system includes other, smaller magnets that perform functions such as measuring the ion dose.

Simulation of ion beam entering magnet.

Understanding the behavior of both electrical and magnetic fields is critical to designing an ion implanter. The system includes two sources of electrical fields. One is the field that is created intentionally by the extraction electrodes. The other occurs because the ions have charge , so they tend to repel each other. This space charge effect must be accounted for in the implanter design so that the beam is refocused as it passes through the implanter. Design issues related to the magnets include determining the saturation in the magnet steel, producing the correct energy range to meet the semiconductor's design specifications, and avoiding aberrations and unwanted distortions of the beam to prevent the inclusion of unwanted components within the ion beam. "Our ultimate goal is to shape the beam so that it delivers a uniform, repeatable, correct dose of dopant and does this 24 hours a day," says James Buff, a senior physicist at Varian. "An understanding of electromagnetic behavior is critical in accomplishing this."

The earliest ion implanters were designed with pencil and paper and optimized by trial and error. More recently, Varian designers have used proprietary electromagnetic simulation codes to move some of this trial and error process to the computer. Although these codes provided insight into magnet behavior, they had several limitations. One was that the codes couldn't model saturation accurately. "This is a critical issue because if the magnetic field goes out into the air instead of being carried through the steel, there can be many unwanted effects," says Buff.

More powerful simulation

To develop the shorter beam line, Varian designers supplemented their own analysis codes with more advanced electromagnetic analysis capabilities such as modeling magnet interactions and evaluating space charge and magnetic fields simultaneously. They chose finite element-based electromagnetic analysis software called OPERA from Vector Fields, Aurora, IL. In addition to the basic OPERA system, the company purchased a module called TOSCA that adds the ability to perform three-dimensional analyses of magnetostatic, electrostatic, and synchronous fields. Another module called SCALA was purchased to permit simultaneous modeling of 3D space charge effects.

Ion path (blue) as they pass through the implanter. Source is at top right.

The Varian designers began the development of a new ion implantation system by sketching the basic concept for the new beam line and using proprietary software to test the general viability of the idea. Next, they modeled each segment of the beam line in detail in OPERA, using the software's CAD facilities. Example--the electrodes that extract ions from plasma were modeled in detail as one segment. The analyzer magnet that selects the desired species was another segment. Each segment was modeled in 3D. After creating the models, the designers specified the material properties using the library of material data contained in the software. The program then automatically divided each model into finite elements.

The researchers next used TOSCA to analyze each individual segment to see the magnetic and electric fields. This step allowed them to design and optimize each individual magnet--the source, the analyzer, and the corrector, as well as the other, smaller ones used in the system. Since some of the magnets were now placed close enough together to interact, the next step was to combine segment models and repeat the analysis to study the interactions. This allowed the designers to achieve their goal of reducing the length of the beam line. Rather than avoiding the issue of magnet interaction by keeping the magnets far apart, they were able to see exactly how close they could place the magnets before observing undesirable effects.

A reduced energy beam was also a design goal for the new implanter because it would make it possible to deposit the dopant at smaller depths, thus supporting customers' demands for more densely packed chips. This low energy beam made space charge a serious concern, however, because as ions travel more slowly in a lower energy beam, they have a greater space charge. Having a shorter beam line would help control space charge, but designers still needed to incorporate ways of neutralizing the charge into the system. They found SCALA to be very helpful because it allowed them to predict space charge and ion trajectories simultaneously with magnetic and electrical fields in the region. By using the results to guide the design, designers were able to resolve the low energy beam space charge problem.

Alternate view of ion paths. Source is now at the left.

One benefit of using these analysis tools on this project, according to Buff, is that it shortened the design process. In the past, it would take many months to design and optimize the magnet for a single segment of the ion implanter. With the insight provided by electromagnetic analysis, this was done in only several days. This new ion implanter is the first in which simulation was used throughout the design process but Buff believes that the system will be completed in less time than the company normally needs to develop a new design. The other benefit of using simulation, he says, is that it "...stops you from pursuing design ideas that look good but are really flawed. You don't go down a wrong path, and you save the time and cost of building and testing prototypes."

The new ion implanters being designed by Varian will provide an advanced tool for chipmakers, making it possible for them to pack more and more functionality onto each device. "Each time we advance our technology to meet our customers' demands, we run into design issues we haven't faced before," says Buff. "Electromagnetic simulation can help to resolve these challenges quickly and cost-effectively. The ion implanter we're working on now couldn't have been designed without simulation software."

For more information:

Circle 470 - Vector Fields, or connect directly to their website via the Online Reader Service Program at http://www.OneRS.net/107df-470

Circle 471 - Varian, or connect directly to their website at http://www.OneRS.net/107df-471