Size Controllable Micro-nano Fluidic Systems Based on Various Angle Sputtering Deposition

Micro-nano fluid systems are widely used in biochemistry and are important to MEMS research. For the fabrication of microfluidic systems, controllable size nanochannels have always been a research challenge. This article proposes a method for fabricating nanochannels based on angle deposition, which can regulate channel size at tens of nanometers. At the same time, DNA molecular stretching simulations and experiments were conducted to verify the application prospects of the sample in the biological field.


Introduction
Micro-nano fluid systems can be widely used in protein analysis [1][2], molecular manipulation [3][4], ion separation and transport [5][6], drug release technology [7] and other fields.They are an important research branch in the field of MEMS.When the channel size reaches the nanoscale, the fluid in it exhibits some unique properties that are different from macroscopic fluids [8][9], thus attracting more and more researchers' interest.The core of the production of micro/nanofluid systems lies in processing micro-nano fluid channels [10].The key issue is how to achieve low-cost, efficient, and high-precision fabrication of micro-nano fluid channels.Researchers favour polymer materials due to their good chemical and mechanical properties and suitability for micro/nano processing.Polymer micro-nano grooves can be fabricated using techniques such as nanoimprinting [11] and electron beam direct writing [12] and combined with sacrificial layer etching technology [13] or thermal bonding technology to achieve top sealing of channels.However, most of the above technologies have certain limitations, making it difficult to form micro grooves, chambers, and nanochannels simultaneously.The two typical structures mentioned above often coexist in the design of microfluidic systems.Therefore, this article proposes a method for manufacturing micro nano fluid systems based on hybrid lithography imprinting templates and angle deposition methods.This method can effectively solve the problems of difficult simultaneous fabrication of micro nano scale structures and uncontrollable nano channel sizes.At the same time, relevant simulations of DNA molecular stretching were conducted on the produced samples to verify their effectiveness.

Design and manufacture process of the micro-nano fluidic chip
The fluidic chip is fabricated on a SiO 2 substrate.It contains nano channels, micro trenches, and four chambers, as shown in Figure 1.The width of the nano channel is designed as 50 nm-100 nm, with detail value determined by fabricate parameters.The chambers are used to store and collect solutions, and micro trenches are used to connect the chamber and nano channels.The solutions contain DNA molecules that are guided and flow into channels.Then the DNA will be stretched.The fabrication process of the fluidic chip can be divided into several steps.Firstly, holographic lithography is used to produce nanochannel templates (as shown in Figure 2), and then the patterns are transferred to the polymer through nanoimprinting.On the other hand, micrometer trench templates (as shown in Figure 3) are made using the traditional photolithography lift-off process, and the patterns are also transferred to the polymer material containing nanochannels through transfer printing.The area surrounded by the red line in Figure 4 is a nanochannel structure surrounded by micron grooves and chamber structures.Finally, the SiO 2 film is deposited onto the top of the groove through a certain coating angle to form a seal.When the incident angle changes, the scale of the nano channel will also transform.As shown in Figure 5, as the incident angle increases, the width of the nano channel decreases.As the key parameter, the incident angle can be adjusted to control the nano channel's scale effectively.In addition, the excess deposition of SiO 2 thin films and the formation of triangular-like channels also contribute to increasing the structural strength of the channels.

Width of channel
Width of channel Width of channel The fabricated sample is shown in Figures 6-8 for different sputtering angles.The width of nano channels is from 210 nm to 44.5 nm as the incident angle is from 45° to 75° and the above fabrication results are consistent with the simulation results.

Simulation and experiment of DNA stretching
The Before DNA molecule is a long chain double helix structure connected by four deoxynucleotides as the basic unit, and the diameter of a single helix is about 2 nanometers.However, the DNA molecular chain will curl into a group under its own interaction, which is not conducive to accurately studying its characteristics.Because DNA molecules can be well stretched in nano channels, which makes it easier for scientists to obtain DNA genetic information more accurately, nano channels have become an important tool for DNA sequencing.
Computer simulation is a good research approach to describe the stretching characteristics of DNA molecules in nanochannels more realistically and accurately.At the beginning of the simulation, we set the DNA molecules to be completely straightened (Figure 9(a)), and the DNA will shrink into clusters under its interaction force (Figure 9(b)).At the same time, the DNA molecular chains will move towards smaller nanochannels under electrophoretic force and expand within the channels (Figure 9(c) and Figure 9(d)).This model achieves the modeling of DNA molecular chains and proves this research method's feasibility.Therefore, based on this model, combined with the characteristics of channel size and optimization of simulation parameters, it is possible to simulate the tensile properties of DNA molecules in different nanochannels.

Conclusions
This article proposes a new method for fabricating nanochannels by combining various MEMS processes to prepare size-controllable nanofluid systems.She has completed the production of hybrid lithography imprinting templates.The production of sample slots and nano groove structures has been achieved.The sealing of different sizes of nanochannels was achieved through the angle deposition method, and the size of the nanochannels was controlled below 50 nm.Based on the size characteristics of nanochannels, DNA motion characteristics were studied using COMSOL multi-physical field coupling software, and biological experiments on DNA stretching were conducted to verify the application prospects of the produced samples in biochemistry.

Figure 1 .
Figure 1.The design of the micro-nano fluidic chip

Figure 2 .
Figure 2. The template of the nano channels Figure 3.The template of the chambers and micro trenches

Figure 4 .
Figure 4.The sputtering deposition process achieves the forming and sealing of the chip

Figure 5 .
Figure 5.The forming simulation results of the nano channels by COMSOL software for the different incident angle

Figure 6 .Figure 7 . 4 Figure 8 .
Figure 6.The sample was fabricated by sputtering deposition with an incident angle of 45°

Figure 9 .
Figure 9.The simulation results of DNA stretch for the nano channel in Figure 8 Scholars have proposed many models for modeling DNA molecules, among which the most popular is the bead spring model shown in the figure below, where each bead represents the shortest DNA

Figure 10 .
Figure 10.DNA stretching experiment We place the stained DNA fragment at the entrance of the microfluidic chip under capillary force.It naturally flows into the channel and stretches, and we use a fluorescence microscope to align with the standard, as shown in Figure 10.