Trace Metal Ion Removal from Ultrapure Water
Resistivity measurements of 18.2 MOhm-cm were once considered an indication that
ultrapure water (UPW) did not harbor any metal ions or other ionic species. With
the advent of ever finer line widths and thinner gate oxides, an increase in defectivity
levels was observed. Although the causes are myriad, one of the areas of focus is
decreasing the levels of trace metal ions such as sodium, calcium, iron and copper.
In fact, the 2006 International Technology Roadmap for Semiconductors (ITRS) has
specified < 50 ppt for these contaminants. Obviously ion exchange polishing bottles
come to mind as the first choice for removing remaining metal ion contamination
not rejected by reverse osmosis or captured by the main mixed resin bed. However,
in a number of cases, they have been shown to pass trace quantities of cations.
These ions can be effectively captured by membrane purifiers, such as the Pall IonKleen™
family of purifier cartridges.
The IonKleen membrane purifiers consist of a microporous ultra-high molecular weight
polyethylene substrate with covalently bonded, strong acid cation exchange groups.
Configured either into standard cartridges or capsules, they have a very large effective
surface area with a small footprint, enabling then to be placed right at the point-of
use.
The IonKleen purifiers are highly efficient because they do not rely on slow diffusion
into a resin bead to achieve ionic adsorption. Rather, due to the intimate contact
of the water with the densely packed ion exchange groups on the membrane, rapid
kinetics occur with immediate and spontaneous removal of the trace contaminants.
This is schematically depicted in Figure 1.
Figure 1: Ion Adsorption ( Resin vs Membrane )
The efficiency of the IonKleen Purifier is relatively independent of flow rate provided
its recommended maximum flow rate is not exceeded. The same cannot be said for ion
exchange columns or polishers.
Since it is extremely difficult to directly detect these trace ions, their existence
and deleterious effects were demonstrated by the following experiments. A Pall IonKleen
ion exchange purifier cartridge was examined after six months of service in UPW.
The purifier was extracted with 10% HCl. The acid was then analyzed using inductively
coupled plasma mass spectrometry (ICP-MS). A variety of cations were detected. The
highest concentrations are shown in
Table I. These represent significant cations removed from the UPW during the service
life of the ion exchange purifier/filter.
|
Cation |
?g per 254 mm/10 in.
IonKleen Purifier |
|
Na
|
85.8 ± 2.7
|
|
Mg
|
277.5 ± 8.9
|
|
Ca
|
147.0 ± 4.7
|
|
Fe
|
267.0 ± 8.5
|
|
Ni
|
75.9 ± 2.4
|
Tests with silicon wafers revealed that significant levels of metals could be detected
on wafers rinsed with standard UPW as compared to the low concentrations on control
wafers exposed to equal amounts of membrane purified UPW. Pall IonKleen™ Purifiers
are being used successfully at point-of-use in a number of UPW systems. Because
of its high metal ion capacity, typical service life ranges from 1-2 years.
Call 1.925.443.9800 for more information or to schedule a visit with a Microelectronics
field representative.
CMP Point of Use Chemical Dilution
Semiconductor manufacturing customers are seeking ways to reduce process costs without
impacting the process itself. Semiconductor production facilities have numerous
Chemical Mechanical Planarization (CMP) tools that use an abrasive process for polishing
the surface of the wafer flat. Planarization areas are opportunities to pursue such
improvements.
At various times during the CMP process, fluids are delivered to the wafer for polishing
or rinsing using a pressurized chemical system. During the rinse phase a chemical,
usually acid or ammonia, is diluted 40:1 with de-ionized (DI) water. McMillan Liquid
Flo-Controllers combine smart electronics with a precision control valve and flow
sensor. The output from the flow sensor is analyzed and compared to the flow rate
set point. The control valve is then automatically adjusted to achieve the required
flow.
The DI water and the chemical are mixed downstream of the Model U801 Flo-Controllers
and delivered to the wafer. The controller uses an external set-point, output signal
and standby mode for process control and monitoring. Standby mode overrides control
functions and freezes valve position, while still providing flow output signals
for monitoring purposes. Normal operation resumes when standby is disengaged.
The Liquid Flo-Controllers provide consistent flow regardless of small pressure
changes and can be configured for available system pressure. The control module
has an LCD display, status and standby LED indicators for visual reference. DIP
switches on the control module allow the user to view flow, choose internal or external
set-point and manually activate standby mode. Benefits to our customers include
a reduction in process cost, reduced maintenance and improved consistency.
Call 1.925.443-9800 for more information or to schedule a visit with a Microelectronics
sales representative.
|
In this issue:
Download this Issue: OCTOBER
2007
Saint-Gobain Microelectronics manufactures custom fluoropolymer valve manifolds
to provide turn-key solutions to semiconductor OEMs.
News provided by Google
|