Eric L. Raab

Use of trilevel resists for high-resolution soft-x-ray projection lithography

A projection optical system with 20:1 reduction has been used with radiation at ∼36 nm to evaluate resists for use in soft‐x‐ray projection lithography. The high absorption of soft x rays by carbon‐based polymers requires that an imaging resist layer be very thin. The sensitivities and contrasts of several such resists are reported. By incorporating a thin imaging layer into a trilayer resist scheme, we have exposed, developed, and transferred features as small as 0.2 μm into silicon.

Soft-x-ray projection lithography : printing of 0.2-μm features using a 20 : 1 reduction

We demonstrate nearly diffraction-limited printing of 0.2-microm features, using soft x rays of approximately 36-nm wavelength. An open-stencil transmission mask with minimum features of 4 microm was imaged by a twentyfold-reduction Schwarzschild-type objective onto silicon wafers coated with various e-beam resists. Implications for soft-x-ray projection lithography are discussed.

Construction and test of phase zone plates for x-ray microscopy

We report the construction and test of an x-ray phase zone plate with 0.1-microm imaging resolution and >14% diffraction efficiency. The zone plate served as the focusing optic in a scanning microscope that operated in the water window, 2.5 nm < lambda < 4.5 nm, and therefore is optimally suited for imaging wet biological specimens. We enhanced the efficiency of the zone plate by using germanium for the absorbing zones, so that some of the incident radiation is transmitted with the correct phase shift to interfere constructively at the focal spot. The zone plate was supported on a relatively transparent membrane, giving the microscope a high absolute throughput.

Analyzing deep-UV lens aberrations using aerial image and latent image metrologies

The technique of direct aerial image metrology (AIM) has been applied to characterize the performance of a microlithographic lens. AIM is potentially faster and more reproducible than measurements obtained by scanning electron microscopy. Direct measurement of the aerial image eliminates both the process variations associated with resist processing as well as the subjective nature of evaluating resist profiles. We have used AIM to evaluate some of the primary aberrations of a 248 nm stepper lens. We compare the results to those obtained with latent image scatterometry, a proven technique for measuring lens performance. We found that AIM, while providing qualitatively good results, contained some slight systematic errors that reduced the accuracy of the data. The sources of error and their remedies are discussed.

20:1 projection soft x-ray lithography using trilevel resist

We demonstrate nearly diffraction limited printing using soft x-ray radiation of approximately 36 and 14 nm wavelength. As an imaging system we used a-20X-reduction Schwarzschild-type objective with iridium coated mirrors for use at 36 nm and Mo/Si multilayer coated mirrors for use at 14 nm. An off-axis aperture and illumination were used to eliminate the central obscuration in the imaging system caused by the primary mirror. Two types of masks were used for exposures: an open-stencil one for 36 nm and a silicon membrane with a Ge absorber for 14 nm. The high absorption of carbonbased polymers at these wavelengths requires that imaging resist be very thin. Thin resist layers are not robust and, by themselves, not very useful for processing. By incorporating them into a tn-level resist, however, we have exposed, developed, and transferred features as small as 0.1 jm into silicon.

X-ray imaging at the diffraction limit

We tested the ability of normal-incidence, multilayer-coated x-ray optics to produce diffraction-limited images. The imaging performance at 14 nm of a spherical mirror substrate coated with a single metal layer was compared with that of the same substrate coated with a Mo–Si multilayer. A knife-edge test was used to quantify the image aberrations. The knife-edge data were fitted with a model based on Fresnel diffraction theory. We found that the multilayer coating introduced a measurable but small figure error into the optic; the change in figure was less than λ/16 peak to valley. We conclude that a properly deposited multilayer coating will not degrade the imaging performance of an optical system designed to operate at the diffraction limit.

Soft x-ray projection lithography: experiments and practical printers

The feasibility of using X-ray projection cameras as a practical lithography tool for making integrated circuits with tenth-micron features was investigated in experiments performed with a 20-fold reduction Schwarzschild camera operating at 36 nm and 14 nm, and with a 1:1 magnification Offner ring field system at 40 nm. The paper examines the requirements on the resist, the source, the camera design and the fabrication of its mirrors, the mask, and the alignment system. The experiments proved that high-reflectance multilayer mirrors are capable of diffraction limited imaging. Some problems exposed by the experiments, such as the deposition of carbon on surfaces exposed to X-rays, are discussed.

Impact of optical thin film effects on CD control in DUV lithography

We examine the effects of optical thin films on CD control in DUV lithography. A number of different substrates typical of device lithography levels were fabricated. Film stack configurations were chosen for their optical properties as well as process integration compatibility. All wafers were then exposed with an open field to measure swing amplitude, followed by a resolution test pattern to measure proximity effect and dose latitude. The resolution test pattern consisted of 250 nm coded features varying in pitch from 500 to 1750 nm. Results will be presented for optimum CD control over four types of film stacks as a function of resist thickness. In general, it is always best to operate at an extremum of the swing curve in resist thickness. However, the underlying film stack effects the CD control differently at Emin versus Emax. We present the optimum exposure conditions for a number of film stacks, resist thickness, and types of illumination (conventional and annular). These results were corroborated by optical imaging software enhanced to incorporate resist thickness and thin film effects. A synopsis of the role of substrate optical properties on CD control will be presented.

Performance of multilayer-coated figured optics for soft x-rays near the diffraction limit

The imaging performance of a spherical mirror was measured at 14nm. A Foucault knife-edge test was used to determine the focal spot size formed by a 400 mm radius mirror at a numerical aperture of 0.04. The image formed by the mirror is compared with a single-layer AuPd coating is compared with that formed when coated with a Mo/Si multilayer. By fitting the results with a diffraction model, it is possible to quantify the change in effective surface figure caused by the multilayer. It is concluded that a multilayer-coated mirror can perform at the diffraction limit under certain conditions.

Experiments in projection lithography using soft x-ray

Abstract We have demonstrated soft x-ray projection lithography using radiation at wavelengths of 14 nm and 37 nm with a commercially available 20X reduction Schwarzschild camera. Line widths as small as 0.05 microns have been printed. The resolution obtained was essentially diffraction limited. Iridium coated mirrors were used with 37 nm radiation and Mo/Si multilayer coated mirrors with 14 nm radiation. A 1:1 magnification Offner Ring-field system with iridium coated mirrors has been used with 42 nm radiation. This optic has imaged line widths as small as 0.2 microns, which is close to the diffraction limit for this system. Transmission masks were used for all these experiments and the radiation was obtained from an undulator in the Vacuum Ultraviolet Synchrotron Storage Ring at Brookhaven National Laboratory.