Ultrapure Water (UPW) in Semiconductor Manufacturing
The production of plasma displays, advanced chips, and integrated circuits demands water of exceptionally high quality for various critical process steps. This water, known as Ultrapure Water (UPW), is crucial to prevent contamination that could compromise device performance and yield. As semiconductor linewidths continue to shrink, the purity requirements for UPW become even more stringent, pushing the boundaries of water treatment technology.
UPW Production System Overview
Achieving the required UPW standards involves an integrated approach combining multiple advanced water treatment technologies. A typical UPW production system is engineered for maximum contaminant removal, including dissolved solids, organics, particulates, dissolved gases, and microorganisms.
Key technologies commonly employed include:
- Reverse Osmosis (RO): For primary removal of dissolved salts and large organic molecules.
- Electro-Deionization (EDI): Continuous deionization for further reduction of ionic impurities without chemical regenerants.
- Ion Exchange (IX): Mixed-bed ion exchangers for polishing to achieve ultra-low conductivity.
- Ultraviolet (UV) Sterilization: For TOC reduction and bacterial control.
- Ultrafiltration (UF) / Microfiltration (MF): For particulate removal.
A well-designed UPW system requires a strategic selection of technologies and equipment, tailored to the specific feed water quality and the end-user's ultrapure water requirements. Various configurations can be applied to meet different demands.
ASTM UPW Quality Guidelines for Semiconductor Industry
The American Society for Testing and Materials (ASTM) provides a framework for classifying ultrapure water quality based on the critical parameters relevant to semiconductor fabrication. These guidelines differentiate several gradations of purity, denoted as Type E.1, E 1.1, and E 1.2, corresponding to decreasing linewidths and increasing purity demands.
The following table outlines the key parameters and their maximum permissible levels for different ASTM types:
| Parameter | Unit | Type E.1 | Type E 1.1 | Type E 1.2 |
|---|---|---|---|---|
| Linewidth | µm | 1.0-0.5 | 0.35-0.25 | 0.18-0.09 |
| Resistivity, 25 °C | MΩ·cm | ≥18.1 | ≥18.2 | ≥18.2 |
| TOC (on-line) | µg/L (ppb) | ≤5 | ≤2 | ≤1 |
| On-line dissolved oxygen | µg/L (ppb) | ≤25 | ≤10 | ≤3 |
| On-Line Residue after evaporation | µg/L (ppb) | ≤1 | ≤0.5 | ≤0.1 |
| SEM particles | ||||
| > 0.1-0.2 µm (0.004-0.008 mils) | Particles/mL | ≤1,000 | ≤700 | ≤250 |
| > 0.2-0.5 µm (0.008-0.02 mils) | Particles/mL | ≤500 | ≤400 | ≤100 |
| > 0.5-1.0 µm (0.02-0.04 mils) | Particles/mL | ≤100 | ≤50 | ≤30 |
| > 10 µm (0.4 mils) | Particles/mL | ≤50 | ≤30 | ≤10 |
| Bacteria in CFU/Volume | ||||
| 100 mL Sample | CFU/100mL | ≤5 | ≤3 | ≤1 |
| 1 L Sample | CFU/L | ≤10 | ≤10 | Not specified |
| 10 L Sample | CFU/10L | Not specified | Not specified | Not specified |
| Silica | ||||
| Total | µg/L (ppb) | ≤5 | ≤3 | ≤1 |
| Dissolved | µg/L (ppb) | ≤3 | ≤1 | ≤0.5 |
| Anions and Ammonium by IC | µg/L (ppb) | |||
| Ammonium | ≤0.1 | ≤0.1 | ≤0.05 | |
| Bromide | ≤0.1 | ≤0.05 | ≤0.02 | |
| Chloride | ≤0.1 | ≤0.05 | ≤0.02 | |
| Fluoride | ≤0.1 | ≤0.05 | ≤0.03 | |
| Nitrate | ≤0.1 | ≤0.05 | ≤0.02 | |
| Nitrite | ≤0.1 | ≤0.05 | ≤0.02 | |
| Phosphate | ≤0.1 | ≤0.05 | ≤0.02 | |
| Sulfate | ≤0.1 | ≤0.05 | ≤0.02 | |
| Metals by ICP/MS | µg/L (ppb) | |||
| Aluminum (Al) | ≤0.05 | ≤0.02 | ≤0.005 | |
| Antimony (Sb) | Not specified | Not specified | Not specified | |
| Arsenic (As) | Not specified | Not specified | Not specified | |
| Barium (Ba) | ≤0.05 | ≤0.02 | ≤0.01 | |
| Boron (B) | ≤0.3 | ≤0.1 | ≤0.05 | |
| Cadmium (Cd) | Not specified | Not specified | Not specified | |
| Calcium (Ca) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Chromium (Cr) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Copper (Cu) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Iron (Fe) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Lead (Pb) | ≤0.05 | ≤0.02 | ≤0.005 | |
| Lithium (Li) | ≤0.05 | ≤0.02 | ≤0.003 | |
| Magnesium (Mg) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Manganese (Mn) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Nickel (Ni) | ≤0.05 | ≤0.02 | ≤0.002 | |
| Potassium (K) | ≤0.05 | ≤0.02 | ≤0.005 | |
| Sodium (Na) | ≤0.05 | ≤0.02 | ≤0.005 | |
| Strontium (Sr) | ≤0.05 | ≤0.02 | ≤0.001 | |
| Tin (Sn) | Not specified | Not specified | Not specified | |
| Titanium (Ti) | Not specified | Not specified | Not specified | |
| Vanadium (V) | Not specified | Not specified | Not specified | |
| Zinc (Zn) | ≤0.05 | ≤0.02 | ≤0.002 |
Note: These values are guidelines provided by ASTM and are subject to revision as the semiconductor industry develops new linewidths and processes. The highest-end use requirements are generally classified under Type E-1.1 and Type E-1.2.
AquaChain Engineering Tip
For critical semiconductor UPW systems, consider deploying advanced real-time particle counters at various stages, especially at point-of-use. While resistivity and TOC are fundamental, sub-micron particle excursions can be fleeting and severely impact yield. Online particle monitoring provides immediate actionable data, allowing for rapid intervention before widespread wafer contamination occurs.
Frequently Asked Questions
Q: Why is ultrapure water quality so crucial for semiconductor manufacturing?
A: Minute impurities, even at parts-per-trillion levels, can cause defects in integrated circuits, leading to device failures and reduced manufacturing yields. UPW ensures a clean environment for sensitive processes like etching, cleaning, and deposition.
Q: What are the primary stages in a typical semiconductor UPW system?
A: A typical system includes pre-treatment (filtration, softening, carbon), primary purification (Reverse Osmosis), intermediate polishing (EDI, degasification, UV), and final polishing (mixed-bed ion exchange, ultrafiltration, and point-of-use filtration).
Q: How often should UPW quality be monitored in a semiconductor facility?
A: Critical parameters like resistivity, TOC, and dissolved oxygen are typically monitored continuously (on-line). Other parameters like specific ions, particles, and bacteria are monitored at frequent intervals (e.g., daily, weekly) depending on the criticality and system stability.
For more detailed information on water quality requirements, explore our article on Ultrapure Water.