Oxide thickness variation

To measure the final oxide thickness in the PMD process, the measurement sites can be set up over field oxide or over the polysilicon interconnections (see Fig. 6). However, since there are fewer variables in measuring over the field oxide, and the field oxide process is relatively well established, the PMD thickness is more accurately measured over the field oxide. If the measurement is over the polysilicon, the resultant PMD thickness measured can be affected by the deposited polysilicon thickness, the polysilicon doping, and the field oxide thickness variation, while if the measurement is over the field, the PMD thickness is affected only by the field oxide thiekness. 

D. Nicolas-Chaubet, C. Haut, C. Picard, F. Millot, A M. Huntz. Linear weight gain and parabolic oxide thickness variations vs. oxidation time The signature of diffusion along two dimensions in A1203 scale formed on 13-NiAl // Mater. Sci.Eng. A.- 1989.-V. 120.-P.83-89. 

The advantage of self-stop is the controllability of the post-planarization film thickness. In CMP case, thicker film than the initial step needs to be removed because of poor ability of CMP planarization. But the oxide removal rate has the within wafer and wafer to wafer non-uniformity. And these non-uniformity affects the post-CMP film thickness variation. On the other hand, the grinding has excellent planarization capability and self-stop phenomenon. The self-stop only remove the oxide on upper area and automatically stops oxide removal after the wafer surface becomes planar. So the grinding does not cause the oxide thickness variation and only the deposition cause the oxide thickness variation as shown in Fig.9. This controllability of postplanarization film thickness is very helpful for the semiconductor manufacturing. In ILD planarization, via-contact depth is not affected by planarization and this helps etch process and reliability of via-contacts. Actually the grinding oxide removal is thickness... 

The pad stack determines the bending of the top pad. Two extreme case are considered perfect pad bending and no pad bending. A typical example for perfect pad bending is when a thin top pad is put on a soft bottom pad. In this case the top pad bends easily and it was concluded that the average pressure on an unit area is equal to the nominal pressure. This results in a large oxide thickness variation after CMP. In the case of a thick top pad on a soft top pad the top pad does not bend. This introduces bulk deformations in the top pad. In this case the oxide thickness variation after CMP is limited. The experiments confirm that a real stacked pad is always in between these two extreme cases. 

Figure 2. Die-level oxide thickness variation remaining after ILD CMP, resulting from topography arising from pattern dependencies in the underlying metalization.

The modeling and characterization of pattern dependent variations in oxide CMP further demonstrate the importance of these effects in the design of a viable CMP process. Given the magnitude of the oxide thickness variation present after planarization in comparison to the variation across the wafer as shown in Fig. 9, it is clear that process optimization should be driven by the die-level variation as much as if not more so than by the wafer-level variation. 

FIGURE 5,56. Rate of oxide thickness variation during current oscillation in 0.86M NH4CI + 0.1 M NH4F + 0.04M NH4AC + 0.06 M HAc at pH 4.5 at 7 V,e. Solid line measured variation of oxide film. Curve a oxide generation rate. Curve b oxide dissolution rate. Curve c sum of contributions a and b. After Chazalviel et al (Reproduced by permission of The Electrochemical Society, Inc.)... 

Figure 2.54 Photocurrent oscillations for n-Si(lll) at 4 = -r6V in 0.1 M NH F, pH 4 (top) and oxide thickness variation determined by in situ Fourier transform...

The second definition of the purpose of a carrier was to remove the overburden of material above a surface above the device plane. For present purposes, we define the device level as the boundary between the material one wishes to remove and the material one wants to keep. It is not necessarily planar, and it moves up with each layer. For oxide CMP, this layer lies within the topmost film layer. For metal CMP, this surface is defined by the topmost surface of the dielectric into which lines and vias are etched for a damascene process. This definition must accommodate a wafer with a modest amount of bow, tilt, warp, and total thickness variation. Furthermore, it must accommodate very modest amounts of bow, warp, tilt. 

The different polish characteristics of each of the traces demonstrate the significance of layout pattern dependencies in oxide CMP. For example, the L3 profile traces the large step density transitions with dramatic variations in the resulting oxide thickness. In contrast, the blocks along L4 polish at the same rate, underscoring the fact that layout pitch is not a first-order determinant of the polish rate. This is the case for a very large range of pitch... 

The wafer-to-wafer thickness nonuniformity (WTWNU) in oxide CMP can be attributed primarily to variation in the CMP polish rate, and secondarily to the thickness variation in incoming wafers. 

For studying supported catalysts, TEM is the commonly applied form of electron microscopy, and today images as shown in Figure 7.1 are obtained routinely. In general, the detection of supported particles is possible provided that there is sufficient contrast between the particles and the support. This may impede applications of TEM on well-dispersed supported oxides [11]. Contrast in the transmission mode is caused not only by the attenuation of electrons due to density and thickness variations over the sample, but also by diffraction and interference. For example, a particle in a TEM image may show less contrast than other identical... 

The color of silicon dioxide is a function of its thickness. Color variation across the dies and the wafer indicates oxide thickness nonuniformity. Ideally, there should be no color variation over the same type of underneath structures. Global color variation across the wafer indicates a CMP uniformity problem. Local color variation at a structure level or arrays of structures within a die reveals a lack of planarization. There is a fine difference between planarization deficiency and nonuniformity. Nonuniformity is revealed in the form of a very gradual thickness variation over 10 mm or more. It is usually not pattern dependent. A rapid thickness variation across arrays of structures less than 5 mm wide is indicative of a planarization problem. This is a pattern-dependent... 

In addition to the nonuniformity in the oxide polish rate, non-uniformity in the oxide deposition process leads to variations in the final thickness of the oxide. Deposition nonuniformities are compounded by the fact that thick oxides must be deposited prior to CMP. For example, if the oxide CMP process must remove 5(X) nm in order to planarize the surface, with a final target oxide thickness of 500 nm, a 1 pm thick film must be deposited. A 10% variation in film deposition rates transfers to a final thickness variation of 100 nm or 20% of the final film thickness (assuming no variation in CMP rate across the wafer). Alternatively, if the CMP process must remove 1 pm of oxide in order to planarize, the deposited oxide must be 1.5 pm thick. The same 10% variation in film deposition rates now results in a 150 nm thickness variation or 30% of the final thickness. Thus, the CMP process affects the final oxide thickness uniformity by virtue of the planarization rate as well as polish rate uniformity. This nonuniformity is a second reason that ceria-based slurries discussed in Section 5.1.3 are undesirable. 

During the oscillation the thickness of the oxide film on the silicon surface varies periodically, the frequency of which coincides with the associated current oscillation. Figure 5.55, for example, shows the variation of oxide thickness, about 2 nm in amplitude (or about 25% of the average thickness), during the photocurrent oscillation of n-type silicon at TVs e in a solution of 0.1 M [F and pH 4.4. ° The anodic current is... 

However, this coverage was not dependent on oxide thickness or reaction time. We feel these values still correspond to mono-layer coverage, with the variation being due to small differences in surface area. This coverage is quite small when compared with the observation of up to 60 molecular layers in the reaction of Pt/PtO with DCSF (10). 

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