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A Three Year Review of Tubular Membrane Filter (TMF) Use for Wafer Backgrinding Water Reclamation in Korea

Abstract

This case study reviews the performance of a backgrinding (B/G) wastewater-recycling system that has been in continuous operation for three years. Located in Cheongju, Korea, this system was first commissioned in July of 2013 by one of the largest semiconductor manufacturing companies in Korea. Functioning at a capacity of 3,000 m3/day, this case study illustrates the excellent filtrate quality, flux rate, and membrane life of Porex Filtration’s Tubular Membrane Filter (TMF™) system. The process continues to make a very positive impact on plant operations and financial savings from recycled water. 

Wafer Backgrinding Wastewater Reclamation with Porex Filtration Tubular Membrane Filter

About wafer processing and backgrinding wastewater

The thickness of a wafer is much larger during the process of forming the microelectronic device than in the final integrated circuit (IC) package. The initial thickness is required to maintain strength, rigidity and flatness during the manufacturing processes. Prior to IC packaging, the wafer is ground from approximately 750µ thickness to less than 20µ in ‘backgrinding’ process. Large amounts of ultrapure water are used for rinsing of the fine silicon particles, and this backgrinding wastewater is discharged from the wafer-packaging workshop. This discharged water contains primarily ultrapure water, fine silicon and colloidal silica particles, and in some cases, a small amount of grinding fluid. If the fine particles can be removed, the reclaimed, filtered water is exceptionally pure due to the resulting low TDS (low conductivity). 

The Project

One of the largest semiconductor manufacturing companies in Korea needed a system for wafer backgrinding wastewater reclamation. In backgrinding wastewater, the influent suspended solids concentration is 500 to 1,000 mg/l DLS (Dynamic Light Scattering). Further analysis demonstrated that most particles are larger than 50nm (0.05µm).

Using POREX® Filtration’s 0.05 µm Tubular Membrane Filter (TMF™) modules, the project engineer, Veolia Water Korea, built a simple system to facilitate the solid/liquid separation process without the need for any chemical dosing. This process works by using the Porex TMF modules to separate the wastewater into two parts: filtrate water and dewatered sludge cake. The filtered water is then stripped of any remaining ionic silica though an RO System before being sent back to the workshop for reuse as ultrapure DI water. The slurry from the PorexTMF System, containing all the suspended solids, is sent to a dehydration system where the squeezed water is sent back to the wastewater treatment system and the sludge cake is transferred out of the workshop for disposal.

Below are the before and after products. On the left, the jar containing mixed raw wastewater has a brown appearance with numerous fine sharp particles in the liquid whereas the jar on the right contains filtrate water from the TMF system.

Wafer Backgrinding Wastewater Recycling: Before and After

Designed for a 95% recovery rate, this system was created to have very high abrasion resistance in order to ensure lasting performance. The system capacity was designed to handle a water capacity of 3,000m3/day with options to increase water capacity by simply adding more modules/skids.

TMF System Information Module Specification: The Porex® TMF 61-tube module configuration was utilized in this system. Specification of this module includes:

  • 61 half-inch membrane tubes in each module
  • Membrane pore size of 0.05 µm
  • 4.25m2 effective membrane filtration area per module
  • PE substrate tube with anchored PVDF membrane
  • 8” diameter PVC module housing

Module Quantity and System Array:

  • 4 skids (3 in service and 1 standby)
  • 36 modules per skid, 4 trains, 9 modules in series in each train (Total of 144 modules in the system).
  • Each skid is designed to treat 1,000 m3/day wastewater.

Wafer Backgrinding Wastewater Recycling Process with POREX® Filtration Tubular Membrane Filters

 

A TMF membrane module is shown below.

Porex® Filtration Tubular Membrane Filter (TMF™) – 61-Tube Module

The installed system is shown below:

Wafer Backgrinding Wastewater Recycling with Porex Filtration technologyWafer Backgrinding Wastewater Reclamation with Porex Filtration – Installed Tubular Membrane Filters (TMF)

 

Operation After 3 Years of Continuous Use

The site was visited and system performance reviewed in April 2016. Operating parameters were documented and are presented in the table below.

 

Skid Designation

 

A

B

C

D

Filtrate Flow Rate (m3/hr)

35-40

OFF

30

34-38

Pressure In (MPA)

0.1

OFF

0.12

0.1

Pressure Out (MPA)

0.08

OFF

0.06

0.06

Filtrate Back Pressure (MPA)

0.04

OFF

0.04

0.04

Cross flowrate (m3/hr)

120-130

OFF

180

150

Filtrate Turbidity (NTU)

0.020

OFF

0.024

0.034

 

Review of the operating data above reveals several interesting details:

  • Operating pressure is very low compared to “typical” operating parameters for a TMF system. The lower operating pressure was selected from experience operating the system to save power. The system flow rate was determined to be satisfactory for the current required flows.
  • Cross flowrate (or cross flow velocity) is much less than typical operating parameters of a TMF system. Again, the flow rate was reduced based upon operating experience to save on required power; flow rate was determined to be more than adequate for this installation.
  • Filtrate quality of the 3-year-old membrane modules was determined to be outstanding.
  • System operation includes shutting down one train at a time on a rotating basis.
  • Chemical cleaning is performed once every two months using caustic soda.

Overall, the system continues to operate at high production rates; operating costs are less than originally designed. The original TMF modules put into service 3 years ago continue to operate as expected with no replacement modules required, and the system has continuously provided reliable, ultrapure recycled water since the original system start-up. One train is shut down on a weekly rotating basis as it is not currently required to meet process flow rate. Since the commissioning of this system in July 2013, the end user estimates that this plant has saved approximately $3,000,000 USD in water usage and sewer discharge fees.

Read more about the initial Backgrinding Wastewater Filtration Project

 

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