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Control of HSG-Si Fabrication Using Film and Surface Technologies

by Clive Hayzelden, Senior Technical Marketing Manager; Albert Bivas, Technical Marketing Manager; Carlos L. Ygartua, Process Module Manager; Kin-Chung Chan, Senior Applications Engineer; Jason Schneir, Product Marketing Manager

The fabrication of hemispherical-grained silicon (HSG-Si) was developed to increase the surface area of capacitor plates and consequently the storage capacitance of high-density dynamic random access memory (DRAM) devices. The increase in surface area (typically 1.8–2.4 times, as compared with smooth polysilicon electrode plates) is extremely sensitive to processing conditions (e.g., seeding and annealing temperatures). Tight in-line process control is, therefore, essential to obtain high yields.

In this article, a KLA-Tencor UV-1250SE spectroscopic ellipsometer was used to measure both the film thickness and the optical properties of seven HSG-Si films fabricated using a range of seeding and anneal temperatures.

Capacitor fabrication was completed by the deposition of a dielectric film on top of the HSG-Si followed by a top polysilicon electrode. We report a strong linear correlation between the HSG-Si film thickness and the completed device capacitance. Additional insight into the discontinuous surface structure of HSG-Si films was provided by high resolution profilometry using a KLA-Tencor HRP-220.

Wafer fabrication

In a typical fabrication of HSG-Si films, a layer of oxide (SiO2) is first deposited on a crystalline silicon (c-Si) substrate. A capacitor plate (or storage electrode) that consists of a layer of doped amorphous silicon is formed on this oxide by low-pressure chemical vapor deposition. Silicon microcrystals – seeds – are then grown from the gaseous phase on the amorphous silicon layer. The wafer is finally annealed in order to grow the amorphous silicon HSG-Si layer using the seeds as nucleation sites. During the 24

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annealing process, which occurs under high vacuum, the HSG-Si seeds grow at the expense of the underlying amorphous Si layer to yield the characteristically rough surface. During annealing, the amorphous Si layer becomes partially crystallized (and is hereafter referred to as polysilicon).

HSG surface topography and film parameters

The cross-section of a typical HSG-Si film stack is represented schematically in figure 1. The HSG-Si layer is composed of islands or “grains” of silicon, and can be described by the mean grain diameter, height and the number of grains per unit surface area. The underlying polysilicon layer is assumed to be smooth.

Figure 2 is a topographic image obtained from an HSG-Si wafer using the profilometer. The area analyzed is 1 x 1 µm2. The maximum grain height is approximately 1000 Å.

HSG-Si Silicon (poly/amorphous) Oxide Silicon (crystaline) Figure 1. Schematic: cross-section of a typical HSG-Si film stack.

F For typical film stacks – composed of homogeneous layers with smooth interfaces – the optical parameters of a film layer are the thickness, t, the refractive index,

Figure 2. Topographic image obtained from an HSG-Si wafer using an HRP-220.

n, and the extinction coefficient, k. The parameters for the film substrate are its refractive index and extinction coefficient. Since n and k depend on the wavelength in a way that is characteristic of the material (a property known as dispersion), it is important to measure the optical properties of the film stack over a broad wavelength range. The UV-1250SE uses broadband light in the wavelength range 240-800 nm.

Spectroscopic ellipsometry (SE) measures the polarization of the light reflected from the surface of a wafer.

This technique has been widely adopted for the nondestructive determination of the thickness and optical parameters of both single- and multi-layer thin film stacks.

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The SE optical path is represented schematically in figure 3. The wafer is illuminated using linearly polarized light at a large angle of incidence. The reflected light is elliptically polarized and its polarization state is analyzed over a selected wavelength range. In this analysis we utilized the wavelength range 320-800 nm. This wavelength range included spectral information from the n and k peaks at 372 nm, while avoiding the effects of scattering and absorption by the HSG-Si layer in the deeper part of the ultra-violet part of the spectrum.

The SE measurement provides the experimental spectra, tanΨe(λ) and cos∆e(λ). Theoretical ellipsometry equations, tanΨt(λ) and cos∆t(λ), represent the expected reflected light polarization for a given set of film stack parameters.

TanΨ and cos∆ are derived from the complex electrical field reflection coefficients, Rp and Rs, of the p and s polarization components of the reflected light, and from the phase difference, ∆, between these two components by the equation: Rp tanΨe • exp(i∆) = (1) cos∆ = Re (exp(i∆)) Rs The quality of the spectral fit (goodness of fit or GOF) based upon the difference between these spectra is provided by the system.

Because the HSG-Si layer is discontinuous, the spectroscopic ellipsometric analysis reports “effective” values of t, n and k for this layer. For the polysilicon layer, both the thickness and degree of crystallinity were determined.

Spectroscopic ellipsometry

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Dielectric

Polysilicon

Polysilicon HSG Figure 4. Schematic: deposition of a dielectric layer and polysilicon top electrode on HSG-Si.

Dispersion models

A good fit between experimental and theoretical spectra requires knowing or calculating the values of n and k at all the individual wavelengths in the spectra. As this is not practically possible, continuous approximation models – with a limited number of variables – are developed to describe the dispersion of the different materials that constitute the film stack. During the calculations, n and k are fitted at the same time as the film thickness until the best fit is obtained.

Figure 3. Spectroscopic ellipsometr y optical path schematic.

The simplest physical model for the n and k dispersions is the harmonic oscillator, which is based on the solution for the dipole moment for a harmonically bound Spring 1999

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variable parameters. The thickness of the oxide was fixed at 1000 Å. A standard n and k table, with no variable parameters, was used for the Si substrate.

Results

We analyzed each of the seven wafers at five sites using SE. The thicknesses of the HSG-Si layer and underlying polysilicon layer were calculated, along with the crystallinity of the polysilicon. After the optical measurements, the wafers were processed further to create capacitors by first depositing a dielectric layer then a polysilicon top electrode on top of the HSG-Si (shown schematically in figure 4). The capacitance was then measured at the same sites that were characterized using SE. An example of tanΨ(λ) and cos∆(λ) spectra for one of the HSG-Si wafers (wafer #3 at site #1) is presented in figure 5. The corresponding dispersion plots for n and k are shown in figure 6.

The calculation method and dispersion models worked well for the entire range of process conditions and the results are summarized in table 1.

Table 1 shows the average capacitance enhancement values (i.e., the ratio of the capacitance with an HSG-Si layer to that of capacitors with a flat electrode). A linear correlation (>95 percent), with a reasonable sensitivity (slope), was found between the HSG-Si thickness and the capacitance enhancement, as shown in figure 7. The calculated percentages of amorphous silicon and voids in the polysilicon layer indicate that this layer was approximately 80 percent crystallized during the growth of HSG-Si. It was also found that wafer #6 had been misprocessed as evidenced by abnormal polysilicon thickness, crystallinity and optical properties.

Figure 5. TanΨ(λ) and cos∆(λ) spectra for an HSG-Si wafer.

electron acted upon by an electromagnetic field. This model was used to represent the effective dispersion of the HSG-Si. The Bruggeman effective medium approximation (BEMA) model represents the film material as a mixture of several components, each defined by a table of n and k values as a function of wavelength. In the calculations, the volume fraction of each component in the mixture is obtained. This model, which combined amorphous Si, crystalline Si and voids, was used to represent the polysilicon layer and account for a wide range of crystallinity.

Conclusion

The thermal silicon dioxide beneath the polysilicon was modeled using a standard n and k table, with no

The characterization of HSG-Si wafers is particularly challenging because the top layer is a rough and discontinuous film with a large sensitivity to process changes.

wafer

HSG t(Å)

1 2 3 4 5 6 7

501.6 362.8 498.2 313.1 448.0 521.5 475.3

HSG Polysilicon n@633 nm t(Å) 2.059 1.913 2.159 1.953 1.904 2.036 1.944

1031.2 1041.3 1037.3 1049.9 1053.5 883.6 1037.2

Polysilicon n@633 nm

Polysilicon % crystallinity

Goodness of fit

Capacitance enhancement

3.488 3.626 3.436 3.676 3.501 4.299 3.534

0.799 0.802 0.786 0.789 0.827 0.578 0.807

0.9814 0.9829 0.9830 0.9840 0.9753 0.9806 0.9803

2.120 1.570 2.102 1.473 1.937 2.032 1.958

Table 1. Summar y of spectroscopic ellipsometr y results for the HSG-Si, underlying polysilicon layer, and capacitance enhancement values for the seven wafers.

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Figure 7. Linear correlation between HSG-Si thickness and the capacitance enhancement.

Figure 6. Dispersion plots for n and k for HSG-Si.

High resolution profilometry is able to assess grain size and spatial distribution of the grains. In this study we demonstrated that spectroscopic ellipsometry can be used to characterize both the HSG-Si and underlying polysilicon layers. A direct correlation between the thickness of the HSG-Si layer and the capacitance enhancement of the fabricated devices was shown. The degree of crystallinity of the bottom electrode was about 80 percent. These two results can be used in production to monitor the fabrication process and to predict DRAM capacitor performance. circle RS#002 + 005

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Visit our website at www.kla-tencor.com, or call 408-875-2098. circle RS#005