Non-contact method of thickness measurement for a transparent plate using a laser auto-focus scanning probe

Quangsang Vo; Yiting Duan; Xiaodong Zhang; Fengzhou Fang

doi:10.1364/AO.58.009524

Applied Optics
Vol. 58,
Issue 35,
pp. 9524-9531
(2019)
•https://doi.org/10.1364/AO.58.009524

Non-contact method of thickness measurement for a transparent plate using a laser auto-focus scanning probe

Quangsang Vo, Yiting Duan, Xiaodong Zhang, and Fengzhou Fang

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Quangsang Vo, Yiting Duan, Xiaodong Zhang, and Fengzhou Fang, "Non-contact method of thickness measurement for a transparent plate using a laser auto-focus scanning probe," Appl. Opt. 58, 9524-9531 (2019)

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Abstract

In this paper we propose a novel non-contact method of thickness measurement for a transparent plate using a laser auto-focus scanning (LAFS) probe. Through the analysis of the focus error signal (FES) from the actual specimen’s upper and lower surface reflectances, a measurement model is introduced to find the best focus position on the corresponding surface of the transparent plate. Thus, the thickness of a specimen can be obtained. Herein, the LAFS probe measurement characteristics by auto-searching the FES method can be analyzed, which can then realize non-contact measurement and focus detection of micro-displacements. Therefore, we have successfully designed a low-cost, high-precision auto-focusing thickness measurement system with linear piezoelectric nano-positioning. To verify the proposed method, some transparent specimens were evaluated, and the thicknesses were rapidly and successfully measured. Experiments confirm the validity of the proposed system, and the results show that the method is effective for the thickness measurement of transparent plates.

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Specimens’ Thickness (µm)	Average Value (µm)	Standard Deviation (µm)	Average Error (µm)	Relative Standard Error (%)
$11.9 \pm 0.5$	10.95	0.60	0.47	5.47
$25.0 \pm 0.5$	24.90	0.77	0.60	3.11
$50.0 \pm 1.0$	49.83	1.02	0.84	2.05
$49.7 \pm 0.3$	49.64	0.66	0.51	1.32

Time	Thickness (µm)	Error (µm)
1	10.17	1.73
2	11.13	0.77
3	11.63	0.27
4	11.77	0.13
5	11.38	0.52
6	11.13	0.77
7	11.34	0.56
8	11.43	0.47
9	10.85	1.05
10	10.32	1.58
11	10.13	1.77
12	9.33	2.57
13	10.56	1.34
14	10.94	0.96
15	11.21	0.69
16	11.24	0.66
17	11.19	0.71
18	11.33	0.57
19	10.61	1.29
20	11.30	0.60

Time	Thickness (µm)	Error (µm)
1	24.68	0.32
2	25.04	0.04
3	24.87	0.12
4	24.56	0.44
5	24.61	0.38
6	26.16	1.15
7	23.59	1.40
8	25.59	0.59
9	25.72	0.72
10	24.95	0.05
11	23.43	1.57
12	24.51	0.50
13	24.40	0.60
14	24.58	0.42
15	24.29	0.71
16	25.90	0.90
17	26.37	1.37
18	24.50	0.50
19	25.44	0.44
20	24.71	0.29

Time	Thickness (µm)	Error (µm)
1	50.72	0.72
2	51.50	1.50
3	51.17	1.17
4	49.85	0.15
5	50.50	0.50
6	49.86	0.14
7	50.30	0.29
8	50.29	0.29
9	50.39	0.39
10	49.78	0.22
11	50.57	0.57
12	50.70	0.70
13	50.49	0.49
14	49.13	0.87
15	48.40	1.60
16	48.74	1.26
17	48.09	1.91
18	48.67	1.33
19	48.02	1.98
20	49.39	0.60

Time	Thickness (µm)	Error (µm)
1	49.67	0.33
2	50.03	0.03
3	49.73	0.27
4	49.97	0.03
5	49.28	0.71
6	49.46	0.54
7	49.00	0.99
8	49.21	0.79
9	50.51	0.51
10	50.63	0.63
11	49.68	0.32
12	49.29	0.71
13	51.03	1.03
14	49.32	0.68
15	50.10	0.10
16	48.40	1.60
17	49.63	0.37
18	50.12	0.12
19	48.85	1.15
20	48.83	1.17

Calibration Axis	Sensor Resolution (µm)	Stage Moving Displacement (µm)	Generate Pulse Number (µm)	Displacement Resolution (nm/pulse)
$x$ axis	0.37	10	30	333
$y$ axis	0.37	9	12	730

Measured Point	True Thickness (µm)	Average Thickness (µm)	Standard Deviation (µm)	Average Error (µm)	Relative Standard Error (%)
1		49.85	0.95	0.80	1.90
2		49.70	0.34	0.28	0.68
3		50.03	0.54	0.47	1.08
4	$49.7 \pm 0.3$	49.53	0.64	0.50	1.30
5	$49.7 \pm 0.3$	49.51	0.47	0.30	0.95
6		49.50	0.54	0.43	1.10
7		49.58	0.90	0.70	1.81
8		49.64	0.66	0.51	1.32

Abstract

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Applied Optics