The use of hard masks instead of photoresist for the Extension implant is an effective way to optimize the amount of dopant that is retained along the fin sidewalls for those fins that border along photoresist edges (as discussed in Part 1 of this series).
However, hard masks do nothing to address the dominant problem driving steeper implant angles, namely the increasing height of fins and the decreasing space between them. As illustrated in figure #1, the fins get taller and closer together at each new node.
This configuration is advantageous because taller fins provide greater W-effective and more closely spaced fins increases transistor density per-unit area. However, tall, closely spaced fins present a serious problem for Extension implants because they dictate the use of very steep implant angles (refer to figure #2).
Such steep implant angles greatly reduce the retention of dopant on the fin sidewall due to ricocheting as illustrated in #3.
Since high-dose and uniform doping of the fin Extension regions is central to FinFET performance, this issues needs to be addressed. The solution is to take advantage of not only the wafer’s tilt during the Extension implant, but also the “twist” of the wafer.
It is important to realize that since all of the fins are formed using Self-Aligned Double patterning (SADP), or Self-Aligned Quadruple patterning (SAQP), the fins consist of a series of parallel straight lines. So it is possible to rotate, or “twist”, the wafer to alter the implant angle in addition to just tilting the wafer away from the vertical. Figure #4 illustrates the difference between wafer tilt and twist.
Because it is much easier to tilt a wafer rather than to tilt the angle of the implant beam, the +/- 25˚ tilt of the Extension implant is accomplished simply by tilting the angle of the wafer.
However, it is also possible to exploit the parallel line nature of the fin orientation and twist the wafer during this implant as well as tilt it. By twisting the wafer during the Extension implant a substantial advantage is gained because it allows the implant beam deeper access into the micro-canyons formed by the tall, adjacent fins.
This is accomplished by breaking down the two Extension implants into four separate implants, two for each side of the fin. The wafer is still tilted (approximately +/-25˚ for a 10nm fin) but between each of the four Extension implants the wafer is twisted, first to 335˚, then to 25˚, then to 155˚ and finally to 205˚ as illustrated in figure #5.
So now both the PMOS and the NMOS Extension implants consist of four implants with the following configurations:
To understand how this implant configuration provides an advantage when realizing the Extension implant, consider the illustration in figure #6.
In figure #6 the NMOS Extension implant is oriented at 25˚ from the vertical. However the wafer is twisted counterclockwise to an angle of 335˚. This allows the dopant to more easily reach into the deep micro-canyons form by the tall fins and at the same time maintain a sufficient vertical angle to minimize ricocheting of dopant off of the fin sidewalls. (Notes that for the sake of clarity figure #6 only illustrates one fin being implanted. In fact all of the fins would experience this implant.) The fact that the dopant is approaching the fin from an angle, and not from a direction that is orthogonal to the fin, is the central advantage of this methodology.
Significant shadowing will occur due to the angle of the implant in relation to the tall Gate Electrode structures, but this issue will be taken care of in the second part of this implant (refer to figure #7).
Figure #7 depicts phase two of the four-part NMOS Extension implant. The implant tilt angle is still 25˚, but the wafer has been twisted clockwise to an angle of 25˚. This implant will compensate for any Gate shadowing that occurred in phase one of the implant and completes the implantation of this side of the fins.
Figures #8 and #9 illustrate the opposite sides of the fins experiencing the phases three and four of the Extension implant. The wafers are twisted to angles of 155˚ and 205˚ respectively.
In figures #8 and #9 the wafer is tilted to -25˚ from the vertical and so the dopant is being implanted from the opposite side of the fin. This is a two-phase implant and the implant angle and the two different twists ensure that the opposite sides of the fins are adequately doped and that any shadowing caused by the proximity of the tall Gate Electrodes is minimized or eliminated.
This same four-step process would be repeated for the PMOS fins.
This process ensures that adequate dopant is implanted into the sidewalls of the fins during the Extension implant with a minimum of shadowing by exploiting the fact that all of the fins form straight, parallel lines and are implanted at a twist angle.
It does involve a slightly more complex process of four implants instead of two for each set of fins, but all four implants could be processed sequentially in the implanter, so the increase in cycle time would be minimal.
Information on this topic and detailed information on the entire process flows for the 10/7/5nm nodes will be presented at the course “Advanced CMOS Technology 2019” to be held on May 22, 23, 24 in Milpitas California.