UV Lithography of Ultrathick SU-8 23 1. Clean Si wafer with acetone, IPA (isopropyl alcohol), and DI (deion- ized) water. 2. Spin coat SU-8 100 at 400 rpm. 3. Level hot plate, bake 10 hours at 110°C, cool down to 60°C inside 1 hour, dwell at 55°C (uncross-linked SU-8’s glass temperature is 50°C to approximately 60°C) for 4 hours, cool down to room temperature inside 3 hours. 4. Expose the sample using a broadband light source (with spectrum total exposure dosage of 2 J/cm 2 , for PMMA filter wavelength selec- tion exposure (with spectrum as shown in Figure 2.6; includes the h-line and g-line) with total exposure dosage 12 J/cm 2 . 5. Postbake at 110°C for 20 minutes, cool down as in step 3. 6. Develop sample using SU-8 developer at 32°C with SONOSYS megasonic actuator driven with a 250 W power supply for 2 hours. The megasonic transducer was placed in a water bath supporting a quartz tank in which the developer and substrate were located. Wafers were facing the megasonic actuator. 7. Rinse sample with IPA several times, dry naturally. To measure the sidewall quality of the microstructures fabricated using filtered a light source and gap compensation, a 20 µm feature-sized micro- structure with a flat edge was removed from the substrate and placed on the measurement stage of the Veeco optical profiler. The R s (roughness of stan- dard deviation) was then measured along the 1150 µm length. It was found that the roughness of standard deviation (in the light incident direction) was 2.72 µm over the entire length of 1150 µm. tions: (a) broadband light source with no air gap compensation, (b) broad- band light source and air gap compensation using a glycerin solution, (c) the filtered light source (i-line eliminated) with no air gap compensation, and (d) filtered light source with gap compensation using a glycerin solution. The minimum designed thicknesses of the crosses achieved are 20 µm for the air gap and 8 µm for the glycerin gap compensation. The dark region in Figure 2.7a was due to the residuals of the development. The theoretical optical resolution of the line and space of the width b can be estimated by the following equation: where b is the width of line or space, λ is the wavelength of the lithography light, s is the air gap between the mask and the photoresist, and d is the bsd min ()=+ 3 2 1 2 λ DK532X_book.fm Page 23 Tuesday, November 14, 2006 10:41 AM as shown in Figure 2.6; includes the i-line, h-line, and g-line) with Figure 2.7 shows a group of microcrosses produced with different condi- © 2007 by Taylor & Francis Group, LLC UV Lithography of Ultrathick SU-8 25 compensation with glycerin have excellent sidewall quality and resolutions. Both structures were developed all through and clearly separated. The top fingers are removed together by the liquid surface tension in the drying process. 2.4 Basic Steps for UV Lithography of SU-8 and Some Processing Tips The standard lithography processing procedures of SU-8 include: (1) pretreat the substrate, spin-coat SU-8; (2) preexposure bake, UV exposure (320 to 450 nm); (3) postexposure bake; and (4) development. The process parameters determine the final quality of the microstructures. The curing process of SU- 8 is completed in two steps: formation of acid during optical exposure and thermal epoxy cross-linking during the postexposure bake. A flood exposure or controlled hard bake is recommended to further cross-link the exposed SU-8 microstructures if they are going to be used as parts of the final prod- ucts. Because most of the publications in the field do not provide detailed lithography conditions, beginners often have to learn from their own expe- riences and the learning curve can sometimes be exceptionally long. Some basic lithography conditions are provided here as guidelines for those read- ers who may need something to start from [21–24]. 2.4.1 Pretreat for the Substrate To obtain good adhesion for SU-8 on a substrates, the substrate needs to be cleaned with acetone, IPA, and DI water sequentially, and then dehydrated FIGURE 2.8 Comb structures made using filtered light source and gap compensation with glycerin. DK532X_book.fm Page 25 Tuesday, November 14, 2006 10:41 AM © 2007 by Taylor & Francis Group, LLC 26 Bio-MEMS: Technologies and Applications at 120°C for 5 to approximately 10 minutes on a hotplate. The substrate may also be primed using plasma asher immediately before spin-coating the resist. In addition, an adhesion promoter may be used as needed. For the applications involving electroplating metals and alloys and stripping of cured SU-8, the vendor of SU-8, MicroChem, recommends using OmniCoat before coating of SU-8. 2.4.2 Spin-Coating SU-8 The thickness of SU-8 film is dependent on several factors: the viscosity of the SU-8 used, the spin speed, and the total number of turns. The vendor of SU-8, MicroChem, provides some spin-coating curves for different SU-8 formulations, such as SU-8 5, SU-8 50, and SU-8 100. Some research labs have also developed their own spin-coat curves based on the particular equipment used. Figure 2.9 shows some typical spin-coating curves of SU-8. FIGURE 2.9 Selective SU-8 spin-speed vs. film thickness curve. (Courtesy of Mark Shaw, MicroChem Corp., Newton, MA.) SU-8 spin speed curves 0 5 10 15 20 25 30 35 40 45 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 Spin speed (rpm) Film thickness (microns) SU-8-2 SU-8-5 SU-8-10 SU-8-25 SU-8 spin speed curves 0 50 100 150 200 250 750 1000 1250 1500 1750 2000 2250 2500 2750 3000 3250 Spin speed (rpm) SU-8-50 SU-8-100 DK532X_book.fm Page 26 Tuesday, November 14, 2006 10:41 AM © 2007 by Taylor & Francis Group, LLC UV Lithography of Ultrathick SU-8 27 Bubbles formed during the spin-coating step may lead to reduced lithogra- phy quality. To eliminate bubbles in resist film, the substrate should be placed on a flat and horizontal plate for 2 to approximately 10 hours before prebake. This is an especially critical step for obtaining good quality of thick SU-8 film. 2.4.3 Soft Bake The spin-coated sample needs to be soft baked to evaporate the solvent on a leveled hotplate or in convection ovens. The heat transfer condition and ventilation are different for the hotplate and the convection ovens, and the preferred soft baking times are therefore different as shown by the curves for measured soft baking times in Figure 2.10. Ramping and stepping the soft bake temperature is often recommended for better lithography results. The glass temperature of the unexposed SU-8 photoresist is about 50 to approximately 60°C. Figure 2.11 shows a typical soft-baking temperature curve used in our laboratory. This soft-bake process consists of multiple steps of ramping up, dwell, and ramping down. The total cooling time is about 8 FIGURE 2.10 Soft bake time vs. SU-8 thickness. FIGURE 2.11 A selected soft bake profile for 1100 mm–thick SU-8 film. 10 8 6 Bake time (hours) 4 2 0 0 200 ickness (µm) Soft bake time vs. thickness 400 600 800 1000 Bake in oven Bake on hot-plate Dwell at 50°C/4 hrs Ramp to 50°C in 40 m Dwell at 75°C/15 ms Ramp to 75°C in 40 mRamp to 110°C in 30 m Dwell at 75°C/15 ms Ramp to 75°C in 30 m 20°C/1~2 hrs Ramp to 20°C in 3 hrs Dwell at 110°C/10 hrs DK532X_book.fm Page 27 Tuesday, November 14, 2006 10:41 AM © 2007 by Taylor & Francis Group, LLC 28 Bio-MEMS: Technologies and Applications to approximately 10 hours for a 1000 µm–thick SU-8 resist. For ultrathick SU-8 film (more than 1000 µm thick), a baking temperature of 110°C is used coated with Cr/Au film (as commonly used in the UV-LIGA process as the plating seed layer), a 110°C bake temperature is suggested instead of 96°C. At the same time, the bake time should be slightly reduced. 2.4.4 Exposure A near UV (320 to 450 nm) light source is normally used for lithography of SU-8. As the wavelength of the light source increases, the absorbance of the light reduces and the transmission increases significantly. The transmission increases from 6% at λ = 365 nm to about 58% as the wavelength increases to 405 nm. SU-8 has high actinic absorption for wavelengths less than 350 nm, but is almost transparent and insensitive for above 400 nm wavelengths. Because of the high absorption of SU-8 for light with shorter wavelengths, a light source dominated by shorter wavelength components often results in overexposure at the surface of the resist and underexposure at the bottom part of the resist layer. This is the main reason that UV lithography of SU using an i-line-dominated light source tend to produce microstructures with T-topping geometric distortions. Thickness of the resist is another key parameter that dictates the required dosage of the exposure. Figure 2.12 shows two curves of required exposure dosage and the thickness of SU-8. MicroChem, the vendor of SU-8, advises that the user filter out the light with a wavelength lower than 350 nm to improve lithography quality. After filtering the light components with wavelengths shorter than 350 nm from the light source of the Oriel UV station used in our laboratory, with its spectrum as shown in kept in a range of 1:7 to approximately 1:10 to achieve perfect vertical side- walls, especially for the SU-8 resist with thickness around 1 mm. For lithog- raphy of a very thick resist, multiple exposures are required to avoid FIGURE 2.12 Exposure dosage vs. film thickness: the preferred exposure dosage should fall between the top and bottom curves. (Courtesy of Mark Shaw, MicroChem Corp., Newton, MA.) 800 600 400 Esposure energy (mJ/cmˆ2) 200 0 0255075100 125 Film thickness (µm) 150 175 200225250 DK532X_book.fm Page 28 Tuesday, November 14, 2006 10:41 AM as shown in Figure 2.11. To improve the adhesion of the SU-8 film on substrate Figure 2.6, the total exposure dosage ratio between the i-line and h-line are © 2007 by Taylor & Francis Group, LLC UV Lithography of Ultrathick SU-8 29 overheating, scattering, and diffusion on the surface of the resist. Typically, exposures need to be separated in 20-second (or less than 400 mJ/cm 2 per time) intervals with 60-second waiting periods in between. For a highly reflective substrate, the effect of the reflection needs to be taken into account in estimating the total exposure time. 2.4.5 Postexposure Bake (PEB) Postexposure bake (PEB) is performed to cross-link the exposed regions of the SU-8 resist. The cross-link, or the curing step of SU-8, can be achieved at room temperature. Postbaking at a raised temperature helps accelerate profile. For resist thickness up to a few hundred micrometers, postbake at 96°C for 15 to approximately 20 minutes is required either on a hotplate or in a convection oven. SU-8’s cross-link process may cause significant residual stress, which may cause cracks or debonding. In order to minimize possible residual stresses, wafer bowing, and cracking, rapid cooling from the PEB temperature should be avoided. For resist films with thicknesses more than 1000 micrometers, ramping the PEB temperature down from 96°C should take more than 8 hours. Another possible way to reduce postbake stress is to use lower PEB temperatures, such as 50°C or 55°C, but longer baking times. This method would result in much lower thermal stress in comparison with using a PEB temperature of 96°C. 2.4.6 Development After exposure and postbake, the sample is then developed by SU-8 devel- oper. Recommended development times can be found in the catalog pro- vided by vendor of SU-8 or your lab’s experiment data. The development process can be optimized based on the experiment’s agitation rate, develop- ment temperature, and SU-8 resist processing conditions. After the sample is developed by SU-8 developer, it is sometimes dipped into a fresh SU-8 developer to rinse, then rinsed with isopropyl alcohol (IPA) for 3 to 5 min- utes. If white spots can be observed in the IPA, the SU-8 is underdeveloped. FIGURE 2.13 A possible temperature profile to be followed in PEB for 1100 µm–thick SU-8 film. Dwell at 50°C/4 hr Ramp to 50°C in 30 m Dwell at 75°C/10 m Ramp to 75°C in 30 mRamp to 96°C in 20 m Dwell at 75°C/10 m Ramp to 75°C in 30 m Ramp to 20°C in 3 hr Dwell at 96°C/20 m DK532X_book.fm Page 29 Tuesday, November 14, 2006 10:41 AM the polymerization process [20]. Figure 2.13 shows a typical PEB temperature © 2007 by Taylor & Francis Group, LLC 30 Bio-MEMS: Technologies and Applications The sample needs to be immersed into SU-8 developer or rinsed with fresh SU-8 developer to further development. After the sample is completely developed, it needs to be rinsed using fresh IPA. If possible, avoiding a deionized (DI) water rinse is preferred. Finally, the sample is dried naturally or by nitrogen gas blow. 2.5 Tilted Lithography of SU-8 and Its Application SU-8 is well suited for the fabrication of three-dimensional microstructures using tilted exposure. A variety of SU-8 resist structures, such as slope, trapezoids, dovetails, as well as various conical shapes, can be fabricated using tilted lithography. In recent years, we have fabricated micromixers [25], out-of-plane microlens [26–28], out-of-plane microlens arrays [30], fiber bundle couplers [31], and three-dimensional hydrofocus components [31]. Because of the refraction of light at the surface of the SU-8 resist, a light beam projected on the resist at an incident angle may propagate at a reduced refraction angle. Based on the refraction index of the SU-8 (n = 1.668 at λ = 365 nm, n = 1.650 at λ = 405 nm), the refraction angle can be approximately calculated to be 25.08° for the i-line with a 45° incident angle as shown in Figure 2.14. The critical angle is about 36.8° at 365 nm. If a larger refractive angle is needed, optical liquid and a coupling prism are used to compensate for the light refraction. The working principle to achieve a bigger refraction angle for SU-8 lithog- the substrate are as shown in Figure 2.15. FIGURE 2.14 The refraction of the SU-8 resist may cause the projected light beam to bend over and therefore leading to reduce angle of the light projection. SU-8’s refraction and the critical angle (critical angle is about 36.8° at 365 nm). 45° 90° 45° 37.3° Incident light Incident light SU-8 SU-8 Air Air Refracted lightRefracted light DK532X_book.fm Page 30 Tuesday, November 14, 2006 10:41 AM raphy is shown in Figure 2.15. The positions of the prism, mask, SU-8, and © 2007 by Taylor & Francis Group, LLC 32 Bio-MEMS: Technologies and Applications (2.10) The substrate therefore needs to be kept at θ = 45° + θ 7 with the horizontal level (because the light beam in the UV station is always in the vertical direction) to completely compensate for the refraction at the interface to obtain a 45° refractive angle inside the SU-8 photoresist. 2.5.1 Micromixer/Reactor As an example of tilted lithography of SU-8, we present a novel passive micromixer/reactor based on arrays of spatially impinging microjets, which takes a three-dimensional approach in design and is based on a fabrication process using UV lithography SU-8 photoresist [25]. To mix microvolumes of fluid samples in microfluidic systems is always a challenging task. Because the flow in all microfluidic systems is laminar and has a low Reynolds number, diffusion is the dominant mechanism. Various efforts have been made to improve the mixing process by introduc- ing geometric irregularities in inflow channels to create localized eddies and turbulent flows. Efforts have also been made to use special actuation mech- anisms to disturb the flow with such noncontact measures as ultrasound waves. Because it is very difficult to obtain high mixing efficiency with a diffusion mechanism, some reported efforts used active disturbance to create turbulence in the microfluidic systems. An obvious approach to increased diffusion efficiency is to maximize the effective interfacial areas of the two samples to be mixed. According to the scaling law, the most effective way to maximize the effective surface area of liquid is to convert it into plumes of stream. This is the approach we have adopted in our design of the micro- mixer. The micromixer/reactor has a simple structure and significantly boosts the mixing efficiency by increasing the interfacial contact with the impinging plumes from two opposite arrays of more, but smaller-sized, micronozzles. The micromixer/reactor is based on large arrays of spatially impinged microjets mixing. The schematic design for the micromixer/reactor is shown micronozzles are parallel with the substrate plane. There are two possible ways to arrange the opposite arrays of nozzles: directly opposite orientations or with a designed offset. The two sample fluids are delivered to inlet A and inlet B, respectively. Then they are converted into plumes of streams by the large micronozzle array and driven into the mixing chamber. The mixing processing is three-dimensional with multiplayer spatially impinged jet arrays. This helps to enhance the Reynolds number; increase the effective interfacial areas; con- vert a higher percentage of the kinetic energy into microscopic molecular θθ=°+45 7 DK532X_book.fm Page 32 Tuesday, November 14, 2006 10:41 AM in Figure 2.16. Two arrays of micronozzles are in opposite directions. The © 2007 by Taylor & Francis Group, LLC 34 Bio-MEMS: Technologies and Applications 2.5.2 Three-Dimensional Hydrofocus Component for Microcytometer Flow cytometric devices are very important for biomedical research and clinical diagnostics. The labeled cell is driven to flow through a nozzle so that light scattering or fluorescence measurements can be used for analyses. Many research efforts have been made to develop microcytometers to reduce the device and sample sizes, develop low-cost and single-use disposable devices, and to improve device portability along with low consumption of sample and buffer fluids, and to reduce the biohazard risk level. The principle of hydrofocusing in a microchannel is based on laminating cells with sheath flow [31]. A small volume of sample flow is injected into a much larger volume of sheath fluid. Both the sheath flow and sample flow require a small Reynolds number. Most of the reported hydrofocusing units for microcytometers based on sheath flow are two-dimensional, which only focuses the samples in the same plane as the substrate. A truly three-dimen- sional hydrofocusing component can focus the cells along the core stream to flow with an almost uniform velocity. Based on the three-dimensional hydrofocusing requirements and the microfabrication limitations of SU-8 UV lithography, a three-dimensional hydrofocusing unit for microflow cytometry was designed as shown by the (a) (b) FIGURE 2.17 Schematic fabrication diagram and SEM image of the fabricated micromixer/reactor. (a) Li- thography principle. (b) SEM image of the prototype mixer. Array of light beams Array of light beams DK532X_book.fm Page 34 Tuesday, November 14, 2006 10:41 AM schematic diagram in Figure 2.19. There are three inlets for the hydrofocusing © 2007 by Taylor & Francis Group, LLC 36 Bio-MEMS: Technologies and Applications glass and sloped bottom help focus the flow upward to a central region in the direction perpendicular to the substrate. The left-side slope, right-side slope, and the two sloped sidewalls perpendicular to the substrate assist in achieving flow to the central region in a horizontal direction [31]. In the fabrication process, all the slopes and the sample injection holes were fabricated using tilted exposure. A total of three exposures were required: (1) a 60° angle tilt exposure to achieve slopes having a 30° angle with the substrate, (2) a 45° angle tilt exposure to obtain a suspended sample injection nozzle in the center position of the sample inlet end, and (3) a conventional contact exposure to produce all of the SU-8 sidewalls. The fabrication procedures were as follows: clean the Si or glass substrate; spin-coat SU-8 100 photoresist to obtain a 500 µm–thick resist layer; soft bake the sample; conduct a 60° tilted exposure of SU-8 with the help of a prism and optical liquid for refraction compensation to obtain slopes tilted at 30° with the substrate; postbake the sample; spin-coat SU-8 100 photore- sist to obtain the second 500 µm–thick resist layer; prebake the sample; use a 45° angle tilted exposure of the SU-8 with a correction prism and optical liquid to obtain a suspended sample injector nozzle in the center of the sample inlet end; expose all of the SU-8 sidewalls; postbake the sample; develop with SU-8 developer; bond cover glass, seal inlet and outlet tubes. Figure 2.20 shows three SEM images of a prototype hydrofocusing unit fabricated using the tilted lithography method. The three-dimensional focusing function of the prototype hydrofocusing show experimental results that clearly demonstrated the three-dimensional hydrofocusing function. The main advantages of this polymer hydrofocusing FIGURE 2.20 SEM pictures for the three-dimensional hydrofocusing components. Sheath flow inlet Sample flow inlet Sample injection nozzle Sloped sidewall Center slope Outlet Sample injection nozzle DK532X_book.fm Page 36 Tuesday, November 14, 2006 10:41 AM unit was tested using a fluorescent dye solution. The images in Figure 2.21 © 2007 by Taylor & Francis Group, LLC [...]... Technologies, 8, 308–313, 20 02 [23 ] E Reznikova, V Namov, and J Mour, Deep photo-lithography characterization of SU-8 resist layers, Microsystem Technologies, 11, 4–5, 28 2 29 1, 20 05 © 20 07 by Taylor & Francis Group, LLC DK532X_book.fm Page 42 Tuesday, November 14, 20 06 10:41 AM 42 Bio-MEMS: Technologies and Applications [24 ] S.J Lee, W Shi, P Maciel, and S.W Cha, Top-edge profile control for SU-8 structural photoresist,... 20 04 [29 ] R Yang, S Soper, and W Wang, Out-of-plane microlens array fabricated using ultraviolet lithography,” Applied Physics Letter, 86, 16, 16111 0-1 –16111 0-3 , April 20 05 [30] R Yang, S A Soper, and W Wang, Microfabrication of pre-aligned fiber bundle couplers using ultraviolet lithography of SU-8, Sensors and Actuators, A, 127 , 1, 123 –130, February 28 , 20 06 [31] R Yang, D L Feeback, and W Wang, Microfabrication... [21 ] R Yang and W Wang, Application of optical refractive index liquid and wavelength selection for ultra-high-aspect-ratio UV-lithography of thick SU-8 resist,” Sensor and Actuator B: Chemical, 110 /2, 27 9 28 8, 20 05 [22 ] Y J Chuang, T G Tseng, and W K Lin, Reduction of diffraction effect of UV exposure on SU-8 negative thick photoresist by air gap elimination, Microsystem Technologies, 8, 308–313, 20 02. .. 5346, 20 04, 151–159, 20 04 [27 ] R Yang and W Wang, Out-of-plane polymer refractive microlens fabricated based on direct lithography of SU-8, Sensor and Actuators A: Physical, 113, 1, P71–77, June 15, 20 04 [28 ] R Yang and W Wang, Numerical and experimental study on an out-of-plane pre-aligned refractive microlens fabricated using UV lithography method, Optical Engineering, 43, 12, 3096–3103, December 20 04... shown in Figure 2. 22b The development rate for the unexposed SU-8, single-exposed SU-8, and double-exposed SU-8 are different By careful control of the exposure dosage and the optimized development time, the double-exposed region formed the microlens or microlens array as shown in Figure 2. 22c Figure 2. 23a shows an SEM image of a sample microlens array fabricated using this method Figure 2. 23b shows the... focus function of the out-of-plane microlens and microlens array A simple application of the out-of-plane prealigned microlens and microlens array [29 ] is to design and fabricate an integrated fiber coupler and fiber bundle couplers The substrate coated with 1100 µm–thick SU-8 100 was soft Unexposed Exposed SU-8 SU-8 Single-exposed SU-8 cylinders Ellipse open Sharp edges Double-exposed region formed the... University/Government/Industry Microelectronics Symposium, 389–390, June 30–July 2, 20 03, Boise, ID [25 ] R Yang, J D Williams, and W Wang, A rapid micro-mixer/reactor based on arrays of spatially impinging micro-jets, Journal of Micromechanics and Microengineering, 14, 10, 1345–1351, October 20 04 [26 ] R Yang and W Wang, Fabrication of out-of-plane SU-8 refractive microlens using directly lithography method, SPIE Photonics... quality of ultrathick SU-8 resist and the method to compensate for it The combination effect of diffraction compensation, wavelength selection, and one-direction agitation development is present by ultra-high-aspect-ratio SU-8 microstructures SU-8 tilted lithography and its application are also presented Some representative applications of UV lithography of SU-8 in microfluidics and micro-optics have also... (ISTM /20 01), Shanghai, China, June 20 01 [7] C Oropeza, K Lian, and W Wang, “Fracture toughness study on LIGA fabricated microstructures,” presented in Micromachining and Microfabrication, Photonics West, San Jose, California, January 20 03 © 20 07 by Taylor & Francis Group, LLC DK532X_book.fm Page 41 Tuesday, November 14, 20 06 10:41 AM UV Lithography of Ultrathick SU-8 41 [8] D E Lee, H.-P Chen, S Soper, and. .. 3999, P1019–1 027 , 20 00 [16] J O’Brien, P J Hughes, M Brunent, et al., Advanced photoresist technologies for microsystems, J Micromech Microeng., 11, 353–358, 20 01 [17] J Zhang, K L Tan, G D Hong, L J Yang, and H Q Gong, Polymerization optimization of SU-8 photoresist and its applications in microfluidic systems and MEMS, J Micromech Microeng., 11, 20 26 , 20 01 [18] C Lin, G Lee, B Chang, and G Chang, . 1000 125 0 1500 1750 20 00 22 50 25 00 27 50 3000 325 0 Spin speed (rpm) Film thickness (microns) SU- 8 -2 SU- 8-5 SU- 8-1 0 SU- 8 -2 5 SU-8 spin speed curves 0 50 100 150 20 0 25 0 750 1000 125 0 1500. 20 0 25 0 750 1000 125 0 1500 1750 20 00 22 50 25 00 27 50 3000 325 0 Spin speed (rpm) SU- 8-5 0 SU- 8-1 00 DK532X_book.fm Page 26 Tuesday, November 14, 20 06 10:41 AM © 20 07 by Taylor & Francis Group,. the mask and the photoresist, and d is the bsd min ()=+ 3 2 1 2 λ DK532X_book.fm Page 23 Tuesday, November 14, 20 06 10:41 AM as shown in Figure 2. 6; includes the i-line, h-line, and g-line) with Figure