OptiScan, Beam Utilization Enhancement on the Purion XE

By: Shu Satoh, Chief Scientist
Axcelis Technologies

Among many semiconductor manufacturing processes, ion implantation seems to be the only process which does not benefit from the round shape of the wafer.  For many other processes, like anneal, etch, CVD, spin coating, CMP in device manufacturing and lapping and polishing in wafer fabrication, it is a real blessing that wafers are round.  

The difference comes from ion implantation’s highly directional interaction with the wafer, which requires not only the dose but direction of the beam to be uniform across the wafer.   Because of these requirements, ion implantation has employed numerous kinds of scanning mechanism from its infancy. 

Purion XE, the high energy single wafer ion implanter on Axcelis’ Purion platform, employs a hybrid scan system; the prevailing scanning system among the modern single wafer implanters.  In the hybrid scan system, the ion beam is scanned in one direction at a high frequency and a wafer is moved in the orthogonal direction at a much slower velocity, as shown in Fig.1.

Fig. 1. Hybrid scan system with side cups.

Although not essential to the hybrid scan system, the addition of one or two narrow faraday cups (side cups), near the edges of scanned ion beam has given enormous advantages to  single wafer ion implantation and  is now considered an indispensable feature of the hybrid scan system.  The constant monitoring of the ion beam with the side cups enables a closed loop dose control system.  The process called glitch repainting is utilized to maintain superb dose uniformity even when the beam current fluctuates due to beam glitch events. On the Purion XE, there are two side cups on either side of the beam. They are called PR cups for its special function during photoresist outgassing.  They are strategically placed upstream from the wafer, as shown in fig. 2.

 

Purion XE side cups (PR cups)

Fig. 2. Purion XE side cups (PR cups)

The scan pattern of the hybrid scan is naturally a rectangle area, combine this with the circular shape of the wafer, the hybrid scan wastes some of the ion beam outside of the wafer area, at least 21 % (=π/4).   With the addition of the side cups, the beam scan width is widened to >400mm for full exposure of the ion beam into the two cups, as shown in Fig. 3.  This leaves more then 50% of ion beam which does not contribute to wafer doping at all. This is the price that is paid for the great benefits  achieved from a  dose control system with the side cups.

Scan shape of hybrid scan with side cups

Fig. 3. Scan shape of hybrid scan with side cups

OptiScan on the Purion XE solves the interesting puzzle presented by the hybrid scan with the side cups:  how to increase the beam utilization while keeping the closed-loop dose system with the side cups, which is indispensable for rigorous requirements of modern dose control.   To solve the puzzlea division of labor is introduced in beam scans.  Namely, wide scans are utilized for beam current monitoring the side cups and narrow scans purely for doping the wafer.  The scan width for the narrow scan can now be set solely for uniform doping of a wafer without worrying about the side cups, in effect considerably shrinking the length of the wide scan.  The two kinds of beam scans, a narrow and a wide scan are interlaced with a fixed ratio. The picture, fig. 4, shows an example of the interlaced scans, two narrow scans for every wide scan are illustrated for an exaggerated imagery.

Interlaced wide and narrow beam scans in OptiScan

Fig. 4. Interlaced wide and narrow beam scans in OptiScan.

The increase of beam utilization with the interlaced beam scans depends on the interlace ratio, the number of narrow scans per every wider scan.  The higher the interlace ratio, the higher the beam utilization will be.  If N is the interlace ratio and R is the width of narrow scan relative to that of wide scan, change of beam utilization relative to non-interlaced scan (all beam scans are wide scans) will be;

Beam utilization improvement = (1 +N) / (1+NR)

For N=3, three narrow scans per every wide scan, with 80% of scan width to narrow scans (R=0.8), the utilization goes up as much as 17.6%.  Naturally, this formula does predict the ultimate 1/R improvement for the case where N will equal infinity, (i.e., if all the beam scans are narrow scans, improvement will be 25% for R=0.8).

One important, although easily missed detail exists on the interlaced scan scheme.  To meet today’s rigorous dose control requirements; the beam current monitored on the side cups has to be a very reliable and stable representation of actual beam current on the wafer.  The ratio between the side cup beam current and the beam current on wafer is called “cup ratio” and every precaution is paid to obtain a stable and reliable cup ratio number, both assumed and actual.  

One challenge posed by the interlacing scan sequence is producing and maintaining a stable “cup ratio”.  The side cups are susceptible to partial beam exposure during the narrow scans which could lead to an unstable “cup ratio”.  This partial beam exposure is due to the “skirt” of the ion beam, which is expected to be highly irreproducible and highly vulnerable to small changes or drifting in the implanters’ conditions.  In order to compensate for the “skirt” of the beam ion implanters will utilize an over scan technique to ensure the beam is scanned completely off of the wafer and ensuring the whole ion beam is measured with the side cups.  This in turn will result in a reliable “cup ratio”.

To avoid this possible contamination to “cup ratio” by narrow scans, OptiScan employs a patented synchronized current gating scheme as shown in Fig. 5 and Fig.6.  During the period of narrow scans, a pair of switches (190A and 190B) disconnect the side cup current(158) from the input of the dosimeter (162) and bypass it to ground.  During  every wide scan, the switches reconnect the side cups to the dosimeter for stable and reliable beam current measurement.

Interlaced wide and narrow scans and snychronized gating signal

Fig. 5. Interlaced wide and narrow scans and synchronized gating signal.

 

Gating of the side cup current

Fig. 6. Gating of the side cup current

The beam utilization improvement by OptiScan does not linearly translate into the same amount of throughput improvement due to  the fixed overhead time from the wafer exchange routine and as well as the throughput improvement are  dose and beam current dependent. An example of throughput improvement is shown in Fig.7.

Throughput improvement with OptiScan

Fig. 7. Throughput improvement with OptiScan

Axcelis’ OptiScan option on the Purion XE can be utilized to increase the throughput of every species at almost any energy.  While the throughput improvements are variable based on species, dose and energy the improvement can be realized anywhere from 3% to 16%, assuming the beam current is not at the mechanical limit of the Purion platform at 500 wafers per hour already.