1When the study was made, the authors were with the Electronic and Electrical Engineering Department, University College London, Torrington Place, London WC1E 7JE, UK.
2A. N. Sloper is now with the M Squared Technology Ltd., 5 Noss House, Bridge Road, Kingswear TQ6 0FB, UK.
Hard, impermeable, glassy, metal phosphate films have been fabricated inexpensively by the use of a spin-coating and low-temperature-curing technique. Films that are suitable for use as monomode waveguides in biosensors have been identified through an examination of the optical and chemical properties of films containing Fe, Al, Ga, In, Cr, or V. The refractive index is controlled over the range 1.49–1.78 by varying the film composition. The film thickness is controlled over the range 50–1200 nm by varying the spin speed and the deposition temperature. Films can be patterned by photolithography or by embossing. Input coupling through an embossed grating of 833-nm pitch is demonstrated.
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Substrate Dependence of the Maximum Intact Film Thicknesses of 1:1 InPO4 and 1:1 FePO4 Films
Substrate
Expansion Coefficient (× 10−6 K−1)
Maximum Intact Film Thickness (nm)
InPO4
FePO4
Cured at 200 °C after 70 °C precuring
Float glass
7.75
550 ± 50
660 ± 50
Microscope slide
5.2
240 ± 30
325 ± 10
Silica
0.55
200 ± 30
225 ± 30
Cured directly at 200 °C
Float glass
7.75
730 ± 10
750 ± 50
Microscope slide
5.2
690 ± 10
700 ± 50
Silica
0.55
680 ± 10
—
Table 2
Curing-Temperature Dependence of the Maximum Intact Film Thicknesses of 1:1 InPO4 and 1:1 FePO4 Films Deposited on Microscope Slides and Precured at 70 °C
Maximum Intact Film Thickness (nm)
Film
125 °C
200 °C
300 °C
InPO4
400 ± 70
240 ± 30
225 ± 30
FePO4
405 ± 55
325 ± 10
240 ± 40
Table 3
Deposition Temperature Dependence of the Maximum Intact Film Thicknesses of 1:1 FePO4 Films on Microscope Slides Cured Directly at 200 °C
Deposition Temperature (°C)
Maximum Intact Film Thickness (nm)
20
700 ± 50
33
780 ± 50
47
850 ± 70
60
1230 ± 100
Table 4
Curing-Temperature Dependence of the Thicknesses, Absorptivities, and Refractive Indices of 1:1 InPO4 Films Cured Directly at 200 °C
Curing Temperature (°C)
Thickness of Films Spun at 2000 rpm (nm)
Refractive Index
λ0 = 543.5 nm
λ0 = 632.8 nm
125
480 ± 10
1.6051 ± 0.0034
1.6002 ± 0.0030
200
575 ± 10
1.5938 ± 0.0044
1.5883 ± 0.0032
300
610 ± 15
1.5903 ± 0.0041
1.5830 ± 0.0042
Table 5
Curing-Temperature Dependence of the Thicknesses, Absorptivities, and Refractive Indices of 1:1 FePO4 Films Cured at 200 °C after a 70 °C Precuring Stage
Curing Temperature (°C)
Thickness of Films Spun at 1000 rpm (nm)
Absorptivity at 255 nm (μm−1)
Refractive Index λ0 = 632.8 nm
125
300 ± 7.5
7.53
1.7122 ± 0.0036
200
290 ± 5
9.80
1.7266 ± 0.0055
300
275 ± 5
11.3
1.7281 ± 0.0075
Table 6
Refractive Indices of 1:1 InPO4 Films Cured Directly at 200 °C
Refractive Index
Cooling Procedure
λ0 = 543.5 nm
λ0 = 632.8 nm
30 °C/min
1.5921 ± 0.0040
1.5881 ± 0.0031
12 °C/min
1.5938 ± 0.0037
1.5886 ± 0.0023
2 °C/min
1.5933 ± 0.0034
1.5899 ± 0.0027
Quenching in ethanol at 20 °C
1.5940 ± 0.0036
1.5890 ± 0.0023
Quenching in acetone and dry ice at −95.4 °C
1.5922 ± 0.0066
1.5906 ± 0.0027
Table 7
Chemical Resistance of FePO4 Films Cured after 70 °C Precuring
Phosphate-buffered saline at 100 mM and pH 7.3.
Films decompose to a reddish-brown gelatinous substance within 3 h.
Ethylene diamine-tetraacetic acid at 5% w/v and pH 7.3.
Table 8
Embossing Times for 550-nm-Thick Films
Metal Phosphate Coating Solution Composition
Optimum Time Spent at 20 °C and 50% Relative Humidity after a 60-s Spin (min)
1:1 Al(NO3)3:H3PO4
70 ± 5
1:1 In(NO3)3:H3PO4
35 ± 3
1:1:1.5 In(NO3)3:H3PO4:HNO3
35 ± 3
1:1 Fe(NO3)3:H3PO4
9 ± 0.5
Tables (8)
Table 1
Substrate Dependence of the Maximum Intact Film Thicknesses of 1:1 InPO4 and 1:1 FePO4 Films
Substrate
Expansion Coefficient (× 10−6 K−1)
Maximum Intact Film Thickness (nm)
InPO4
FePO4
Cured at 200 °C after 70 °C precuring
Float glass
7.75
550 ± 50
660 ± 50
Microscope slide
5.2
240 ± 30
325 ± 10
Silica
0.55
200 ± 30
225 ± 30
Cured directly at 200 °C
Float glass
7.75
730 ± 10
750 ± 50
Microscope slide
5.2
690 ± 10
700 ± 50
Silica
0.55
680 ± 10
—
Table 2
Curing-Temperature Dependence of the Maximum Intact Film Thicknesses of 1:1 InPO4 and 1:1 FePO4 Films Deposited on Microscope Slides and Precured at 70 °C
Maximum Intact Film Thickness (nm)
Film
125 °C
200 °C
300 °C
InPO4
400 ± 70
240 ± 30
225 ± 30
FePO4
405 ± 55
325 ± 10
240 ± 40
Table 3
Deposition Temperature Dependence of the Maximum Intact Film Thicknesses of 1:1 FePO4 Films on Microscope Slides Cured Directly at 200 °C
Deposition Temperature (°C)
Maximum Intact Film Thickness (nm)
20
700 ± 50
33
780 ± 50
47
850 ± 70
60
1230 ± 100
Table 4
Curing-Temperature Dependence of the Thicknesses, Absorptivities, and Refractive Indices of 1:1 InPO4 Films Cured Directly at 200 °C
Curing Temperature (°C)
Thickness of Films Spun at 2000 rpm (nm)
Refractive Index
λ0 = 543.5 nm
λ0 = 632.8 nm
125
480 ± 10
1.6051 ± 0.0034
1.6002 ± 0.0030
200
575 ± 10
1.5938 ± 0.0044
1.5883 ± 0.0032
300
610 ± 15
1.5903 ± 0.0041
1.5830 ± 0.0042
Table 5
Curing-Temperature Dependence of the Thicknesses, Absorptivities, and Refractive Indices of 1:1 FePO4 Films Cured at 200 °C after a 70 °C Precuring Stage
Curing Temperature (°C)
Thickness of Films Spun at 1000 rpm (nm)
Absorptivity at 255 nm (μm−1)
Refractive Index λ0 = 632.8 nm
125
300 ± 7.5
7.53
1.7122 ± 0.0036
200
290 ± 5
9.80
1.7266 ± 0.0055
300
275 ± 5
11.3
1.7281 ± 0.0075
Table 6
Refractive Indices of 1:1 InPO4 Films Cured Directly at 200 °C
Refractive Index
Cooling Procedure
λ0 = 543.5 nm
λ0 = 632.8 nm
30 °C/min
1.5921 ± 0.0040
1.5881 ± 0.0031
12 °C/min
1.5938 ± 0.0037
1.5886 ± 0.0023
2 °C/min
1.5933 ± 0.0034
1.5899 ± 0.0027
Quenching in ethanol at 20 °C
1.5940 ± 0.0036
1.5890 ± 0.0023
Quenching in acetone and dry ice at −95.4 °C
1.5922 ± 0.0066
1.5906 ± 0.0027
Table 7
Chemical Resistance of FePO4 Films Cured after 70 °C Precuring
Phosphate-buffered saline at 100 mM and pH 7.3.
Films decompose to a reddish-brown gelatinous substance within 3 h.
Ethylene diamine-tetraacetic acid at 5% w/v and pH 7.3.
Table 8
Embossing Times for 550-nm-Thick Films
Metal Phosphate Coating Solution Composition
Optimum Time Spent at 20 °C and 50% Relative Humidity after a 60-s Spin (min)