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Micro/Nano Lithography

Controlling CD

As feature sizes shrink, critical dimension control becomes increasingly important in chip manufacturing

From oemagazine March 2005
28 March 2005, SPIE Newsroom. DOI: 10.1117/2.5200503.0004

The characteristic feature size of integrated circuits (ICs) has been shrinking for more than two decades. Indeed, 25 years ago, few believed that the minimum feature size would go below 0.5 µm; now, feature sizes have reached 65 nm and continue to shrink. With decreasing feature size, control of linewidth (critical dimension (CD)) in the exposed pattern becomes more and more important if the IC is to perform as designed. The accepted tolerance for CD is roughly 10%; for example, 60-nm features must be controlled to within ±6 nm.

Optical lithography typically uses step-and-scan systems in which a laser source illuminates a rectangular section of a mask of the IC pattern (also called a reticle). The pattern is imaged by a projection lens with a 1:4 reduction ratio to expose a photoresist-coated wafer. During exposure, the reticle and the wafer are scanned in opposite directions relative to the lens until one shot of exposure is completed. The scanning motion moves the slit area over the entire reticle to expose the whole pattern, with the reticle scan moving four times as fast as the wafer scan due to the 4X magnification. The scanner then steps the wafer to the next exposure area and repeats the exposure. This cycle continues until the full area of the wafer is exposed.

Across-chip linewidth variation (ACLV), widely called CD uniformity, is one of the most appropriate parameters by which to measure the total performance of the stepper and scanner. Errors introduced by each unit of the tool worsen CD uniformity. The main contributors to CD error are the tool body, including the reticle and wafer stages; the projection lens; and the illumination system.

Mechanical Performance

Synchronization of reticle scanning and wafer scanning is the critical body-performance parameter for effective CD control. Both the wafer stage and reticle stage float on air bearings and move with linear motors. High-throughput performance requires very-high scanning speed; for example, our most advanced scanner features a wafer scanning speed of more than 500 mm/s and a reticle scanning speed of four times that (2.0 m/s). During scanning, the lateral position of the stage is continuously controlled to an on-wafer accuracy of less than 10 nm.


Figure 1. This reticle stage design reduces vibration by eliminating the tubes and cables attached to the slider and using a counter mass to counteract stage motion.

Ensuring such high accuracy control requires reducing vibration error. Ironically, the moving stages constitute the primary sources of the vibration. To minimize vibration, we use a counter mass to reduce the force conveyed from the stage to the body (see figure 1). The counter mass, which floats on air bearings, is attached to a linear motor guide (stator). When the linear motor positions the moving stage (a reticle slider), the reaction force transfers to the linear motor guide. Since the guide is attached to the counter mass, the reaction force is channeled to move the counter mass in the opposite direction; thus, no force goes out from the stage unit. The same concept is used for the wafer stage.

Another issue for vibration control is the existence of cables and tubes attached to the moving portion. A conventional reticle stage features vacuum lines for reticle chucking, electronic cables for fine-positioning schemes like voice coil motors, and so on. These cables and tubes generate small but very complicated forces and degrade the positioning accuracy of the reticle.

The newest reticle stage solves this vibration issue by removing all cables and tubes from the reticle slider. An innovative technique for vacuum supply to the slider allows operation with no vacuum tubes, which lightens the slider. The reduced mass enables the stage to perform fine positioning without any additional scheme, which eliminates the need for the additional cable.

Optical Performance

We can also improve CD uniformity by reducing optical aberrations in the projection lens. Resolution (minimum resolution half pitch) of the projection lens is given by

R = k1 λ/NA

where λ is the wavelength of the light source, NA is the numerical aperture of the projection lens, and k1 is a constant known as the process factor. To obtain smaller resolution, the wavelength should be shorter, the NA should be higher, and the k1 factor should be smaller. The most advanced Nikon scanner has a wavelength of 193 nm (from an argon-fluoride (ArF) excimer laser), an NA of 0.92, and a k1 factor of 0.26 to 0.30. Note that while reducing the k1 factor increases resolution, it also increases the negative impact of aberrations on CD uniformity.

For best CD uniformity, a projection lens should be as perfect as possible. Though increasing NA itself is a challenge for optics, required lens performance improvement is another challenge; we are solving both problems simultaneously. The aberration level of the current Nikon projection lens is about 1/30 of that of the Nikon's first g-line stepper lens from 25 years ago, even though it has more than twice the NA. Our most advanced projection lens has an NA of 0.92 with a wavefront aberration of only 0.006λ rms (1 nm rms). The design achieves this performance with the use of aspherical surfaces and optomechanical mounts that introduce the lowest possible deformations.

Illumination Performance

A third means of improving CD uniformity is via polarization control of the illuminator. A laser source used in a typical stepper produces linearly polarized light that is de-polarized for illumination purposes. Half of the intensity of the light consists of s-polarized light, which increases image contrast, and the other half is p-polarized light, which tends to degrade image contrast because of the vector imaging effect.1 As the NA of the lens increases, the adverse effect of p-polarized light tends to increase even if no aberrations exist.

To achieve a high-contrast image, we restrict our illumination light to s-polarized light by converting the laser output to s-polarized light directly with an optical element. In the case of Nikon's illumination system, there is no direct energy loss due to the generation of polarized light. The transmittance of the main optics of the scanner, such as the projection lens, has some dependency on polarization. There may, therefore, be a several-percent energy loss on the wafer. Since the birefringence of the projection lens is well controlled, there is little wavefront error due to polarization. The resulting high-contrast image is robust against various kinds of errors, such as dose error and mask-pattern-width error. The image is thus less sensitive to lens effects like changes in best focus across the lens, and will therefore yield better CD uniformity.

Figure 2: A series of 60-nm line-and-space photoresist images exposed by a 0.92 NA ArF scanner with dipole illumination show that the image formed with s-polarized illumination (top) has a 25% wider depth of focus than the image formed with de-polarized illumination (bottom).

With the development of a polarized illumination function, we have succeeded in changing illumination light into s-polarized light that boosts image contrast without losing exposure power. Experiments with this s-polarized illumination imaging, which used a 0.85 NA ArF scanner (NSR-S307E), confirm 20% improvement in dose latitude and about 20% mask error enhancement factor (MEEF) improvement when compared to conventional de-polarized illumination. A comparison between exposures using de-polarized illumination and s-polarized illumination at 0.92 NA on our ArF exposure tool (NSR-S308F) show about a 25% depth-of-focus improvement for the s-polarized case, meaning that focus margin is improved by 25% (see figure 2). This not only improves overall process latitude but improves CD uniformity. We estimate that CD uniformity can be improved by as much as 20% by using polarized illumination.

The use of polarized illumination makes birefringence a factor in optical performance. Such illumination requires birefringence control for all of the optical components in the system, from the laser output coupler to the wafer. Of course, the tool manufacturer takes responsibility for birefringence in the illumination and projection systems, but tool users should consider the birefringence performance of their reticles. With de-polarized illumination, reticle birefringence does not affect imaging performance. For a polarized illumination system, however, birefringence of a reticle may change the polarization status of the illumination light, and the distribution of the birefringence of the reticle directly affects CD uniformity.

In birefringence maps, the length of the small lines indicates the amount of birefringence at the measurement position and the azimuthal angle of the lines indicates the axis. Conventional reticle blanks have about 10 nm of retardation. In contrast, a new annealed blank designed for polarized illumination has less than 1 nm of retardation, low enough to make an insignificant contribution to CD uniformity error. For best performance, we recommend birefringence levels of less than 0.8 nm for isolated patterns and less than 1.3 nm for dense patterns.

Linewidth roughness (LWR), a kind of very localized CD uniformity on a single line, is a new topic for CD control. This may be a resist-performance-oriented issue; however, exposure tool improvement using s-polarized illumination also can reduce the amount of LWR because of corresponding increases in image contrast. Tests with a 0.92 NA, s-polarized light system have shown an improvement of approximately 30% in LWR.

In conjunction with immersion technology, deep UV tools should remain in use for critical layers until at least the half-pitch 45-nm node and below. CD control continues to be a challenge and requires ongoing technology improvement for each tool. Bridging the region between different tools like the exposure tool and wafer truck may yield many more opportunities to improve CD control. One field of technology may easily solve very difficult issues for another, so collaboration and cooperation between different fields are becoming ever more important. oe

References

  1. T. Brunner et al, Proc. SPIE 4691, p.1 (2002).

Toshikazu Umatate, Tomoyuki Matsuyama
Toshikazu Umatate is executive headquarters staff and Tomoyuki Matsuyama is junior executive staff, Optical Design Department, Development Headquarters, Nikon Corp., Kumagaya, Saitama, Japan.