JPH07198935A - Selecting method of temperature coefficient for wavelength shift of multilayer film filter and multilayer film filter having almost zero temperature coefficient of wavelength shift - Google Patents

Abstract

PURPOSE:To decide the temp. coefft. of wavelength shift of a multilayered film filter according to decision of the coefft. of linear expansion of a substrate and to obtain a multilayered film filter having almost zero temp. coefft. of wavelength shift. CONSTITUTION:Dense multilayer films are formed on different substrates by vapor deposition by an ion or plasma process. The temp. coefft. of wavelength shift of each multilayer film is plotted on the coordinate of linear expansion coefft.-temp. coefft. of wavelength shift. Based on the obtd. plot, the temp. coefft. shift line intrinsic to each multilayer film is obtd. The substrate for a multilayer film having a specified temp. coefft. of wavelength shift is selected according to this temp. coefft. shift line. Further, by using a substrate having coefft. of linear expansion of 75-150(1/10<7>), a multilayered filter having almost zero temp. coefft. of wavelength shift is obtd. Thereby, the temp. coefft. of wavelength shift can be determined from the temp. coefft. shift line. A multilayered filter having almost zero temp. coefft. of wavelength shift can be obtd. by using a substrate having the coefft. of linear expansion in a specified range.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a wavelength shift temperature coefficient selecting method for a multilayer filter used for optical communication or the like.
A desired wavelength shift temperature coefficient value is selected on the basis of the coefficient of linear expansion peculiar to a multilayer film-wavelength shift temperature coefficient coordinate, and in particular, a multilayer filter having a wavelength shift temperature coefficient of substantially zero is provided.

[0002]

_2. Description of the Related Art A multilayer bandpass filter (hereinafter referred to as a multilayer filter) in which a dielectric film such as TiO ₂ or SiO ₂ is laminated on a substrate such as quartz is a substrate heated to 250 ° C. in a vacuum chamber. It is vapor-deposited by an electron beam on the top. In this case, the multi-layered film has a low packing density (in other words, scathers), and the film structure has a columnar structure. In the environment of use, moisture absorbs and desorbs in the multilayer film, changing the refractive index of the film, which causes fluctuations in the wavelength shift temperature coefficient. The peak transmission wavelength shifts to the short wavelength side when the temperature rises, and shifts to the long wavelength side when the temperature falls. Ion-assisted method to provide dense multilayer film,
An ion-plasma process vapor deposition method such as an ion plating method or an ion beam sputtering method is adopted. High packing density multilayer filters are released from the influence of humidity, but narrow band multilayer filters still have the problem of peak wavelength shift due to temperature.

[0003]

Today, dense wavelength division multiplexing communication and coherent optical communication are adopted. Since the transmission peak wavelength of the narrow band multilayer filter by the ion-plasma process method shifts according to the fluctuation of the ambient temperature, there is an extremely strong demand for a multilayer filter having a wavelength shift temperature coefficient of almost zero. We are looking for materials, physical properties, vapor deposition methods, etc. of the multi-layer film itself, but the temperature coefficient of the refractive index is difficult.
The inventors of the present invention focused on the linear expansion coefficient of the substrate and established a theory for selecting a wavelength shift temperature coefficient in consideration of volumetric strain from the substrate to the multilayer film, thereby providing a multilayer filter having a wavelength shift temperature coefficient of substantially zero. Try.

[0004]

[Means for Solving the Problems] In addition to the temperature coefficient of the refractive index of the multilayer film itself and the film thickness fluctuation due to thermal expansion, the volume distortion of the multilayer film due to the thermal expansion of the substrate is also taken into consideration, and the wavelength shift temperature coefficient value The linear expansion coefficient of the substrate is determined in consideration of the monotonically decreasing relationship with the linear expansion coefficient value of the substrate, and a multilayer filter having a desired wavelength shift temperature coefficient value is provided. On a substrate with a linear expansion coefficient value in the range of 75 to 150 (/ 10 ⁷ ),
TiO ₂ and SiO ₂ films or Ta ₂ O ₅ and SiO ₂ films are alternately and repeatedly laminated to fabricate a multilayer filter with a wavelength shift temperature coefficient of almost zero.

[0005]

OPERATION FIG. 1 shows the coordinates of the substrate linear expansion coefficient-wavelength shift temperature coefficient, and the temperature coefficient shift straight lines a to d for each multilayer film are drawn. For example, the wavelength shift temperature coefficient multilayer films of the straight line a is a multilayer film filter of substantially zero coefficient of linear expansion may be employed substrate taking 110/10 ⁷ neighborhood. It is understood that the temperature coefficient of wavelength shift monotonically decreases in proportion to the linear expansion coefficient of the substrate. A volume strain model considering the volume strain to the multilayer film due to the thermal expansion of the substrate will be described below. temperature
The average refractive index of the deposited film at t ₀ and t is n ₀ , n
_t , the average refractive index of the vapor deposition material portion of the vapor deposition film is N ₀ , N _t ,
The packing density of the deposited film is P ₀ , P _t , and the thickness of the deposited film is d ₀ , d _t ,
The volume of the cube in a certain thin film region is V ₀ , V _t . The Poisson's ratio of the multilayer film is s, the average linear expansion coefficient is β, the temperature coefficient of the refractive index is δ, and the linear expansion coefficient of the substrate is α. n ₀ = N ₀ P ₀ +1 −P ₀ V _t = V ₀ [1 + 2 (α−β) × (1−2 s) / (1−s) +3 β] From this, P _t = P ₀ (1 + 3 β ) / [1 + 3 β + 2 (α−β) (1−2 s) / (1−s)] d _t ＝ d ₀ [1−2 s (α−β) (1 −s) + β] where temperature of refractive index coefficients, when defined as δ = (1 ÷ n) ( dN / dt), from which _{n t = n t P t +1} -P t = (n 0 + n 0 δ) P t +1 -P t, the multilayer filter The wavelength shift temperature coefficient (TSC) at the wavelength λ is represented by TSC = λ · Δ (nd) / n ₀ = n _t d _t / n ₀ d ₀ −1. Figure 2 shows the calculation results of the dependence of the refractive index on the temperature coefficient (δ) based on the equation. Packing density P =
1.0, Poisson's ratio s = 0, coefficient of linear expansion of film β = 1/10 ⁷ , average refractive index N = 1.85. Comparing with the experimental results in Fig. 1, it is understood that the temperature coefficient of the refractive index takes a value of about 1.0 / 10 ⁵ . This calculation result, in which the temperature coefficient of wavelength shift monotonically decreases in proportion to the linear expansion coefficient of the substrate, is the proof of the correctness of the volume strain model considering the volume strain to the multilayer film due to the thermal expansion of the substrate.

[0006]

EXAMPLE The coefficient of linear expansion of the substrate of the multilayer filter is plotted on the horizontal axis, and the coefficient of wavelength shift (TSC) of the transmission peak wavelength is plotted on the axis of ordinate.
Shown in the figure. The temperature coefficient shift line a in the figure is TiO ₂
This is a wavelength shift temperature coefficient straight line of a multilayer film in which 33 layers of / SiO ₂ are alternately repeated (the cavity layer has a primary cavity length, is made of SiO ₂ , and the half-value width is about 0.5 nm). This multilayer film is vapor-deposited on each substrate shown in Table 1 to produce each multilayer filter. 20 ° C for each multilayer filter
The transmission peak wavelength is measured at two points at 60 ° C, and the resulting wavelength shift temperature coefficient is plotted (marked with a circle) in the same figure. The temperature coefficient shift line a is created based on this plot. If there are two plot points, it is possible to draw a temperature coefficient shift straight line.

[Table 1] Next, 31 layers of TiO ₂ / SiO ₂ are alternately laminated to form a multilayer film. The difference from the above example is that TiO ₂ was used for the cavity layer having the primary cavity length. The design center wavelength is 1540 nm and the half-width is about 1.0 nm. The wavelength shift temperature coefficient was similarly measured for each substrate (each substrate described in Table 1) on which this multilayer film was deposited and plotted in Fig. 1 (marked with ●). Temperature coefficient shift line from each of these points
b is required. The difference between the two temperature coefficient shift lines a and b is
This is due to the difference in temperature coefficient of refractive index between SiO ₂ and TiO ₂ which are cavity layers. In the case of a multilayer film in which 41 layers of Ta ₂ O ₅ / SiO ₂ are repeatedly laminated alternately and SiO _{2 is used} for the cavity layer having the primary cavity length, the temperature coefficient shift line c is drawn. This straight line c can be obtained from each plot (marked with Δ).
Similarly, in the case of a multilayer film in which Ta ₂ O ₅ / SiO ₂ is repeatedly laminated 39 layers alternately and Ta ₂ O _{5 is used} for the cavity layer having the primary cavity length, the temperature coefficient shift line d is be painted. This straight line d can be obtained from each plot (square mark). The difference between the two temperature coefficient shift lines c and d is the difference in the cavity layers. Ta ₂ O rather than SiO _{2 in} one cavity layer
_{This is because 5} has a slightly higher temperature coefficient of refractive index. From these measurement results and the above equation, in the multilayer film deposited on the substrate with a large linear expansion coefficient, the film itself spreads two-dimensionally as if it was dragged by the thermal expansion coefficient of the substrate, resulting in volume distortion. , The volume strain theory that the packing density decreases and the wavelength shift temperature coefficient fluctuates.

From the temperature coefficient shift straight lines a to d in FIG.
It is understood that the wavelength shift temperature coefficient value decreases monotonically with the linear expansion coefficient value of the substrate. A multilayer filter having a certain wavelength shift temperature coefficient can be obtained by selecting the type of multilayer film derived from this straight line and the linear expansion coefficient of the substrate. To fabricate a multilayer filter with a wavelength shift temperature coefficient of almost zero, a TiO ₂ / SiO ₂ film or Ta ₂ O ₅ film should be formed on a substrate with a coefficient of linear expansion within the range of 75 to 150 (/ 10 ⁷ ). / SiO ₂
The films may be laminated alternately and repeatedly.

Although TiO ₂ , SiO ₂ , and Ta ₂ O ₅ have been described as vapor-deposited films, Ta ₂ O ₅ / SiO ₂ / Al ₂ O ₃ or TiO ₂ / SiO ₂ / A
In the case of a multilayer film in which l ₂ O ₃ is alternately and repeatedly laminated, the temperature coefficient shift lines a to d have almost the same tendency. This Al ₂ O
Not limited to ₃ , a film suitable for the design transmission peak wavelength of the multilayer filter is appropriately adopted. The vapor deposition materials in the above examples are SiO ₂ , TiO ₂ , Ta ₂ O ₅ , and Al ₂ O ₃ , but in addition, ZrO _2, Si, ZnS, HfO, Ge, Nd ₂ O _6, Nb ₂ O ₅ and CeO ₂ are selectively adopted. In particular _, a long-wavelength filter consisting of _a Ge (germanium) film with a wavelength shift temperature coefficient of almost zero is
Expected for space observation equipment. These deposition materials are sputtered by accelerated electrons or ions, and deposited on the selected substrate directly or through the plasma region.
The vapor deposition film is irradiated with oxygen and nitrogen ions by an ion gun. The substrate is heated to 100 to 120 ° C by the radiant heat from the evaporated substance. The multi-layer film produced by this ion or plasma process method is a dense film with a packing density close to 1. Due to such a high packing density, the multilayer film suffers volume distortion due to the linear expansion coefficient of the substrate, leading to a decrease in packing density.
The wavelength shift temperature coefficient fluctuates. In a multilayer filter with a film (squashed state) that has an extremely low packing density, the temperature coefficient of wavelength shift is not significantly affected by the linear expansion coefficient of the substrate.

TiO ₂ / SiO ₂ / Al ₂ O ₃ or TiO ₂ / SiO ₂ / A
In order to fabricate a multilayer film filter having a wavelength shift temperature coefficient of substantially zero, which is a multilayer film in which l ₂ O ₃ is alternately and repeatedly laminated, a linear expansion coefficient value of 75 to A substrate within the range of 150 (/ 10 ⁷ ) will suffice. Similarly, ZrO _2, Si, ZnS, HfO, Ge, Nd ₂ O _6, Nb ₂ O ₅ ,
Even with a CeO ₂ film, a linear expansion coefficient of 75 to 150 is required to fabricate a multilayer filter with a wavelength shift temperature coefficient of almost zero.
Use a substrate within the range of (/ 10 ⁷ ). However, the absolute value of the wavelength shift temperature coefficient value is the same and its sign (positive or negative)
The wavelength shift temperature coefficient becomes 0 if two kinds of multilayer filters different in only the difference are used together. With reference to the temperature coefficient shift lines a to d, it is possible to select a multilayer film filter having different positive and negative signs.

[0010]

In summary, the present invention considers not only the temperature coefficient of the refractive index of the multilayer film itself and the film thickness variation due to thermal expansion but also the volume strain to the dense multilayer film due to the thermal expansion of the substrate, and the wavelength shift temperature. In order to determine the linear expansion coefficient of the substrate based on the relationship that the coefficient value monotonically decreases with respect to the linear expansion coefficient value of the substrate,
A multilayer film filter having a predetermined wavelength shift temperature coefficient value can be selected. The multilayer film and the substrate are selected with reference to the temperature coefficient shift straight line. Especially, the coefficient of linear expansion is 75 to 150 (/ 10 ⁷ )
If a substrate within the range is adopted, it is possible to provide a multilayer filter having a wavelength shift temperature coefficient of substantially zero. Also, if the linear expansion coefficient of a certain substrate is known, the multilayer film of the multilayer filter having a predetermined wavelength shift temperature coefficient can be determined from a large number of temperature coefficient shift lines, and if the multilayer film is specified, the multilayer film can be determined. The wavelength shift temperature coefficient of the multilayer filter is known in advance.

[Brief description of drawings]

FIG. 1 is a graph showing a relationship between a linear expansion coefficient of a substrate and a wavelength shift temperature coefficient of a multilayer filter.

FIG. 2 is a temperature coefficient shift straight line derived from a calculation formula that considers volume strain to a multilayer film due to a linear expansion coefficient of a substrate.

[Explanation of symbols]

 a temperature coefficient shift straight line b temperature coefficient shift straight line c temperature coefficient shift straight line d temperature coefficient shift straight line

Claims (11)

[Claims] 1. The wavelength shift temperature coefficient value is a line of the substrate in consideration of the temperature coefficient of the refractive index of the multilayer film itself and the film thickness variation due to thermal expansion, as well as the volume strain to the multilayer film due to thermal expansion of the substrate. A method for selecting the wavelength shift temperature coefficient of a multilayer filter, which determines the linear expansion coefficient of the substrate based on the relationship of monotonically decreasing with respect to the expansion coefficient value. 2. In a multilayer filter in which a dense multilayer film is laminated on a substrate by using an ion or plasma process method, the wavelength shift temperature coefficient value monotonically decreases with respect to the substrate linear expansion coefficient value. A method for selecting the wavelength shift temperature coefficient of a multilayer filter that determines the linear expansion coefficient of the substrate based on this. To 3. A linear expansion coefficient of 75 to 150 (/ ¹⁰⁷⁾ on the substrate in the range of, formed by repeatedly laminating TiO ₂ and SiO ₂ film alternately, wavelength shift temperature coefficient of substantially zero Multilayer filter. 4. The wavelength shift temperature coefficient, which is obtained by alternately and repeatedly laminating Ta ₂ O ₅ and SiO ₂ coatings on a substrate having a linear expansion coefficient value in the range of 75 to 150 (/ 10 ⁷ ). Zero multilayer filter. 5. A wavelength obtained by alternately laminating Ta ₂ O ₅ , SiO ₂ and Al ₂ O ₃ coatings on a substrate having a coefficient of linear expansion within the range of 75 to 150 (/ 10 ⁷ ). Multilayer filter with a shift temperature coefficient of almost zero. 6. A wavelength shift temperature obtained by alternately and repeatedly laminating TiO ₂ , SiO ₂ and Al ₂ O ₃ coatings on a substrate having a linear expansion coefficient value within the range of 75 to 150 (/ 10 ⁷ ). A multilayer filter with a coefficient of almost zero. 7. The deposition material is TiO ₂ , SiO ₂ , Ta ₂ O ₅ ,
Al ₂ O ₃ , ZrO _2, Si, ZnS, HfO, Ge, Nd ₂ O _6, Nb ₂ O ₅ ,, CeO ₂
Wavelength shift obtained by densely depositing the above vapor-deposited film on a substrate having a coefficient of linear expansion within the range of 75 to 150 (/ 10 ⁷ ) by an ion or plasma process method by employing one or more of A multilayer filter with a temperature coefficient of almost zero. 8. A multilayer filter of two or more kinds is manufactured by laminating the same multilayer film on different substrates by an ion or plasma process method, and the wavelength shift temperature coefficient value of each multilayer filter is set to a linear expansion coefficient-- Wavelength shift temperature coefficient A method of selecting the wavelength shift temperature coefficient of a multilayer filter, which is plotted on the coordinates and draws a temperature coefficient shift straight line specific to the multilayer film, and a substrate having a predetermined linear expansion coefficient is adopted from this straight line. 9. A substrate having a coefficient of linear expansion within the range of 75 to 150 (/ 10 ⁷ ) is employed, and the linear temperature coefficient of the refractive index of the multilayer film itself is adjusted to obtain a predetermined wavelength shift temperature coefficient. A method for selecting a wavelength shift temperature coefficient of a multilayer filter according to claim 1. 10. A multilayer filter having a wavelength shift temperature coefficient of substantially zero, which employs a substrate having a linear expansion coefficient value derived from a temperature coefficient shift straight line peculiar to the multilayer film. 11. The multilayer filter according to claim 10, wherein a temperature coefficient shift straight line is obtained from the linear expansion coefficient-wavelength shift temperature coefficient coordinate values of the respective multilayer filters having different substrates.