Μm Contact Holes Research Articles

DUV lithography for 0.35 μm CMOS processing

The performance of positive and negative tone resists on the critical levels of a 0.35 μm CMOS process has been evaluated. The use of a darkfield reticle suppresses reflections in lens and resist. Therefore reflection problems are reduced for poly gate patterning with a negative tone resist. If a positive resist is used it should be combined with a bottom ARC. For the contact hole level a positive tone resist is required. Good performance, including batch uniformity is obtained for 0.5 μm holes. For 0.4 μm contact holes, latitudes needed to be improved and this has been realised by either the combination of mask biasing and focus drilling or the use of an attenuated phase shifting reticle. For metal patterning, again positive and negative resists have been studied. An important issue here is the interaction between the acid formed upon exposure and the underlying TiN layer.

Improvement of the Interface between Selectively Deposited Aluminum and Silicon by Annealing

The interaction between Al deposited selectively by chemical vapor deposition (CVD) and Si has been investigated. The structure of Al depended on the size of contact holes. Single-crystalline Al was formed in 0.7 µm contact holes, but polycrystalline Al in 1.2 µm and 2.0 µm holes. In the case of single-crystalline Al, Si at the bottom of contact holes was eroded uniformly by Al after deposition and after annealing at 400° C. In the case of polycrystalline Al, Si was eroded non uniformly and Al spikes were formed. However, after annealing at 400° C, Al spikes disappeared and the surface of eroded Si became extremely flat. Junction leakage current of the 2.0 µm n+ contact was found to increase after Al deposition. However, after annealing at 400° C, increase of junction leakage current was not found to 10 V. This change may be due to the improvement of the interface. These results indicate that selective Al-CVD can be used as a contact-hole-filling method.

Cross shaped pattern on chrome mask for 0.5 μm contact hole fabrication

Abstract This paper presents a cross shaped pattern on a chrome mask for the fabrication of 0.5 μm contact hole. Based on simulation, in comparison to the conventional 0.5 μm square mask, the contrast of aerial image of this mask gained 1.41% at no defocus; 8.81% at defocus 1.5 μm. Exposure latitude gained 22.8% under no defocus. From the experimental pattern transfer studies on positive tone photoresist using i-line 5X stepper, this mask has an useful focus range of -1.2 ∼ +1.2 μm while conventional has a range of -0.6 ∼ +0.9 μm. The drawback is that exposure dose needed is 1.2∼1.5 times higher than conventional mask in our study.

Process issue improvement of surface image transfer technique: Depth-of-focus characteristics and their comparison with simulation results

There are several disadvantages concerning surface image transfer technique for contact hole fabrication in KrF excimer laser lithography utilizing absorbing photoresist SAL601 (Shipley, chemically amplified negative type). A long development time (MF622, Shipley, 10 min) is the first disadvantage, but this can be improved by using concentrated developer (MF312-CD38, Shipley) without loss of focus margin. Pattern transfer thicknesses limited to an oxide layer of less than 0.2 μm is the second disadvantage, but this can be improved to a thickness of 0.8 μm by using more anisotropic etching conditions. Small depth of focus (DOF) is the third and the most critical disadvantage, and this can be improved by employing low numerical aperture (NA) and high σ (coherency factor) exposure conditions (NA=0.35, σ=0.9). Comparison between the DOF characteristics for a 0.4 μm contact hole using SAL601 and a two-dimensional photointensity simulation has resulted in a simple metric to predict the DOF by mask edge intensity loss from perfect focus to defocus. Although the correlation between the loss and the DOF is weak, the loss is ∼0.025–0.045. The observed DOF results for 0.35 and 0.32 μm contact holes were in the range of the prediction obtained by the metric except NA=0.45 and σ=0.3 with 0.35 μm contact hole case.

Optimization of aerial image quality

In this study, a metric of aerial image quality for two dimensional images is presented. Modeling is used to predict feature shape. The metric then imposes ‘bounds of acceptable deviation’ on the feature perimeter, and measures the exposure latitude which these bounds allow. The figure of merit is applied to the optimization of mask parameters for x-ray lithography of a 0.25 μm contact hole. The findings of this trial are presented.

Step coverage of tungsten films deposited by germane reduction of WF 6

The step coverage of tungsten films deposited by the germane reduction of WF 6 is investigated. The step coverage modulus for the germane process is influenced only by the deposition temperature. It hardly depends on the partial pressures of the reactive species. These results are confirmed by experiments on structures with a large internal surfaces as compared to the access area. The stepcoverage of the films deposited by this process is excellent and sufficient to fill small (0.5 μm) contact holes with a high aspect ratio.

The SEM in present-day semiconductor FABS

Computers, cars, HDTV, and microwave ovens: what great modern conveniences! Why do these marvels of technology work the way they do? The Scanning Electron Microscope has a lot to do with it. For the last decade, the shrinking geometries of integrated circuits have necessitated the use of the Scanning Electron Microscope (SEM) for inspection and metrology. The current device feature sizes in production are as small as 0.5μm, and current feature sizes under development are 0.25μm or smaller. The semiconductor industry depends on the SEM to tell us if our processes, whether production or developmental, are correct. Neither leading edge fabs nor developmental fabs could function very well without the SEM.The uses of the SEM are many and varied, both inside and outside the fab. Inside the fab, there are many inspection steps to be performed. Although the optical microscope is still a viable option for, and is still routinely used for, in-line wafer inspection, the SEM is indispensable for inspection (Fig. 1) of small geometries such as 0.35μm lines and spaces and 0.5μm contact holes.

Characteristics of an improved chemically amplified deep-ultraviolet positive resist

Chemically amplified positive resist formulations have been shown to exhibit high photospeed, excellent resolution, and tolerance to process parameters such as softbake, exposure, postexposure bake, developer concentration, and temperature. Many chemically amplified positive resists, however, adhered poorly to some substrates (e.g., Si3N4), required considerable optimization of the etch process to achieve desired etch selectivities and were sensitive to airborne basic contaminants. Many chemically amplified negative resists while not as sensitive to contaminants in the clean room air, show retrograde wall angles especially on antireflection coatings, demonstrate poor latitude in defining contact holes and are difficult to strip after pattern transfer steps. In this article we discuss our efforts toward designing new deep-ultraviolet (UV) matrix resins and resist formulations as well as efforts toward defining an optimized process. The optimized resist process demonstrates 0.25 μm line and space (L/S) and 0.30 μm contact hole resolution in 0.8 μm thick resist films with a GCA deep-UV exposure tool. The resist also exhibits excellent adhesion on most semiconductor substrates (e.g., Si, polysilicon, SiO2, Si3N4), thermal stability to at least 140 °C, an order of improvement in postexposure delay latitude over that of CAMP1 (poly t-butoxycarbonyloxystyrene-sulfone formulated with photoacid generators) and etch selectivity comparable to that of novolac based resists. In addition, the polymers developed were designed for ease of manufacture with regard to reproducibility, low metal concentration, and cost.

Diffraction effects in x-ray proximity printing

The influence of diffraction on the shape and size of features printed using x-ray proximity printing with a collimated x-ray source (measured beam divergence of ∼0.2 mrad full width at half-maximum) at mask to wafer gaps of 25 μm and above is described. Three major conclusions can be drawn from the results: (1) Diffraction can distort the shape of a printed feature, and the systematic shape changes observed in resist images can be explained using simple scaling based on Fresnel diffraction; (2) The linewidth change with exposure dose is independent of feature type and size, and depends only on the square root of the mask to wafer gap; and (3) The bias of each printed feature varies when all of the feature types and sizes are printed at the same dose. Resist images of 2.0–0.25 μm contact holes printed at mask to wafer gaps ranging from 25 to 515 μm are presented. The square contact holes on the mask print diamond shaped at a Fresnel number of 2.5. Isolated lines, spaces, and line-space arrays ranging from 1.0 to 0.25 μm were printed in thin PMMA (0.1 μm thick) resist at gaps ranging from 25 to 80 μm over a series of exposure doses. A 10% increase in exposure dose results in a 16, 20, and 30 nm change in linewidth at gaps of 25, 40, 80 μm, respectively. When the 1.0–0.25 μm lines, spaces, and line-space arrays are printed with a single dose at a 40 μm gap, the bias variation among the features appears to be approximately ±20 nm. However, this bias measurement is very sensitive to the metrology technique used, and systematic errors in these absolute measurements may be a significant contributor to the observed bias variation. All the experimental results are compared to calculated aerial images. Exposures in thick PMMA (0.5 μm) show trends similar to those observed in thin resist. However, as the thickness of the resist increases, the dose at which the printed linewidth has zero bias decreases, and approaches the dose required to clear a large open area.

High-performance electron beam lithography for 0.5 μm semiconductor device fabrication

High-performance electron beam (e-beam) lithography is applied for fabricating contact holes with submicron size, using a new trilayer resist process. This technology consists of proximity effect correction software and the H+ ion shower irradiation technique onto a trilayer resist. The proximity effect correction software based on a dose modulation method was developed to control the dimensional accuracy of submicron patterns, by using the calculation of the double Gaussian exposure intensity distribution (EID) function. The dimensional accuracy of 0.5 μm contact hole and line and space patterns were kept within 0.05 μm. The H+ ion shower irradiation technique was adopted to decrease the charging problem. Ion shower was irradiated onto the bottom layer of the trilayer resist at 40 keV, 1×1016 ions/cm2. The sheet resistance of the bottom layer resist was decreased from 1013 Ω/⧠ to 107 Ω/⧠ by the H+ ion shower irradiation. The field butting error due to the charging problem was improved to 0.1 μm/3σ, and also the overlay accuracy was improved to 0.12 μm/3σ. When this high performance e-beam lithography was applied to fabricate 0.5 μm size contact holes, the yield of the 106 contact hole chains was improved to 90% by using selectively deposited W film on the bottom of the contact holes.

Submicrometer contact hole delineation with a two-layer deep-UV portable conformable masking system

The two-layer deep-UV portable conformable masking (PCM) system using a novolac resist as the imaging layer on a PMMA planarizing layer was chosen to delineate 1.0×1.25 μm contact holes with a 0.32 NA 405 nm 5× projection lens to support 1 μm MOS circuit fabrication. Measured SiO2 images, delineated with CF4+H2 reactive ion etching, exhibited insignificant biases and small size variations for critical features down to 0.87 ×0.87 μm over typical wafer topography when a near-UV dye was incorporated in the planarizing layer. A comparison of the dyed and undyed deep-UV PCM and a single-layer soaked process will also be given.