(_~_u_m_m_e_r_S_c_h_o_o_z ______)
LAB-BASED UNIT OPERATIONS IN MICROELECTRONICS PROCESSING
CHIH-HUNG (ALEX) CHANG, MILO D. KORETSKY, SHO KIMURA, SKIP ROCHEFORT, CYNDIE SHANER Oregon State University • Corvallis, OR 97331-2702 he semiconductor industry has grown rapidly in the last three decades, and chemical technologies have The most recent AIChE placement survey T played a central role in its continuing evolution. His shows that the number of BS graduates torically, chemical engineering has focused on petrochemi placed in the electronics industry cal and bulk chemical production, but more and more chemi has increased since 1998 from cal engineers are working in the microelectronics and related 7.0% to 15.9% in 2001 industries. The most recent AIChE placement survey shows that the number of BS graduates placed in the electronics microelectronics processing. industry has increased since 1998 from 7.0% to 15.9% in 2001 (it decreased dramatically last year to 4.2% due to the Many chemical engineering pioneers in this field have rec 2 4 economic slowdown). ognized this symbiotic relationship[ - l and a number of schools have started incorporating microelectronics process The percentage of ChE graduates with advanced degrees ing into their curriculum. For the most part, however, the ma hired into the semiconductor industry is even larger. For ex terial tends to be presented in specialized, elective courses ample, more than 25% of PhD graduates have been hired by without a laboratory component. One common approach to 1 the electronics industry since 2000.[ l Chemical engineers have provide hands-on experience is to have students make a tran the advantage of a solid background in chemical kinetics, sistor in a lab typically offered by the electrical engineering reactor design, transport phenomena, thermodynamics, and department. Students go through a series of microfabrication process control that allows them to meet the challenges in processes (either hands-on or observed) to build a complete
Chih-hung (Alex) Chang is Assistant Professor of Chemical Engineering integrated circuit, or a set of test transistors. at OSU. He received his BS degree from National Taiwan University and While this lab can be an exciting and successful approach his PhD degree from the University of Florida, both in chemical engineer ing. His research interests include phase equilibria, photovoltaics, X-ray to introduce the students to the field of microelectronic pro absorption, fine structure, electronic materials, and nano- and cessing, there are some limitations. First of all, it is often microtechnology Milo D. Koretsky is Associate Professor of Chemical Engineering at OSU. taught with a cookbook approach, and second, there is lim He received his BS and MS degrees from UCSD and his PhD from UC ited time devoted to each process, so process analysis is usu Berkeley, all in chemical engineering. His research interests are in thin film materials processing, including plasma etching, chemical vapor depo ally ignored. The resulting device functionality supersedes sition, electrochemical processes, and chemical process statistics. the process engineering. Finally, not all schools have the nec Sha Kimura is Professor of Chemical Engineering at OSU. His research essary resources to offer this lab to their chemical engineer interests cover high-temperature materials synthesis, nano-sized materi als synthesis, surface modifications, applications of high-temperature flu ing undergraduates. idization technology, reaction kinetics, catalytic effects on gas-solid reac tions, and reactor design and simulations. A complementary approach is to incorporate the unit op Skip Rochefort is Associate Professor of Chemical Engineering at OSU. erations in microelectronics processing into the existing He received his BS degree from UMass, his MS degree from Northwest ern, and his PhD from UCSD, all in chemical engineering. His research chemical engineering curriculum. An example of such an interests are in all areas of polymer engineering and science and in engi approach is the incorporation of thermal oxidation of silicon neering education. into the unit operations lab at Georgia Tech. [5l This approach Cyndie Shaner served as the Linus Pauling Distinguished Engineer at OSU from 2000 to 2002. She received her BS degree in chemical engi can provide students with depth as well as breadth by pre neering from Northwestern University and has had 22 years industrial ex senting these unit operations in the context of core chemical perience with Chevron. 6 7 engineering science. [ , l © Copyright ChE Division of ASEE 2003
188 Chemical Engineerring Education (..______R_o_st_e_r_S_e_s_s,_·o_n_A_w_a_rd_R_a_rp_e_r_)
Development of programs in this area has led to innova skill as well as to demonstrate technical understanding of tive and improved education practices_[s-ioJ A successful ex the unit operation. The instructor, the student, and the ample is the curriculum developed by the Chemical and student's peers assess each student's work process skills, Materials Engineering Department at San Jose State Uni safety performance, and team behavior. versity. The essence of their project is to abandon the tradi tional laboratory cookbook instruction method and create a INTEGRATION OF MICROELECTRONICS team-oriented and open-ended laboratory where students de UNIT OPERATIONS INTO THE velop the types of skills they will later use in industry. The CHE CURRICULUM content of their laboratory includes having students make a field effect transistor and perform open-ended experiments Hundreds of individual process steps are used in the manu to improve the process. [ioJ facture of even simple microelectronics devices, but the fab rication sequence uses many of the same unit processes a While the approach at San Jose relies on coordination be number of times. A list of unit operations that are common tween students in three different engineering disciplines for the fabrication of microelectronic devices, along with (electrical, material, and chemical), at Oregon State Univer the curricular material related to each topic, are given in sity (OSU) we are implementing the same type of learning Table 1. These unit operations rely on core chemical en environment solely within chemical engineering. In this way, gineering science. we can leverage off the fundamental research in microelec tronics processing to develop unit operations accessible to We are developing unit operations modules for integration undergraduate students based on their core engineering sci into the chemical engineering curriculum and unit operations ence background. laboratories at OSU, including plasma etching, chemical va por deposition (CVD), spin coating, electrochemical deposi Integration of unit operations into microelectronics has oc tion, and chemical mechanical planarization (CMP). These curred in conjunction with a transformation in the senior unit unit operations contain complex systems that involve inter operations laboratory that began during the 2000-2001 aca action of many physical and chemical processes. Fortunately, demic year. A newly created endowed chair, the Linus Pauling there have been extensive re Engineer, was hired from search efforts in these areas, industry to identify and in and many of the fundamen corporate the highest prior TABLE 1 tal mechanisms have been ity professional practices in Unit Operations in Microelectronic Device Fabrication elucidated. For example, the seniorlab. She serves as plasma etching processes "project director" for this Unit Operations ChE Core Courses have been modeled based on class that helps graduates Bulk Crystal Growth from Melt Fluid Dynamics; Heat Transfer; Mass the fundamental transport become prepared for indus Transfer; Thermodynamics; Reaction and reaction processes oc Engineering; Process Control trial practice. Professional curring within the glow dis practices are incorporated Surface Reactions Kinetics; Fluid Dynamics; Mass Transfer charge to understand issues Cleaning and Oxidation into the lab through lectures, of etch rate, selectivity, uni Etching Mass Transfer; Kinetics; Reaction 2 15 classwork assignments, and formity, and profileY · l Plasma Etching. Wet Etching Engineering; Process Control homework assignments. Similarly, chemical vapor Eight lectures cover project Thin Film Deposition Kinetics; Fluid Dynamics; Mass Transfer; deposition reactors have • Physical Vapor Deposition Heat Transfer; Thermodynamics; management, meeting • Chemical Vapor Deposition Electrochemical Engineering; Reaction been modeled in analogy to skills, technical writing, oral • Electrochemical Deposition Engineering; Process Control porous catalysts,P 6l incorpo presentations, safety, ratio Lithography Fluid Dynamics; Mass Transfer; Polymer rating transport and reaction 17 20 nal management processes • Photoresist spin coating Rheology; Kinetics; Process Control processes. [ - l Control (situational, problem, deci • Photoresist baking schemes have been based on sion, and potential problem • Photoresist exposure and development the fundamental CVD reac analysis), personality self Doping and Dopant Mass Transfer; Heat Transfer; Process tor modelsY1l The fluid dy assessment, and conflict Redistribution Control namics of photoresist spin 1 • Ion Implantation resolutionY l All students • Thermal Diffusion coating has been modeled complete writing assign and studied experimentally Planarization Fluid Dynamics; Mass Transfer; Electro ments and oral presentations • Chemical Mechanical Polishing chemical Engineering; Process Control to predict coating thickness to practice the professional and uniformity as a function
Summer 2003 189 (_~_u_m_m_e_r_S_c_h_o_o_z ______)
22 of spin-speed, fluid properties, and spin duration. [ - In the processing labs (plasma, CVD, electrodeposition, and oxidation), 25l Similarly, fluid-dynamics-based models of chemi students are introduced to the unit operations. For efficacy, these labs are 6 28 cal mechanical polishing are being developed_l2 - l conducted in a well-prescribed manner where the process parameters and lab procedure are given to the students. Each group runs the process at We are synthesizing the research results in the different parameter settings, and once the entire class has rotated through a literature and applying them to the five unit op given unit operation, the students are presented with the data collected erations discussed above to make them accessible from the entire class for analysis. to undergraduate chemical engineers. At the same time, we are reinforcing the fundamental engineer For example, the silicon nitride deposition is operated at three tempera ing science taught in the curriculum. To accom tures and three flow rates (NH3 rich, stoichiometric, and NH3 lean). Stu plish this objective, we are developing both lab dents measure growth rate, film thickness (from which they calculate den- based and classroom based instruction. Integration into the lab occurs through the two required unit operations laboratories (ChE 414 and Reactor Design 415) as well as an elective, Thin Film Materials ChE 432 Detailed Design Project - LPCVD Silicon Nitride Processing. The first quarter of the two-quarter Spring,2002 lab sequence is highly structured and focuses on Introduction the students completing three unit operation ex This project is designed for senior students in ChE 432 who are interested in periments. We intend to have each student com experiencing a detailed design/simulation project in microelectronics processing. plete at least one microelectronics unit operation The topic selected is LPCVD (Low Pressure Chemical Vapor Deposition) that has been used for the deposition of silicon nitride on silicon wafers in the process for during this rotation. The second quarter of the se producing ICs. nior lab course builds on the work done in the first Requirements quarter. The focus is on working independently, Students who work on this project are required to developing a project proposal, completing experi 1. Propose a design idea for a piece of equipment that handles 200 silicon wafers mental work, and writing a final technical memo of 300mm in diameter randum that includes recommendations for future 2. Develop a model to simulate the performance of the equipment 3. Determine proper operating conditions, i.e., temperature distributions, operat work. The microelectronics unit operations are de ing pressure, feed rates, and reaction time for controlling the deposit thickness signed to be flexible enough so that each year the at 1000A with variations across wafers and from wafer to wafer within ±3%. group of students has a new, unique, and creative experience. The first four unit operations were in Figure 1. The CVD senior design-project assignment. tegrated into the second-quarter lab in the spring of 2002 and are described below. The unit opera TABLE2 tions experiment in chemical mechanical Implementation Grid of planarization was added in the spring of 2003. Microelectronics Unit Operations and ChE Classes
Students who are interested in pursuing high Chemical Electro- Chemical tech careers can obtain a transcript visible micro Plasma Vapor Spin chemical Mechanical electronics or materials science and engineering Course Etching_ Deeosition Coating_ Deeosition Polishing option in the chemical engineering department. Materials Balances X X In either of these options, Thin Film Materials Energy Balances Processing is required. Starting in the winter of Chemical Process Statistics X 2003, the thin films course was expanded from 3 Thermodynamic Properties & Relationships X X to 4 credits to enhance the laboratory component Phase and Chemical Reaction Equilibrium X X X and in the lab the students rotated through six ex Applied Momentum and Energy Transfer X periments. Three experiments-plasma etching Chemical Engineering Laboratory I X X X and spin coating, silicon nitride deposition, and Chemical Engineering Laboratory II X X X X copper electrodeposition-are based on the unit Chemical Plant Design I X operations mentioned earlier. Students also study Chemical Plant Design II X wet and dry silicon oxidation, have a vacuum com Chemical Reaction Engineering X X X ponents lab, and undergo a "virtual" lab based on Thin Film Materials Processing X X X X X the semiconductor device applets developed by Polymer Engineering and Science X Wie and coworkers at SUNY, Buffalo.l29l
190 Chemical Engineerring Education ( ______R_o_s_te_r_S_es_s_i_o_n_A_w_a_rd_R_a_r.p_e_r_) sity ), and index of refraction for their particular run. Then they look at the entire literature. Each of the four unit operations data set of the (nine) groups in the class to explore the effect of temperature and listed above will include at least two ex concentration on nitride deposition. ample exercises or homework problems to The vacuum lab gives students experience with constructing a vacuum system be integrated into a core chemical engineer and shows them the basic components in any vacuum system, including pumps, ing science or design course. By integrat pressure measurement, mass flow control, and different types of flanges. The ing the technical content in this manner, virtual lab provides a workshop on carrier physics, crystal structure, and inte the future process engineers in this indus grated circuit processing with subtractive pattern transfer. For more detail, go to try will be able to draw upon core funda
terials using SF6 and 0 2 feed gases by us Figure 3. Silicon nitride CVD reactor and schematic. ing Design of Experiments (DOE) and ana-
Summer 2003 191 (_~_u_m_m_e_r_S_c_h_o_o_z ______)
lyzing the problem using a well-mixed reactor model; the effects of wafer spacing on etch rate; the effect of the num 0 ber of substrates, i.e., loading, on etch rate; and transient analysis of temperature effects on the etching rate. In the spring of 2002, two groups of three students each were given an assignment to develop a process that mini mized interwafer, as well as wafer-to-wafer, variation in etching rate. This lab included several processes to pattern and etch a wafer, including cleaning, spin coating, photoli thography, and plasma etching. Additionally, students de veloped their own artwork to serve as a mask in photoli thography. The experimental design focused on etching pa rameters, however, while the other processes were un ® changed. One group did a 2x2 design in which they varied Spin Coating of Photoresist-Fluid, Mass, and Heat Transfer Problems pressure and wafer spacing while the other group varied Spin coating of photoresists is found in most photolithography steps in microelec power and spacing. Thickness measurements before and tronics processing. One general formulation of photoresist involves a low molecular weight polymer dissolved in a low boiling point (high vapor pressure) solvent at after etching were made using a profilometer. The plasma various concentrations (typically 60-80wt% polymer). The photoresist is spun onto a reactor does not have temperature control. Thus the tem silicon wafer ( currently 6-, 8-, or 12-inch nominal diameter) at various spin speeds perature rises as power is input during the etch process. for various spin times. The material properties (such as the viscosity) change with This facet provided a good opportunity to use chemical time as the solvent evaporates. Typical coating thickness values after spinning are l- 5µm, with residual solvent in the film. Residual solvent removal is achieved by a engineering analysis to understand the system. The group high-temperature "baking" step. There are several interesting and challenging trans was advised to record temperature during the process, but port problems that can be derived from this process that are suitable for undergraduate needed faculty help to develop an averaging method based courses. These problems are not "plug and chug" but rather require that the students on the Arrhenius expression for the activated process. The reason through the physical situation and apply simplifying assumptions to arrive at reasonable models (see two recent textbooks by Middlemanl3.34. 35l for examples). plasma lab analysis can be found at
192 Chemical Engineerring Education ( ______R_o_s_te_r_S_es_s_i_o_n_A_w_a_rd_R_a_rp_e_r_) fers, varying with temperature-profile settings, reactant feed This unit process has been used as a project in several out rates, and operating pressures. reach activities. Classroom examples for momentum, heat, • Spin Coating and mass transfer are given in Figure 4b. Spin coating has come into widespread use in the micro • Electrochemical deposition electronics industry for coating the photoresists used to de The electrochemical deposition system includes a computer fine patterns in the films on a silicon wafer. It will also be controlled bipotentiostat, PineChem software, rotator, elec used in future technologies as polymers become incorporated trodes, and a standard voltammetry cell (see Figure 5a). A as dielectric materials. The underlying principles of spin coat variety of experiments could be designed using this system. ing (fluid flow, fluid properties, surface phenomena) and the Examples of such experiments are process itself make it a natural for inclusion in the chemical engineering curriculum. The precursors to coating, surface • Diffusion coefficient determination by rotating elec wetting, and adhesion are also classical problems. The spin trode cyclic voltammetry coating of solid substrates with various viscous liquids and • Measurement ofthe kinetics and the flux ofcopper ions surface wetting phenomena (surface tension, contact angle, to an electrode surface by means ofrotating ring-disk viscosity) is done using a "state-of-the-art" programmable electrode laboratory spin coater from Specialty Coating Systems (SCS • Study ofmass transfer using rotating electrodes Model P6700) (see Figure 4a) and highly polished oxide coated 6-inch wafers. Examples of engineering projects in • The effects ofadditives on deposition rates clude: experiments on viscous, Newtonian liquids to test the • Leveling effects ofadditives Emslie model and compare data to published spin coating • Superfilling phenomena results for Newtonian liquids;[22J coating photoresist on sili • Resistive seed effect con wafers as the first step in the photolithography process. In the spring of 2002, the student team was taught how to use this sys tem by going through a "cookbook" experiment using cyclic voltammetry and the rotated disk electrode to char Sietclhi,cxrtrobtor e::q,erinerts on.~ •. acterize the redox reaction of potas lairg~, sium ferricyanide solution. After the training, they were asked to propose IQwlre-goingtDthe pWruTl n:bthg1cilk an experimental plan using this setup. ....K~l'fl'ire..plgtothecoppe!'. Relerenc::l!BednJdeWff. They decided to study the copper mass transport using different copper elec
) trolytes (CuCl2 and CuSO4 and the influence of a sulfur-containing addi tive (thiourea). The experiments were ChE 312 Phase and Chemical Reaction Equilibrium performed using acid copper solutions prepared from CuCl and CuSO with Electroplating is used to deposit the copper metal lines in integrated circuit processing. In this case, 2 4 dissolution of a copper anode is used to provide cupric ions for reaction, i.e., the solid copper-cupric ion and without thiourea. The electro reaction is used both at the anode to generate cupric and at the cathode to grow the resulting film. Please chemical reactions were characterized calculate the minimum electrode potential to achieve copper growth for the following cell by sweeping the voltage and measur Cu(s)ICuSO.(1,0.lM)IICuSO.(l,0.0lMICu(s) ing the current. The boundary layer using (a) ideal solution model, and (b) modified Debye-Hiickel theory thickness was controlled by the rotat A number of concepts in chemical reaction equilibrium can be taught through this classroom example, ing speed of the working electrode and including • How to write electrochemical reactions the Levich equation was used to de • How to incorporate the concept of electrical work into the equilibrium formulation. How to calculate the termine the diffusivity. electrochemical potential from the Gibbs energy • How to estimate activity coefficients for ionic species in an electrolyte solution (Debye-Hiickel theory) A classroom example based on cop per electrodeposition is given in Fig Figure 5. (a) Copper electrodeposition reactor and schematic, and ure 5b. In addition, many interesting (b) electrodeposition classroom example. laboratory problems on copper plat-
Summer 2003 193 (_~_u_m_m_e_r_S_c_h_o_o_z ______) ing can be found in a manuscript by Talbot. [3oJ • The students will demonstrate communication skills. For • Chemical Mechanical Polishing example, they will be required to master written and oral reports. The experimental setup of a bench-scale CMP module is • The students will demonstrate technical synthesis in each shown in Figure 6a. The setup is adopted from the research of the unit operations. For example, in CVD they will use literature,l31·33l which has been useful in understanding the kinetic data in reactor design problems. reaction mechanisms during CMP. In this setup, the copper • The students will demonstrate professional practices. For CMP will be studied in a three-electrode electrochemical cell example, they will be required to demonstrate project by using a copper-plated rotating disk electrode. The polish planning before performing experiments. ing downward force will be measured by a balance, which supports the entire electrochemical cell. The DC electrochemi Each of these outcomes will be assessed by three methods: cal measurement will be carried out by using a potentiostat. • Student self-assessment and peer-assessment, e.g., sur A variety of experiments can be designed to study the copper vey of effectiveness of educational materials. CMP process. For example, the effect of HN03 or NHpH • Evaluation of student performance by instructors. on the chemical etching mechanism, the effects of additives • Feedback from industrial constituency, e.g., survey of stu (e.g., inhibitor, oxidizer), the relation between downforce, and dent performance from industrial employers. removal rates through the Preston equation. The Preston equa tion[31l relates removal rate to driving force in the same way SUMMARY that mass transfer coefficients relate mass transfer to driving The integration of microelectronics-based unit operations force. Studies on the bench-scale system will be scaled up to into the chemical engineering curriculum at Oregon State the industrial-scale system shown in Figure 6b. University has been presented in this paper. We are develop ing both lab-based and classroom-based instruction. Five new OUTREACH unit operations are being implemented in the senior lab, in cluding plasma etching, chemical vapor deposition, spin coat- Three modules, including plasma etching, spin coating, and copper electrodeposition, were implemented in the outreach programs that are currently in place in the chemical engineering department:
~ in1:DiJkt:1n;t...... , 1) Summer Experience in Science and Engi ( Workioel'.lttlrode) neering for Youth (web site at
ASSESSMENT PLAN The measurable student outcomes for each unit operation will include Figure 6. (a) Bench-scale CMP and schematic (b) industrial-scale CMP.
194 Chemical Engineerring Education ( ______R_o_s_te_r_S_es_s_i_o_n_A_w_a_rd_R_a_rp_e_r_) ing, electrochemical deposition, and chemical mechanical II. Koretsky, M.D., C.-H. Chang, S. Kimura, S. Rochefort, and C. Shaner, planarization. These labs are also included in an elective "Integration of Microelectronics-Based Unit Operations into the ChE Curriculum," Session 1313, Proceedings of the 2003 ASEE Annual course, Thin Film Materials Processing. Classroom examples Conference are being integrated into chemical engineering core courses. 12. Bell, AT., "Fundamentals of Plasma Chemistry," in Techniques and In addition, the students are learning professional practices Applications ofPlasma Chemistry, A.T. Bell and J.R. Hollahan, eds., that include effective oral and written communication, project John Wiley & Sons, New York, NY (1974) 13. Graves, D., and K.F. Jensen, "A Continuum Model of DC and rf Dis planning, time management, interpersonal interaction, team charges," IEEE Trans. Plasma Sci., P5-14, 78 (1986) work, and proactive behavior. These modules are also being 14. Alkire, R.C., and D.J. Economou, "Transient Behavior During Film used effectively in outreach programs. Removal in Diffusion-Controlled Plasma Etching," J. Electrochem. Soc., 132, 648 (1985) ACKNOWLEDGMENTS 15. Plumb, LC., and K.R. Ryan, "A Model of the Chemical Processes Occuring in CF/O2 Discharges Used in Plasma Etching," Plasma The authors are grateful for support provided by the Intel Chem. Plasma Proc., 6,231 (1986) Faculty Fellowship Program and the National Science 16. Levenspiel, 0., The Chemical Reactor Omni book, Oregon State Uni versity, Corvallis, OR (1993) Foundation's Course, Curriculum, and Laboratory Improve 17. Hess, D.W., K.F. Jensen, and T.J. Anderson, "Chemical Vapor Depo ment Program under grantDUE-0127175. We also acknowl sition: A Chemical Engineering Perspective," Rev. Chem. Engg., 3, 97 edge the Dreyfus Special Grants Program (SG-97-075), OSU (1985) Precollege Programs, Kelley Foundation, and Bridges Foun 18. Jensen, K.R., and D.B. Graves, "Modeling and Analysis of Low Pres sure CVD Reactors, J. Electrochem. Soc., 130, 1950 (1983) dation for their support of the outreach program Summer 19. Roenigk, K.F., and K.F. Jensen, "Low Pressure CVD of Silicon Ni Experience in Science and Engineering for Youth, and Pete tride, J. Electrochem. Soc., 134(7), 1777 (1987) and Rosalie Johnson for the endowment of the Linus Pauling 20. Fogler, H. Scott,Elements ofChemicalReactionEngineering, 3rd ed., Engineer. SEH America graciously donated the highly pol Prentice-Hall PTR, 789 (1999) 21. Edgar, T.F., S. Butler, W.J. Campbell, C. Pfeiffer, C. Bode, S.G. Hwang, ished and oxide-coated 6-inch silicon wafers. Helpful dis K.S. Balakrishnan, and J. Hahn, "Automatic Control in Microelec cussions with Emily Allen of San Jose State University tronics Manufacturing: Practices, Challenges, and Possibilities," and Chuck Croy of Intel Corporation are greatly appreci Automatica, 36, 1567 (2000) ated. The authors would also like to acknowledge the re 22. Emslie, A.G., F.T. Bonner, and L.G. Peck, "Flow of a Viscous Fluid on a Rotating Disc," J. Appl. Phys., 29, 858 (1958) viewers for the critical reading and suggestions they made 23. Flack, W.W., D.S. Soong, A.T. Bell, and D.W. Hess, "A Mathematical regarding this paper. Model for Spin Coating of Polymer Photoresists," J. Apply. Phys., 56 1199 (1984) REFERENCES 24. Bornside, D.L., C.W. Macosko, andL.E. Scriven, "Spin Coating: One Dimensional Model," J. Apply. Phys., 66, 5185 (1989) 1. "Initial Placement of Chemical Engineering Graduates,"
Summer 2003 195