## 5 Conclusions  
A water jet is frequently employed to assist laser etching of polysilicon materials. This study assessed the influence of water-jet incident angle and velocity on the depth and width of the etching groove. At a water jet incident at 60\({}^{\circ}\), the effects of processing parameters (i.e., water-jet velocity, laser pulse width, repetition frequency, and input current) on the surface quality of the composite etching were investigated using the orthogonal test method. The processing parameters were optimized and verified. By comparing the verification test results with those of the orthogonal test, the processing parameters that optimize each evaluation index were obtained. Our primary findings are summarized as follows.  
(1) The effects of 30\({}^{\circ}\) and 60\({}^{\circ}\) water-jet-assisted laser etching of polysilicon materials on the depth and width of the etched trenches were investigated. It was found that when the water jet angle was  
Figure 16: Surface and section morphologies of groove formed by composite machining under final optimized combination of processing parameters (\(\times\)30): (**a**) sectional topography and (**b**) surface topography.  
Figure 15: Cross-sectional morphology of samples after composite processing at processing parameters that optimize each evaluation index (\(\times\)30): (**a**) section taper and (**b**) section depth.  
maintained at one value, the depth of the machining tank initially increased and then decreased as water-jet velocity increased, and the width of the machining tank increased gradually. As the velocity increased from 16 to 24 m/s, the depth of the processed groove increased, but a further velocity increase (28 m/s) reduced the groove depth. The width of the processing groove was a gradually increasing function of the water-jet velocity. When the water-jet velocity was maintained at one value, the 60\({}^{\circ}\) water jet assisted the laser etching of the polysilicon material to obtain a tank with depth and width greater than that obtained by the 30\({}^{\circ}\) etching.  
(2) Taking the 60\({}^{\circ}\) water-jet-assisted laser etching of polysilicon as an example, the orthogonal experimental method was used to optimize the processing parameters. In this study, the processing quality was evaluated relative to the depth, taper, and surface topography of the processing tank (the deeper the depth of the tank, the smaller the taper and the better the surface topography).  
The experimental results demonstrated that the section taper was minimized under a 1.1-ms laser pulse width, 40-Hz frequency, and 180-A current when assisted by 28-m/s water-jet velocity. Here, the minimum section taper was 0.3\({}^{\circ}\). The tank depth was maximized with a 1.1-ms laser pulse width, 40-Hz repetition frequency, and 150-A input current when assisted by a 20-m/s water-jet velocity. Here, the maximum tank depth was 2.05 mm; however, the section taper was not ideal. The optimal processing parameters were a laser pulse width of 1.1 ms, repetition frequency of 40 Hz, water-jet velocity of 24 m/s, and input current of 180 A. Under these conditions, the cross-section taper and groove depth were 1.2\({}^{\circ}\) and 1.88 mm, respectively, and the groove surface demonstrated no defects.  
[PERSON] designed the study. [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] developed the methodology. [PERSON] wrote manuscript.  
This research was funded by Provincial Key Project of Natural Science Research Anhui Colleges (grant number: KJ2015A013 and KJ2015A050), Anhui Province Outstanding Young Talent Support Program Key Project (grant number: xyqZD2016153), National Natural Science Foundation of China (grant number: 51175229).  
We thank all the authors for their joint efforts to complete the experiment. We also thank Jiangnan University for providing experimental equipment.  
The authors declare no conflicts of interest.