6

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

N
E
W
S
L
E
T
T
E
R


 

DEPOSITION IN LUNG: Inhalation Dosimetry/Exposure Index of Fiber Aerosol in Human Respiratory Tract

Clarkson Distinguished Professor Goodarz Ahmadi and Professor Philip Hopke (the Bayard D. Clarkson Distinguished Professor), together with their students and scientists from Lovelace Respiratory Research Institute and NIOSH, are working to provide a better understanding of particle and fiber deposition in the human lung. Exposures to airborne particles and fibers increase the incidence of asthma, lung cancer and other respiratory diseases. In particular, ethical constraints severely limit the use of fibers in human volunteer studies, such that no data have been published on controlled studies of fiber deposition in human subjects. This lack of information hampers the understanding of the etiological process of fiber-related lung diseases, and the development of an exposure index to assess and control exposure to fibers in the workplace.

This project has three objectives: (1) to develop experimental information on the deposition of particles and fibrous aerosols using a realistic human respiratory tract replica, (2) to develop a computer model for the prediction of particle and fiber deposition sites in the human lung, and (3) to define a size-selective exposure index based on the fiber penetration data obtained.

The group plans to measure depositions of particles and fibers in realistic replicas of the human respiratory tract. These replicas include the nasal passage, an oral cavity, larynx, trachea, and three generations of bronchi. The researchers also plan to numerically simulate the motion of fiber particles and their deposition in the oral, nasal, and TB airways. A computational fluid dynamics (CFD) technique will be used to develop a theoretical flow pattern in the airway and to calculate fiber trajectory and deposition efficiency in different airway regions. The results will be used to define a thoracic fiber fraction as an index of exposure to fiber aerosols.

Combined Research and Curriculum Development Project

Clarkson Distinguished Professor Goodarz Ahmadi and his collaborators ( Professors McLaughlin, Cetinkaya, Taylor, Dahniyala, and Doheny-Farina) are developing a sequence of web-based courses on particle transport, deposition and removal, as part of an NSF Combined Research and Curriculum Development (CRCD) project. The objectives of these courses are:

  • To provide a fundamental understanding of aerosol transport and removal in laminar flows.


  • To provide a fundamental understanding of particle adhesion and removal from surfaces.

  • To develop expertise with the computational modeling of particle resuspension in laminar flows.

  • To provide a fundamental understanding of particle transport and deposition in turbulent flows.

  • To develop expertise with the computational modeling of dilute two-phase flows.

  • To provide experience with the modern experimental techniques in aerosol transport analysis.

  • To provide information on the industrial applications of aerosols' transport, deposition and removal.

The preliminary course materials are available on the web at the following sites.

http://www.clarkson.edu/fluidflow/courses/
me437/syllabus.htm

http://www.clarkson.edu/fluidflow/courses/
me537/syllabus.htm

http://www.clarkson.edu/fluidflow/courses/
me637/syllabus.htm

 

 

Modeling of the Chemical-Mechanical Polishing Process

CAMP Professor R. Shankar Subramanian is interested in the process by which mechanical removal of material occurs at the microscopic level. Here, the issues are the role of the mechanical properties such as the relative hardness of the wafer, abrasive particle, and the pad, the role of asperities on the pad, and the coupling of the chemistry to the mechanical removal process. He and graduate students Lirong Guo and Qingjun Qin have been studying the mechanical removal of copper in an alumina slurry as a function of relative velocity, applied pressure, and particle concentration using a Struers Benchtop polisher. Experiments have been performed using IC-1000 and Suba 500 pads. The results clearly demonstrate the inadequacy of the Preston model in describing mechanical removal rates over a wide range of velocities and pressures, especially in the case of the IC-1000 pad. The removal rates initially increase with increasing pressure or velocity, but tend to level off at sufficiently large values of these parameters. In a recently completed M.S. thesis, Lirong Guo empirically identified the scaling laws that appear to apply to this particular mechanical removal process. Further experiments and theoretical analysis will be performed by doctoral student Qingjun Qin.

Graduate student Rajesh Appat is working with Professor Subramanian on predicting the polishing rates of steps on patterned wafers. The initial modeling work is focused on fixed abrasive pads used for polishing oxide films in applications such as shallow trench isolation (STI). In fixed abrasive pads, an abrasive such as ceria is incorporated into pillars on a flat polymer substrate, and is held together by a binder. The binder disintegrates during polishing, exposing the abrasive particles. Two important reported features of fixed abrasive polishing of STI oxide films are that the rate at which a step is polished is a stronger function of pattern density than is the case when a slurry containing abrasive particles is used, and that the polish rate of blanket films is extremely low. A preliminary model was developed that captures these two features, and an article that describes the model and provides sample predictions was published in Electrochemical and Solid State Letters in 2001. In this model, it is assumed that the removal of material from a step during its traverse is limited by the available supply of alkali in the neighboring recesses, and that this alkali diffuses rapidly to the top of the step where, in conjunction with the abrasive action, it helps to remove material. Appat has been developing a more detailed model in which the alkali transport in the recess is described by a suitable convective diffusion equation, which is solved along with the associated initial and boundary conditions.

CAMP'S SEVENTH CMP SYMPOSIUM

TOP
PREVIOUS PAGE
NEXT PAGE
INDEX PAGE

Page
6