1.
Introduction
In most traditional thin film transistors and high frequency circuits, the complex layer with silicon dioxide and silicon nitride layer is in common use, which can cover the shortage of poor compactness of pure silicon dioxide, because the surface of the inorganic insulation layer, such as silicon dioxide or tantalum oxide is hydrophilic, and it can probably react with the active layer material and directly affect the performance of the device.
Since early 1990s, organic material began to be used as a modification layer. Philips proposed HMDS (hexamethyldisilazene vapors, C6H19NSi2) as the modification layer of silicon dioxide substrate[1, 2]. Chen introduced organic materials OTS (octadecyltrichlorosilane) and PMMA (poly methyl methacylate) as the modification layer based on the traditional silicon dioxide[3]. All of the device, but the production process of the microcircuit is still relatively complex.
In recent years, photosensitive polyimide has been widely used as encapsulants or insulation material, such as buffer coating, passivation layer, shielding material, wafer packaging materials etc. The organic material polyimide can form a uniform surface morphology after treatment with suitable gas and temperature conditions, which can provide excellent performance as the insulating layer of a microcircuit[4–16]. The best heating rate of PI is 5 °C/min[5]. However, the properties of photosensitive polyimide are still worth researching.
In this paper, we use positive type photosensitive polyimide as the modification layer in the thin film transistors’ production process. The photosensitive polyimide is not only used as the second insulating layer, it can also be used instead of a mask because of the photosensitivity. The microcircuit fabrication process can be improved. In the experiments, the photosensitive polyimide is treated under different temperature conditions, and the ability of protecting silicon dioxide is analyzed.
2.
Process of microcircuit
In our microcircuit fabrication process shown in Fig. 1, the description of the fabrication process of the interconnecting metal wire is omitted, and only the production steps related to the photosensitive polyimide are considered. So it is divided into five process steps:
Step 1: Depositing a silicon dioxide layer and a photosensitive polyimide layer on a substrate having metal wiring; Step 2: Lithographing photosensitive polyimide pattern; Step 3: The production of the second layer of interconnecting metal wire, silicon dioxide layer and polyimide layer; Step 4: Lithographing the second layer of photosensitive polyimide pattern; Step 5: Etching the silicon dioxide pattern to facilitate the electrical formation of the metal in the microcircuit.
In the traditional process microcircuit production, the photoresist is used as a mask to produce graphics, and the photoresist need to be removed after the patterns are formed, so the production process is very complex.
However, in the process of this paper, as can be seen from Fig. 1, the photosensitive polyimide itself can be patterned by photolithographing, and the remainder does not need to be removed, because the remainder can be used as a modification layer of the silicon dioxide layer.
In the microcircuit fabrication of this paper, the first and third steps, the deposition of photosensitive polyimide process, we studied the photosensitive polyimide’s thickness and curing conditions. Through the fifth step, we can evaluate the quality of the photosensitive polyimide film by comparing the quality of the silicon dioxide after etching, and obtain the most suitable deposition and curing conditions for the photosensitive polyimide. Specific experiments will be described in the following sections in this paper.
3.
Experiments
In the experiments, thirty pieces of substrate are used to form the microcircuit. The microcircuit process is shown in Fig. 1, where in the first and third steps, the deposition of the photosensitive polyimide process, we studied the photosensitive polyimide’s thickness and curing conditions.
3.1
Film thickness
In order to get more precise data, first of all, we should point out that every substrate is coated with the same thickness of photosensitive polyimide liquid, and the thick differences between and in the pieces can be accepted. In the experiment, we interview the thickness of photosensitive polyimide and its uniformity. As shown in Table 1 and Fig. 2, the photosensitive polyimide is coated in the speed from 2000 r/s (low) to 3000 r/s (high), considering the thickness difference in pieces, we test five pieces of substrate in each coating rate. As shown in Table 1 and Fig. 2, the photosensitive polyimide films’ thickness is average and changes from 2.02 to 1.1 μm when the coating rate was changed from 2000 to 3000 r/s, and the difference in piece goes straight down from 0.28 to 0.1. Consider that the thickness is not too thin, and the difference in piece should be smaller than 0.15 at best. Taken together, we choose the rate of 2600 r/s to be the best condition in the curing process, and the thickness is 1.42 μm.
| Rate (r/s) | Coating thickness (μm) | ||||||
| y1 | y2 | y3 | y4 | y5 | Average | Stedv | |
| 2000 | 1.7 | 1.8 | 2 | 2.2 | 2.4 | 2.02 | 0.29 |
| 2200 | 1.7 | 1.7 | 1.8 | 1.9 | 2.2 | 1.86 | 0.21 |
| 2400 | 1.5 | 1.6 | 1.7 | 1.4 | 1.8 | 1.6 | 0.16 |
| 2600 | 1.2 | 1.4 | 1.4 | 1.5 | 1.6 | 1.42 | 0.15 |
| 2800 | 1.1 | 1.2 | 1.3 | 1.3 | 1.4 | 1.26 | 0.11 |
| 3000 | 1 | 1 | 1.1 | 1.2 | 1.2 | 1.1 | 0.10 |
Table1.
Thickness on different coating rate.
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| Rate (r/s) | Coating thickness (μm) | ||||||
| y1 | y2 | y3 | y4 | y5 | Average | Stedv | |
| 2000 | 1.7 | 1.8 | 2 | 2.2 | 2.4 | 2.02 | 0.29 |
| 2200 | 1.7 | 1.7 | 1.8 | 1.9 | 2.2 | 1.86 | 0.21 |
| 2400 | 1.5 | 1.6 | 1.7 | 1.4 | 1.8 | 1.6 | 0.16 |
| 2600 | 1.2 | 1.4 | 1.4 | 1.5 | 1.6 | 1.42 | 0.15 |
| 2800 | 1.1 | 1.2 | 1.3 | 1.3 | 1.4 | 1.26 | 0.11 |
| 3000 | 1 | 1 | 1.1 | 1.2 | 1.2 | 1.1 | 0.10 |
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Figure2.
Thickness on different coating rate.
3.2
Curing
Curing is the imidization process of polyimide, which is the key step to keeping heat resistance, corrosion resistance, high-impedance, and reliability．Therefore, the photosensitive polyimides’ curing conditions are tested and presented in this paper. Twenty pieces of substrate are coated by photosensitive polyimide liquid in step 1 in Fig. 1 with the proper coating, and the substrates are divided into four groups. Each group is cured in an oven with nitrogen. In addition, each group is treated under different curing conditions. In the experiment, the curing rate of condition A and B is 0.4 °C/min, and the curing rate of condition C and D is 0.9 °C/min. Each condition consists of different curing temperature and duration. The C condition is widely used in tradition, the others are chosen to make comparisons in the experiment.
Polyimide film would not be dissolved in organic solvent. Hence, in step 5 of the process in Fig. 1, two groups are tested in organic solvent to verify whether the curing process is completed. Since there are some patterns on photosensitive polyimide films during the lithography and developing progress, the change of patterns’ area could be noticed when the photosensitive polyimide film began to be dissolved.
We found that photosensitive polyimide film treated with both A and C conditions does not dissolve, because the r of the hole in step 5 in Fig. 1 is equal to our design value R, as shown in the photo in Fig. 4(a).
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Figure1.
Photosensitive polyimide as modification layer in microcircuit.
From curing conditions experiments and the results of corrosion listed in Table 2 and Fig. 3, we know that, the best curing A condition has only three temperature gradients, and achieves the same effect of the traditional curing conditions.
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Figure3.
Temperature on the different curing time.
| Sample | T (°C) | Time (min) | |
| A | 120 | 30 | r = R |
| 180 | 60 | ||
| 250 | 240 | ||
| B | 120 | 30 | r > R |
| 180 | 60 | ||
| 250 | 240 | ||
| 300 | 60 | ||
| C | 80 | 120 | r = R |
| 150 | 60 | ||
| 180 | 60 | ||
| 250 | 60 | ||
| D | 80 | 120 | r > R |
| 150 | 60 | ||
| 180 | 60 | ||
| 250 | 60 | ||
| 300 | 60 |
Table2.
Changes of r on different curing rate.
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| Sample | T (°C) | Time (min) | |
| A | 120 | 30 | r = R |
| 180 | 60 | ||
| 250 | 240 | ||
| B | 120 | 30 | r > R |
| 180 | 60 | ||
| 250 | 240 | ||
| 300 | 60 | ||
| C | 80 | 120 | r = R |
| 150 | 60 | ||
| 180 | 60 | ||
| 250 | 60 | ||
| D | 80 | 120 | r > R |
| 150 | 60 | ||
| 180 | 60 | ||
| 250 | 60 | ||
| 300 | 60 |
However, the temperature B and D have higher temperature than A condition and C condition, and we found that r will be larger than R, which is shown in the photo in Figs. 4(b) and 4(c). Because such thermal imidization process would cause volume contraction because of the loss of low-molecular substance during the cyclization process. As soon as the temperature of photosensitive polyimide reaches to a certain value, the carboxyl group and acylamino react with polyimide membranes and steam.
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Figure4.
The photo of silicon dioxide pattern in process of Step 5. (r is the actual diameter of the film hole; R is the design diameter of the film hole)
4.
Conclusions
From the experiments and result, the proper thickness is 1.42 μm with the coating speed of 4600 r/s, and the best curing condition to form high quality photosensitive polyimides’ film are obtained. The proper curing rate of the photosensitive polyimide is 0.4 or 0.9 °C/min, and the curing temperature T should not be higher than 250 °C. The photosensitive polyimide is also used as the modification layer and has the best protecting ability in the conditions above.
