Improvement of Column Spacer Uniformity in a TFT LCD Panel

Transcription

1 Journal of the Korean Physical Society, Vol. 48, No. 2, February 2006, pp Improvement of Column Spacer Uniformity in a TFT LCD Panel Jung-Hyuk Cho and Jung-Min Sohn LCD Business, Samsung Electronics Co.,Cheonan Sung-Hyuck Kim Semiconductor Business, Samsung Electronics Co., Yongin Jong-Sun Kim, Young-Hoon Kim and Hye-Keun Oh Department of Applied Physics, Hanyang University Ansan (Received 10 October 2005) The photoresist of a column spacer is very sensitive to UV light, and its uniformity depends on the exposure process. In a proximity-type aligner, the exposure gap can control the light diffraction and intensity, which affect the column spacer thickness and the critical dimension. The limitation of non-flat mask and work stage causes a non-uniform exposure gap. Applying an attenuated phase shift mask can compensate for this handicap. By using commercial software, we analyzed the difference between a conventional mask and a phase-shift mask as a function of the exposure gap and examined changes in the column spacer s thickness loss and the critical dimension. In order to reduce the diffraction effect and to get a uniform intensity, we varied the transmission in an opaque mask area and found that the thickness uniformity could be improved by about 30 %. Consequently, we expect the attenuated phase shift mask to have a larger possibility of improving the column spacer s thickness uniformity without any mechanical alteration. PACS numbers: 42 Keywords: TFT LCD, Column spacer, Diffraction, Lithography, Phase-shift mask I. INTRODUCTION The trend of a thin film transistor liquid crystal display (TFT LCD) panel is toward a larger size and a higher resolution. As the size of the LCD panel gets larger, keeping the same cell gap between the TFT and the color filter glass with a bead spacer is getting harder, so another type of spacer is needed to replace the bead spacer. There are many approaches to obtain this goal in LCD panel industry, and a column spacer [1] (JSR JNPC 67-R3) is one that satisfies the customer s needs. A column spacer is located on the color filter glass, exactly on the blue pattern. Its figure is usually a circle, but that differs among various model types. The thickness of a column spacer is about 3.0 µm in the patterned vertically alignment model and 4.2 µm in the twisted nematic model. The critical dimension (CD), which is the diameter of column spacer s bottom, is usually µm, and the density is cm 3. Though it looks very easy and simple to make such a pattern, there are some problems in making a good-quality LCD panel. jh001.cho@samsung.com; hyekeun@hanyang.ac.kr Defects of LCD panel such as an active unfilled area (AUA) or a gravity gap (G-GAP [2], the liquid crystal in the panel is pulled down by gravity and it enlarges the cell gap.) come from the nonuniformity of the column spacer thickness, which depends on the liquid crystal s filling state. If the column spacer thickness is different in some areas on the same panel, the liquid crystal will be over-filled (G-GAP) or unfilled (AUA), which will be inversely proportional to the column spacer thickness. This makes an abnormal LCD picture, and G-GAP can occur even a few months after the LCD was made. The best way to be free from these problems is to improve the uniformity of the column spacer. There are many factors affecting column spacer uniformity. The Coating process is important: for example, the spin rpm, the soft bake temperature, and so on. However, nothing is more influential than the exposure process. The exposure process must be done precisely because the photoresist of column spacer is very sensitive to UV light. In a proximity aligner, the major parameter is an exposure gap between the mask and the color filter glass because the exposure gap affects the degree of light diffraction and the intensity [3]. The bending of mask and the flatness rate of work stage affect the exposure

2 Improvement of Column Spacer Uniformity in a TFT LCD Panel Jung-Hyuk Cho et al gap, which controls the column spacer thickness and the CD. There is a technical limit to making the mask and the work stage perfectly flat, so we should find a solution in the optical parts. In order to get a uniform intensity transferring through the mask, we can change the mask type to a phase-shift mask (PSM) [4] and use a shorter UV wavelength (248 nm) from a KrF excimer laser, but changing the UV light requires a different kind of photoresist and optical structure. Thus, we chose to vary the transmission and the phase of the opaque mask area and simulated the effect of the column spacer thickness and the CD as functions of the exposure gap by using a commercial Solid-C program by Sigma-C. We found that the attenuated PSM was not effective for controlling the uniformity of the CD, but thickness loss was reduced about 30 %. Consequently, the quality of the TFT LCD panel can be upgraded, and the manufacturing process can be much easier. 1. Role of the Column Spacer A spacer is a material used to maintain a uniform cell gap between the TFT and the color filter glass. There are two kinds of spacer applied in LCD panel, a bead spacer and a column spacer. A bead spacer is not adequate for larger-sized LCD panels because of the nonuniformity of the cell gap and many handicap in manufacturing process. For this reason, bead spacers are being changed to column spacers to satisfy today s manufacturing trend and customer needs. A column spacer is patterned on the color filter glass, exactly on the blue pattern, by using a negative photoresist, and the process includes cleaning, coating, exposure, developing and oven baking. Figures 1 and 2 show cross section of an LCD panel. We can see a bead spacer and a column spacer in the center of the panel. Figure 3 is a top-down microscopic picture. Fig. 3. Top-down microscopic picture of a column spacer on a color filter glass (magnification: 100 X). 2. AUA and G-GAP If cell gap uniformity is not good during the liquid crystal process, there will be some defects, such as AUA or G-GAP, which come from liquid crystal overfilling or unfilling. Figure 4 shows a TFT LCD panel with two kinds of defects that make an abnormal picture. We can see that AUA causes spots on the display and G-GAP causes some area to be whiter than other areas. Both come from a nonuniformity of the column spacer thickness. As Figure 5 shows, the normal state has a liquid crystal filling level that is the same as column spacer thickness, but G-GAP has been over filled. On the contrary, AUA shows a behavior opposite to that of G-GAP. In order to reduce the effects of these defects, we should Fig. 4. Abnormal pictures caused by AUA and G-GAP. AUA causes spots on the display, and G-GAP causes some area to be whiter than other areas. Both come from the nonuniformity of the column spacer thickness. Fig. 1. Bead spacer in a TFT LCD panel. Fig. 2. Column spacer in a TFT LCD panel. Fig. 5. Normal state has a liquid crystal filling level which is the same as the column spacer thickness, but G-GAP has been too much filled. On the contrary, AUA shows an opposite behavior compared to G-GAP.

3 -242- Journal of the Korean Physical Society, Vol. 48, No. 2, February 2006 find the vital process affecting the column spacer uniformity 3. Proximity-type Aligner in the Column Spacer Process First of all, we are going to deal with a proximity-type aligner, which is widely used in the color filter manufacturing process. It usually has an i-line lamp whose intensity is about 30 mj/cm 2 and an exposure unit, such as work stage and mask stage. Unlike the projectiontype stepper, there is no lens below the mask, so in the proximity-type aligner, we can control diffraction by adjusting the exposure gap. The exposure gap is the gap between the mask and the photoresist on the LCD panel. If this gap is large, the intensity transferring through the mask will be small, and the area exposed by the diffracted light will be large. As a result, the CD of the column spacer is large, and its thickness is small. On the contrary, if the exposure gap is small, the behavior will be opposite. This behavior can be approximated as [5] CD u + λd/u, where u is the mask hole size, λ is the wavelength, and D is the gap between the mask and the glass. Also, the light intensity, I, is related with the gap as [3] I = ε A u 2 /(2D 2 ), where ε A is the source strength per unit area. Figure 6 is the result of measuring the column spacer thickness and the CD as functions of the exposure gap. We can find that the lines of thickness and CD have opposite behaviors with increasing exposure gap. Figure 7 is a microscopic picture, which supports the measurement result. 4. Vital Factor Affecting Column Spacer Uniformity in the Exposure Process A photomask used in the column spacer process has a mm 3 size and weighs over 10 kg. Because of its big size and largeweight, the mask surface is not flat when it is attached to the mask stage. Gravity causes the center of the mask to sag about 40 µm, which is the reason the exposure gap varies. The surface of the work stage is not flat either. There are ups and downs in some areas, which make the exposure gap nonuniform. Figure 8 is a mask-bending simulation result from NSK Co. In this figure, we see that the CD of the column spacer around the center of the mask is smaller than those of other areas of the mask and that the column Fig. 8. Simulation result for mask bending. The CD of the column spacer around the center of the mask is smaller than those of other areas of the mask, and the column spacer thickness is larger around the center because of diffraction. Fig. 6. Column spacer thickness and CD as functions of the exposure gap. The lines for the thickness and the CD show reverse trends (measured by SOKKIA SMIC-11 & DEKTAK OSP-1100). Fig. 7. Microscopic picture of the column spacer CD as a function of the exposure gap (magnification: 100 ). Fig. 9. Measurement result of the column spacer CD. The range of the CD variation caused by diffraction shows the same trend as the mask bending simulation (measured by DEKTAK OSP-1100).

4 Improvement of Column Spacer Uniformity in a TFT LCD Panel Jung-Hyuk Cho et al Table 1. Experimental and simulation conditions; three major parameters, such as the mask transmission, the phase and the defocus, are varied. Process unit Parameters Specifications JSR JNPC 67-R3 Photoresist Photoresist (negative photoresist) fixed & Primary thickness 4.0 µm fixed Coating Refractive index 1.53 fixed Soft bake (temperature/time) 90 C/100 s fixed Exposure Defocus µm variable Wavelength i-line (365 nm) fixed Numerical aperture 0.04 fixed Transmission on opaque area 0 3 % variable Phase on opaque area variable Coherence 1 fixed Intensity 30 mj/cm 2 fixed Exposure dose 100 mj fixed Resolution 1.2 µm fixed Mask pattern size µm 2 fixed Development Model Mack4 fixed spacer thickness is larger around the center because of diffraction. Figure 9 is a measurement result for the CD of a column spacer, and we can see that the range of CD variation caused by diffraction shows the same trend as the mask bending simulation shows. II. SIMULATION CONDITIONS Knowing how to prevent changes in the column spacer thickness from causing variations in the exposure gap is a key. There might be many possible ways to achieve this goal, but we tried to solve this problem by using an attenuated phase-shift mask because it has already been applied to the semiconductor process and is very effective in reducing diffraction. An attenuated PSM is coated with shifter materials, such as MoSiON, CrON, WSi, SiN, and so on, instead of chrome in order to transmit a little light with a 180 phase shift on the opaque area. Table 1 shows the experimental condition and contains the three major parameters, the mask transmission, the phase, and the defocus. In Solid-C program, the exposure gap is fixed, so we varied the defocus instead, which gave us results similar to those obtained by varying the gap. III. RESULTS 1. Pattern Profile as a Function of Opaque Area Transmission and Phase Figure 10 is a pattern profile resulting from the Solid-C program. As we can see, the thickness loss is large with Fig. 10. Pattern profile of the column spacer from Solid-C program. The thickness loss is larger for a larger defocus. large defocus, but the CD is too large for the case of 3.0 % transmission and a defocus over 320 µm. Unlike the semiconductor process which uses 5 10 % transmission and a 180 phase, the best pattern profile was made for the case of % transmission and a 0 phase. When a 180 phase is applied, the thickness loss is worse than it is for 0 phase for every transmission, which is opposite to what we had expected.

5 -244- Journal of the Korean Physical Society, Vol. 48, No. 2, February 2006 Fig. 11. Simulation results for the thickness losses of a conventional mask and an attenuated mask. In the case of a conventional mask (0 % transmission), the thickness loss ranges %, but in case of an attenuated mask ( % transmission), the thickness loss s range is reduced down to %. 2. Thickness Loss as a Function of Opaque Area Transmission Figure 11 shows simulation results for the thickness losses of a conventional mask and an attenuated mask. In case of a conventional mask (0 % transmission), the thickness loss ranges from %, but in the case of an attenuated mask ( % transmission), the thickness loss range is reduced down to %. This result shows that the thickness loss can be minimized by 30 % when attenuated mask is used. 3. Critical Dimension as a Function of Opaque Area Transmission Figure 12 shows simulation results for the CD at the substrate in a conventional mask and an attenuated mask. As we can see in this figure, the range of CD is µm for 0 % transmission, µm for 1.0 % transmission, and µm for 2.0 % transmission. For the 3.0 % transmission mask, some intervals are overexposed, so the pattern became too large. As a result, an attenuated mask is not effective in reducing the range of the column spacer CD. 4. Future Work We expect the column spacer uniformity to be improved by using an attenuated mask, but there are some undesirable behaviors. Because of the light transmission in the opaque area, the CD became 20 % larger than that of a conventional mask. For further works, we will study the following. Fig. 12. Range of CD is µm for 0 % transmission, µm for 1.0 % transmission, and µm for 2.0 % transmission. a. improving the CD uniformity, b. a CD increase in the phase-shift mask, and c. a phase shift in a negative photoresist. IV. CONCLUSIONS The column spacer plays an important role in a TFT LCD panel. It has the merit of maintaining the cell gap in a large-sized LCD panel, but unlike a bead spacer, defects such as AUA or G-GAP will occur if the column spacer thickness is not uniform. The major process that affects the column spacer uniformity is exposure. The intensity and the diffraction of light transferring through mask must be uniform everywhere on the exposure field, but in case of a proximity-type aligner, which has largesized mask and work stage, it is very difficult to maintain exposure gap uniformly because of mask bending and the flatness of work stage. In order to minimize variations in the column spacer thickness with the exposure gap, we adopted an attenuated mask. We expected that it would be effective in controlling diffraction in a negative photoresist, and we obtained a hopeful result by using Solid-C program. We analyzed the difference between the conventional mask and the attenuated mask as a function of the defocus and examined the behaviors of the column spacer thickness loss and the CD. The main variables are the transmission and the phase of opaque mask area. The thickness uniformity can be improved by 30 % for % transmission and 0 phase compared to 0 % transmission and 0 phase. However the thickness uniformity for a 180 phase is worse than it is for a 0 phase by over 30 % at every transmission. As for a CD, there is no difference between a conventional mask and an attenuated mask. The uniformity of the CD is not affected very much by AUA or G-GAP, but we should study this behavior in a future work. Consequently, we

6 Improvement of Column Spacer Uniformity in a TFT LCD Panel Jung-Hyuk Cho et al expect the attenuated phase shift mask to offer a great possibility of improving column spacer uniformity. REFERENCES [1] S. H. Chen, H. S. Koo, W. Y. Chen, C. H. Kang and D. Y. Goang, SID Symposium Digest 36, 539 (2005). [2] S. Choi, J. Korean Institute of Electrical and Electronics Material Engineers 17, 871 (2004). [3] E. Hecht, Optics, 4th edition (Addison Wesley, San Francisco, 2002), p [4] H. J. Levinson, Principles of lithography (SPIE Press, Washington, 2001), p [5] D. Halliday, R. Resnick and J. Walker, Fundamentals of Physics (Wiley, Hoboken, 2005), p. 966.