In the field of micro-nano processing, electron beam lithography (EBL) has become one of the key technologies for preparing high-precision micro-nano structures due to its high resolution and flexibility. In this process, precise control of exposure dose is not only related to precise pattern formation, but also directly affects the performance and reliability of the final device. This article will introduce a very important parameter in the conditions of the exposure process of electron beam lithography: exposure dose.
1. What is electron beam lithography?
Electron beam lithography uses high-energy focused electron beams to directly illuminate a substrate coated with photoresist. Through the interaction of electrons and photoresist materials, chemical or physical changes are induced to form the desired pattern on the photoresist technology. This process is much more complex than traditional optical lithography because it involves the direct interaction of electrons and matter at the nanoscale, and the various effects that result. The current direct writing method of our common Gaussian beam e-beam lithography system is based on vector scanning technology. Its beam can be ideally simplified as a circular beam, its light intensity has a Gaussian distribution, and the beam generally travels along a straight line in direct writing.
2. What is the exposure dose?
Among the many parameters of the electron beam lithography process, the exposure dose is undoubtedly crucial. Exposure dose refers to the number of electrons received per unit area, which directly determines the degree of exposure of the photoresist and the quality of subsequent pattern formation. Different photoresist types, thicknesses and pattern characteristics require different exposure doses. Therefore, precise determination and control of the exposure dose constitutes a major challenge in optimizing the electron beam lithography process.
3. Gaussian beam e-beam lithography system
Currently, Gaussian beam e-beam lithography system has become the common choice for direct-write e-beam lithography due to its efficient vector scanning technology. In this system, the electron beam is focused on an approximately circular beam point and its light intensity distribution follows a Gaussian function, i.e. the intensity is highest at the center and gradually weakens towards the environment. The beam point moves linearly along the predefined path. By controlling the speed of movement of the beam spot and the dwell time at each pixel (i.e. “dwell time”), precise control of the exposure dose is achieved.
4. Calculation and control of the exposure dose
1. Formula for calculating the exposure dose:Exposure dose = exposure light intensity * duration
For different types of exposure graphs (such as area scan, single pixel line, single point), the exposure dose calculation formula will be different. But the basic idea is the same: determine the exposure dose by calculating the total number of incident electrons required. These formulas generally take into account parameters such as the effective beam spot radius (i.e. the size of the zone of influence), scan speed, dwell time and beam current d ‘electrons.
The above formula reveals that upon exposure to the electron beam, the dose is determined by the intensity of the beam over a specific period of time. This dose accumulates on the surface of the photoresist, reflecting the build-up of charge. It should be noted that the beam point is not a zero-dimensional point feature, but has a specific range of influence, i.e. the effective radius (S, also called step size), which defines the area of action of the beam spot. Based on this understanding and the time required for the beam point to stay at each pixel point to deliver an effective dose (i.e. dwell time Tdwell), the scanning speed during the process of exposure is determined. Therefore, the step size and dwell time settings jointly determine how fast the beam spot moves during the exposure process.
2. Analysis of influencing factors: type of photosensitive resin
Different types of photoresists have different sensitivities to electrons and therefore require different exposure doses to achieve the same exposure effect.
Type and conditions of developer Type, concentration, temperature, etc. of the developer will affect the development rate and quality of the photoresist, which will indirectly affect the selection of the exposure dose.
Cooking temperature
The baking process can remove the solvent from the photoresist and improve its adhesion to the substrate. It also affects the response characteristics of the photoresist to electrons.
Accelerating Voltage of the Electron Beam Exposure System The accelerating voltage determines the energy of the electron beam, which in turn affects the depth and efficiency of its interaction with the photoresist.
3. Determination of dosage in practice:
In actual operation, although we can make preliminary calculations based on the above theory, a more common and efficient method is to use the reference dose provided by commercial electron beam photoresist products, combined with the parameters specific to the electron beam exposure system’s own (such as acceleration voltage), the thickness of the required photoresist and the structural characteristics of the exposure pattern, the appropriate exposure dose can be quickly determined through simple experiments dose. This process usually involves multiple exposure experiments to find the optimal exposure dose value by comparing the patterning quality at different doses.
5. The relationship between exposure dose and pattern quality
Precise control of exposure dose is crucial to ensure pattern quality. Too low an exposure dose may result in incomplete exposure of the photoresist and blurred or missing pattern edges, while too high an exposure dose may cause overexposure of the photoresist, resulting in distortion of the pattern or a reduction in its size. Therefore, finding a “golden point” of exposure dose that can ensure clear and complete patterns while avoiding overexposure is the goal pursued by every e-beam lithography engineer.
With the advancement of science and technology, electron beam lithography technology is moving in a more intelligent and automated direction. The application of advanced exposure dose prediction models, real-time monitoring systems and automated optimization algorithms will make exposure dose determination more accurate and efficient.
At the same time, the development of new photoresist materials has also brought new possibilities to electron beam lithography technology, such as materials with higher sensitivity and better resolution, which will further improve the performance limits electron beam lithography.
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