Isamu Funahashi, Takuya Nagafuchi, Bipin Parekh, Mykrolis Corporation
Introduction
The complexity of semiconductor device manufacturing processes is increasing as scaling of ICs continues with shrinking feature sizes. With this complexity, contamination control of nanosize particles is increasingly becoming more important during fabrication. The ITRS guidelines in Table 1 show the stringent purity requirements for liquid chemicals to be used in the manufacture of next-generation semiconductors.1
As semiconductor device design rules become more constrained, controlling yield-reducing contaminants in the wet-etch and cleaning process chemicals becomes more critical. An optimally designed filtration process can prevent wafer recontamination from the chemical delivery and cleaning systems in the fab. The filters should be designed to efficiently remove nanoparticles from the cleaning chemicals and also provide high liquid flows at low differential pressures to meet the needs of expanding wafer size from 200 mm to 300 mm.
Importance of stringent particle filtration: Wafer defects caused by undetected sub-0.10-μm particles
Currently, the sensitivity of liquid particle counters for detecting particles in chemicals is 0.065 μm. Any particle smaller than 0.065 μm not detected in the chemical could wind up on the wafer surface and adhere to it. Such a particle can be detected by enhancement with an oxide/nitride deposited film.
 Figure 1. Sub-0.065-µm substances on an IC device surface |
Figure 1 shows a scanning electron microscope (SEM) image of surface contamination detected on an IC device during its surface evaluation tests. This image shows magnified residual particles on the wafer surface. After the device was cleaned and dried, an oxide/nitride film was deposited on it. The SEM picture clearly shows that the deposited film enhances the presence of smaller size particles, which were probably undetected in the chemical. Based on the film thickness measurements, this device maker estimated the size of residual particles to be less than 0.05 μm. This observation clearly indicates the need for more stringent filtration.
Such surface analysis is appropriate for measuring particle contamination on wafer surfaces. To characterize particle removal efficacy of chemical filters, we have developed several test methods that measure membrane pore size (by bubble point) and determine particle retention using an appropriate particle challenge, in conjunction with either an in-line particle counter or by analytical methods. In the following, we will briefly explain the principle of these test methods and present experimental results that demonstrate the removal of 0.034-μm particles by Teflon® 0.03-μm filters. Additional references are cited to understand the details of the methods.2
Filter pore size and nanoparticle retention test methods (SEM method)
There are several available methods of measuring membrane pore sizes, such as SEM imaging to view it directly or the membrane bubble point test. The particle removal efficiency can be determined by challenging the filters with test particles of the appropriate size and counting the particles before and after filtration using a particle counter or an appropriate analytical detection method.
 Table 1. Yield enhancement/wafer environment contamination control (2004 ITRS update) |
Figure 2 shows a SEM image of two different membrane surfaces. The left image shows a film-stretched membrane, and the right shows a track-etch membrane. The track-etch membrane has a fairly well-defined circular pore structure that facilitates measurement of the pore size. The PTFE membrane, made by the stretch method, has the pore structure formed between the stretched fibers. Since its pore structure has an undefined, tortuous shape, SEM imaging cannot be used to precisely quantify its pore size.
 Figure 2. SEM images of PTFE (left) and track-etched (right) membranes. |
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Pore size rating by bubble point
Bubble point testing is widely used by filter makers since it can be done without damaging the membrane. The test method is as simple as wetting the membrane with a solvent, such as an alcohol, that completely fills all the pores. The wetted membrane is then pressurized with an inert gas such as nitrogen, gradually raising the pressure. The gas pressure level at which the solvent from the largest pore is completely pushed out is termed the bubble point (rated pore size) of the membrane. The bubble point is directly proportional to the solvent surface tension, and inversely proportional to membrane pore size.3 The mathematical expression of the bubble point is as follows:
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P = Bubble point pressure
δ= Surface tension
θ= Liquid-solid contact angle
d = Pore diameter
K = Shape correction factor
The bubble point test method is extensively relied upon in the development of membrane filters.
Particle retention with a monolayer-coverage PSL-bead challenge test
This test involves challenging the membrane with a high concentration of polystyrene latex (PSL) bead solution (monodispersed particle size distribution), containing an added surfactant to cover the equivalent of one layer of PSL beads on the membrane. The light absorption technique is used to measure particle concentration in feed and filtrate. The particle retention measured by this method is attributed to the presence of large diameter pores. (Special fluorescent beads can be used for this challenge test.) This method is used as a quality tool to check the retention performance and not necessarily for retention claim.
pictures have been abusively taken from published papers by other authors, without mentionning references.
for instance picture 1 (i recognize my own picture).
so, at least you need to add references where you’ve taken a copy paste of the data !