Laser Technique Cleans Polymers
Maria Lester, Associate Editor -- Semiconductor International, 4/1/1999
GaSonics International (San Jose, Calif.) announced
it has received a multi-system order for its Performance Enhancement
Platform (PEP) 3510A system from a leading North American semiconductor
manufacturer. The PEP 3510A systems, configured with dual photoresist
removal modules, will be used to manufacture advanced logic and wireless
communication devices with 0.35- and eventually 0.18-micron design
rules.
Lumonics Inc. (Ottawa, Ontario,
Canada) announced it incurred a loss of $8.3 million (49 cents per share)
before restructuring charge for the 12 months ended December 31, 1998 on
revenue of $213.9 million. The results are in keeping with the company's
previously announced (January 14, 1999) expectations.
The 6th International Symposium and
Technical Exhibition by SCP Global Technologies (Boise, Ida.) will
be held May 10-12 at the Boise Centre on the Grove Convention Facility.
This year's theme, 'Meeting Surface Preparation Challenges of the New
Millennium,' is segmented into topics such as 'Strategies for New Back End
of Line Materials' and 'New Strategies for Reducing CoO.'
At a time when structures are more
complex and technologies more advanced, wet chemical processes dominate most
cleaning applications. However, new demands are being made on wafer cleaning
technologies with the development of metalization materials such as multi-metal
and multi-level interconnect schemes for sub-micron processes. In particular,
during reactive ion etching, polymers on the bottom and sidewalls of via holes
become hardened. When the etch reaches the underlying metal, the metal is
sputtered further, hardening the polymers. The challenge is removing the
etch-induced polymers without affecting the underlying metal layer. Though wet
chemistries using proprietary solvents are being developed to address this
issue, laser cleaning may provide particular advantages.
Company News
Chartered Semiconductor Manufacturing Ltd. (Republic of Singapore) has developed a non-contact, dry laser-cleaning technique that removes the etch-induced polymers at fluences (pulse energy per unit beam area) controlled to avoid damage to underlying layers (damage threshold). By eliminating direct contact and chemical reactions, laser cleaning can reduce the likelihood of damage to submicron via structure and the underlying layers. Through selective cleaning, especially of via holes, potential damage of surrounding areas of the wafer can be avoided. The problem of toxic chemicals and waste disposal is no longer an issue, and laser cleaning can be implemented at any point of the manufacturing process.
This technique uses a 248 nm KrF excimer laser to irradiate the wafer surface, removing submicron particulates and absorbed hydrocarbon films. The 23 nsec pulse has a maximum repetition rate of 30 Hz and maximum pulse energy of 300 mJ. The laser beam is focused onto the wafer by a quartz lens with the best exposures for sidewall polymer removal achieved at a 45° angle (Figure).
The next step in the development of laser cleaning was determining the damage threshold, which is dependent on the wafer's device structure. Chartered Semiconductor studied the damage threshold for removal of etch-induced polymers using their laser clean technique. Wafer samples were prepared by PVD, followed by metalization of Al-Cu and TiN antireflective coating. The samples were then intentionally overetched to simulate a worst case scenario.
The study showed polymer removal at fluences of 250 mJ/cm2 with no damage threshold to underlying metal film and antireflective coating. While most of the polymers were removed after just 120, 200 mJ/cm2 pulses and at a 45° angle, a range of 150-200 mJ/cm2 is needed for efficient cleaning. The effectiveness of this technique was demonstrated with etch residue removal clearly defined. Simulations of the laser interaction with the via structure correspond with and confirm the experimental results. This dry laser technique brings the technology closer to meeting the new demands of wafer cleaning.
Fig. 1. This cross-sectional view of the via hole structure shows the via-etch induced polymers formed on the sidewalls and bottoms and laser irradiating at an incident angle of s .
