JP4358053B2 - Surface acoustic wave filter and manufacturing method thereof - Google Patents

Description

  The present invention relates to a surface acoustic wave filter using a nitride-based semiconductor and a method for manufacturing the same.

  In recent years, the number of mobile communication users centering on mobile phones has increased rapidly. Under such circumstances, development of a high-performance filter that operates stably even in a high frequency region is underway. In addition, with recent needs for downsizing of mobile communication devices, filters have been changed from conventional LC filters to dielectric filters, and surface acoustic wave (abbreviated as SAW) filters. Currently.

  In order to realize a SAW filter that operates in a high frequency region, a crystal material having a high surface acoustic wave propagation velocity (hereinafter referred to as a SAW propagation velocity) is required. The resonance frequency of the SAW filter can be estimated by the following equation.

f = Vs / (2 × L)
Here, f is a resonance frequency, Vs is a SAW propagation velocity, and L is a distance between comb-shaped electrodes (IDT: Interdigital transducer). From this equation, it can be seen that an increase in the SAW propagation speed of the crystal material forming the SAW filter is effective for increasing the frequency of the SAW filter.

Conventionally, quartz, LiNbO ₃ , LiTaO ₃ or the like has been used as the crystal material of the SAW filter. Recently, a nitride-based semiconductor such as GaN or AlN has attracted attention as a crystal material having a high SAW propagation speed (for example, see Patent Document 1 below). The following table summarizes the SAW propagation speed of these materials.

この表から同じ櫛型電極（以下ＩＤＴ電極と称す）の間隔の場合には、従来の水晶、ＬｉＮｂＯ_３、ＬｉＴａＯ_３と比較してＧａＮやＡｌＮの窒化物系半導体の方がより高い周波数で動作すると期待できる。また、特に窒化物系半導体を用いた結晶材料の場合には、電子デバイスの作製も可能であることから、窒化物半導体を用いたフィルターとアンプの集積化が期待されている。

From this table, in the case of the same comb-shaped electrode (hereinafter referred to as IDT electrode) spacing, nitride semiconductors such as GaN and AlN operate at a higher frequency than conventional quartz, LiNbO ₃ , and LiTaO _3. Then you can expect. In particular, in the case of a crystal material using a nitride-based semiconductor, since an electronic device can be manufactured, integration of a filter and an amplifier using a nitride semiconductor is expected.

  Conventionally, crystal materials such as GaN and AlN use sapphire as a substrate, and crystals are grown by metal organic vapor phase epitaxy (hereinafter referred to as MOCVD method) or molecular beam epitaxy (hereinafter referred to as MBE method). However, since there is a lattice mismatch of 10 to 20% between the sapphire substrate and the nitride semiconductor, when the nitride semiconductor is grown directly on the sapphire substrate, three-dimensional growth occurs, surface roughness occurs, There was a problem that many crystal defects occurred.

  In order to solve this problem, growth of a nitride semiconductor on a sapphire substrate is generally performed by the following procedure. After the sapphire substrate is first cleaned in a hydrogen atmosphere at a high temperature of 1000 ° C. or higher, the substrate temperature is lowered to about 500 ° C., and a GaN layer or an AlN layer (hereinafter referred to as a low temperature buffer layer) is grown at a low temperature. The target nitride semiconductor is grown by raising the substrate temperature to about 1000 ° C. (see, for example, Non-Patent Document 1 and Non-Patent Document 2 below).

  Further, the SAW filter is manufactured by forming an IDT electrode on the crystal material by a photolithography process and vapor deposition of a metal material such as Al.

JP 2001-203561 A S. Nakamura, “GaN growth usi GaN buffer layer”, Jpn. J. et al. Appl. Phys. Vol. 30, L1705 (1991) H. Amano, N .; Sawaki, I .; Akasaki and Y.K. Toyoda, “Metalorganic vapor phase epidermal growth of a high quality GaN film using an AlN buffer layer”, Appl. Phys. Lett. Vol. 48, 353 (1986)

  As described above, the growth of nitride-based semiconductors, which are expected as SAW filter materials that operate in a high frequency region, requires a process of raising and lowering the growth temperature in three stages, so the growth process is complicated. There is a problem that it is difficult to fabricate a high resistance layer with good reproducibility, that is, with a high yield, and the growth time becomes long. In addition, the growth process that frequently changes the substrate temperature in the temperature range of 500 to 1000 ° C. or more causes the deterioration of the heater material of the growth apparatus, resulting in a problem of shortening the component life.

  The present invention has been made in view of the above problems, and the problem to be solved by the present invention is a nitride-based semiconductor having a high surface acoustic wave propagation velocity and operating effectively at a high frequency. To provide a surface acoustic wave filter used and a high-efficiency, high-yield, low-cost manufacturing method thereof.

In order to solve the above problem, in the present invention, as described in claim 1,
Sequentially forming a first layer of sapphire or Ranaru on a substrate _{(AlN) x (Al 2 O} 3) 1-x, and a second layer of AlN in this order, further, the second A surface acoustic wave filter having a thin film structure formed by forming a third layer made of GaN, AlN, or a mixed crystal of GaN and AlN on the layer, and having a comb-shaped electrode on the thin film structure In the surface acoustic wave filter, the value of x in the composition formula of the first layer increases from the substrate side toward the second layer side.

Moreover, in this invention, as described in Claim 2,
Sapphire or Ranaru on a substrate _{(AlN) x (Al 2 O} 3) a first layer consisting of _1-x, and a second layer of AlN, this order an intermediate layer of _Al 2 _{O 3} And a thin film structure in which a third layer made of GaN, AlN, or a mixed crystal of GaN and AlN is formed on the intermediate layer, and a comb is formed on the thin film structure. Surface acoustic wave filter having a mold electrode, wherein the value of x in the composition formula of the first layer increases from the substrate side toward the second layer side Configure the filter.

Moreover, in this invention, as described in Claim 3,
The surface acoustic wave filter according to claim 1 or 2, wherein at least one element of magnesium, zinc, beryllium, carbon, and iron is added to the third layer.

Moreover, in this invention, as described in Claim 4,
A first step of sequentially forming a second layer of the first layer and the AlN sapphire or Ranaru on a substrate made of _{(AlN) x (Al 2 O} 3) 1-x in this order, wherein A second step of forming a third layer of GaN, AlN, or a mixed crystal of GaN and AlN on the second layer; and a third step of forming a comb-shaped electrode on the third layer. A surface acoustic wave filter having a process, wherein when the first layer is formed, a supply amount of a nitrogen raw material and a supply amount of an oxygen raw material are changed over time, and the first layer is formed. A method of manufacturing a surface acoustic wave filter is provided, wherein the first layer is formed such that the value of x in the composition formula of the layer increases from the substrate side toward the second layer side.

In the present invention, as described in claim 5,
Sapphire or Ranaru on a substrate _{(AlN) x (Al 2 O} 3) a first layer consisting of _1-x, and a second layer of AlN, this order an intermediate layer of _Al 2 _{O 3} And a second step of forming a third layer made of GaN, AlN, or a mixed crystal of GaN and AlN on the intermediate layer, and a step of forming the third layer on the third layer. A method of manufacturing a surface acoustic wave filter having a third step of forming a comb-shaped electrode, wherein when the first layer is formed, a supply amount of a nitrogen raw material and a supply amount of an oxygen raw material are passed over time. The surface elasticity is characterized in that the first layer is formed such that the value of x in the composition formula of the first layer increases from the substrate side toward the second layer side. The manufacturing method of a wave filter is comprised.

In the present invention, as described in claim 6,
5. The method according to claim 4, wherein, in the second step, the third layer is formed by adding at least one element of magnesium, zinc, beryllium, carbon, and iron to the third layer. 5. A method for producing the surface acoustic wave filter according to 5 is configured.

In the present invention, as described in claim 7,
7. The surface acoustic wave filter manufacturing method according to claim 4, wherein an ECR plasma film forming apparatus is used for forming the first layer and the second layer.

In the present invention, as described in claim 8,
7. The surface acoustic wave filter manufacturing method according to claim 5, wherein an ECR plasma film forming apparatus is used to form the first layer, the second layer, and the intermediate layer.

In the present invention, as described in claim 9,
The surface acoustic wave filter according to any one of claims 4 to 8, wherein the layer formation in the first step is performed without using a heating means for raising the temperature of the substrate. Configure the manufacturing method.

By implementing the present invention, it is composed of (AlN) _x (Al ₂ O ₃ ) _1-x (where 0 ≦ x ≦ 1, where x increases with an increase in distance from the substrate surface) on the sapphire substrate. By forming the first layer and the second layer made of AlN, and further forming the third layer made of GaN, AlN, or a mixed crystal of GaN and AlN, high surface acoustic wave propagation velocity It is possible to provide a surface acoustic wave filter using a nitride-based semiconductor that operates effectively at a high frequency, and a high-efficiency, high-yield, low-cost manufacturing method thereof.

  Hereinafter, the layer configuration of the SAW filter using the nitride-based semiconductor of the present invention and the details of the manufacturing method thereof will be described.

FIG. 1 is a schematic cross-sectional view showing the layer structure of a SAW filter according to an embodiment of the present invention. In FIG. 1, 1 is a sapphire substrate, 2 is a first layer made of (AlN) _x (Al ₂ O ₃ ) _1-x , 3 is a second layer made of AlN, and 4 is a third layer made of GaN. Reference numeral 5 denotes a comb electrode (IDT electrode) made of Al. In the (AlN) _x (Al ₂ O ₃ ) _1-x layer 2, 0 ≦ x ≦ 1, and x is increased with an increase in distance from the surface of the substrate 1 along the thickness direction of the layer. It has changed to do. Such a layer is a layer whose lattice constant changes along the thickness direction of the layer, and is extremely effective for reducing inconveniences (three-dimensional growth, generation of lattice defects, etc.) due to lattice mismatch. is there.

In the present embodiment , the GaN layer 4 is used as the third layer, but a layer made of AlN or a mixed crystal of GaN and AlN may be used as the third layer.

The (AlN) _x (Al ₂ O ₃ ) _1-x layer 2 as the _first layer is made of Al, nitrogen, and oxygen as raw materials by an ECR (Electron Cyclotron Resonance) plasma film forming apparatus, and at room temperature. Deposition was performed in Ar plasma so as to have a film thickness of about 10 nm without using a heating means for increasing the temperature. At this time, the oxygen gas flow rate is increased from 6.1 sccm to 0 sccm, the supply rate of the nitrogen raw material, that is, the nitrogen gas flow rate is increased stepwise from 0 sccm to 6 sccm, and (AlN) _x (Al ₂ O ₃ ) ₁₋ The value of x in the _x layer 2 was increased from 0 to 1 in this case from the substrate 1 side toward the AlN layer 3 side as the second layer.

  On the other hand, the AlN layer 3 as the second layer was deposited to a thickness of about 10 nm in the same apparatus using Al and nitrogen as raw materials and fixing the nitrogen gas flow rate at 6 sccm. It is not necessary for the value of x to change from 0 to 1. If the value of x increases from the substrate 1 side toward the AlN layer 3 side as the second layer, the effect of the present invention is Appear.

For example, the (AlN) _x (Al ₂ O ₃ ) _1-x layer 2 as the _first layer can be designed with a film thickness of _{1 to} 200 nm, preferably about 5 to 50 nm. The AlN layer 3 as the second layer can be designed with a film thickness of 1 to 200 nm, preferably about 5 to 50 nm.

Further, on the AlN layer 3 which is the second layer, an intermediate layer made of Al ₂ O _{3 is} laminated in an amount of about 1 to 100 nm in the same apparatus, and the GaN layer 4 which is the third layer is formed thereon. The same effect could be obtained even if the structure was used.

The step of forming the (AlN) _x (Al ₂ O ₃ ) _1-x 2 and the AlN layer 3 or the step of adding the step of forming the intermediate layer to the step is the first step. .

  Although the ECR plasma film-forming apparatus is used for the layer formation in the first step, a film-forming apparatus that does not require a heating means for raising the temperature of the substrate, such as a plasma CVD apparatus, is used. Also good.

  The GaN layer 4 as the third layer was epitaxially grown using an MOCVD apparatus. At this time, trimethylgallium and ammonia were used as source gases. Hereinafter, the growth procedure of the GaN layer 4 will be described. The sample having the above laminated structure was placed in the growth chamber of the MOCVD apparatus, and the temperature was raised to 1050 ° C. in an ammonia atmosphere. Next, trimethylgallium was introduced into the growth chamber, and the GaN layer 4 was grown by adjusting the flow rate and time so that the film thickness was 2 μm. After completion of the growth, the temperature was lowered to room temperature in an ammonia atmosphere and a sample was taken out. The step of forming the GaN layer 4 as the third layer is the second step.

  In order to explain the effect of the present invention, FIG. 2 shows the time change of the growth temperature when GaN is grown. For comparison, FIG. 3 shows the time change of the growth temperature when a GaN layer is grown using the above-described conventional low-temperature buffer layer. In the conventional method, the temperature must be temporarily reduced to 500 ° C. in order to form the low-temperature buffer layer. As is clear from the comparison between FIG. 2 and FIG. 3, it can be seen that the growth process is simplified and the growth time can be shortened by the present invention.

The resistivity of the GaN layer grown by the method of the present invention was about 3 × 10 ⁶ Ω · cm. However, n-type impurities may be mixed during GaN growth due to contamination in the growth chamber and the like, leading to a decrease in resistivity. In addition, such a decrease in resistivity decreased the electromechanical coupling constant, which is one of the performance indexes of the SAW filter. When the resistivity of the GaN layer is reduced in this way, doping is performed using a gas containing at least one element that becomes a p-type impurity such as magnesium, zinc, beryllium, carbon, and iron during the growth of GaN. This effectively improved the resistivity.

  For example, the film thickness of the GaN layer 4 as the third layer can be designed in the range of 1 to 20 μm, preferably 0.5 to 5 μm. Further, as the third layer, instead of GaN, AlN having a high SAW propagation speed or a mixed crystal of AlN and GaN may be used. As for the growth method, MBE or gas source MBE can be used in addition to MOCVD.

  After forming the third GaN layer 4, as shown in FIG. 1, the IDT electrode 5 was formed on the GaN layer 4 to a thickness of 200 nm by a photolithography process and Al deposition. The step of forming the IDT electrode 5 is a third step.

  Further, the SAW propagation speed obtained from the analysis of the characteristics of the SAW filter produced by the implementation of the present invention is 5800 m / s, and the electromechanical coupling constant is 4.2%, and the characteristics are sufficient in practical use. It was confirmed that there was.

  In the present invention, as described above, the steps of cleaning the substrate and growing the low-temperature buffer layer when growing the nitride semiconductor become unnecessary. As a result, the time required for the growth of the nitride semiconductor can be shortened and the heater life can be extended. As a result, a low-cost SAW filter can be produced in a short time with good reproducibility, that is, with a high yield. Make it possible.

  That is, by implementing the present invention, a surface acoustic wave filter using a nitride-based semiconductor that has a high surface acoustic wave propagation velocity and operates effectively at a high frequency, and its high efficiency, high yield, and low cost manufacturing. It becomes possible to provide a method.

  The present invention has a great effect in promoting practical application and application of SAW filters using nitride semiconductors.

It is sectional drawing explaining embodiment of this invention. It is a figure which shows the temperature change in the process in the manufacturing method of the surface acoustic wave filter which concerns on this invention. It is a figure which shows the temperature change in the process in the manufacturing method of the conventional surface acoustic wave filter.

Explanation of symbols

1 ... sapphire _{substrate, 2 ... (AlN) x (} Al 2 O 3) 1-x layer, 3 ... AlN layer, 4 ... GaN layer, 5 ... IDT electrode.

Claims (9)

Sequentially forming a first layer of sapphire or Ranaru on a substrate _{(AlN) x (Al 2 O} 3) 1-x, and a second layer of AlN in this order, further, the second A surface acoustic wave filter having a thin film structure formed by forming a third layer made of GaN, AlN, or a mixed crystal of GaN and AlN on the layer, and having a comb-shaped electrode on the thin film structure In the surface acoustic wave filter, the value of x in the composition formula of the first layer increases from the substrate side toward the second layer side. Sapphire or Ranaru on a substrate _{(AlN) x (Al 2 O} 3) a first layer consisting of _1-x, and a second layer of AlN, this order an intermediate layer of _Al 2 _{O 3} And a thin film structure in which a third layer made of GaN, AlN, or a mixed crystal of GaN and AlN is formed on the intermediate layer, and a comb is formed on the thin film structure. Surface acoustic wave filter having a mold electrode, wherein the value of x in the composition formula of the first layer increases from the substrate side toward the second layer side filter.   3. The surface acoustic wave filter according to claim 1, wherein at least one element of magnesium, zinc, beryllium, carbon, and iron is added to the third layer. A first step of sequentially forming a second layer of the first layer and the AlN sapphire or Ranaru on a substrate made of _{(AlN) x (Al 2 O} 3) 1-x in this order, wherein A second step of forming a third layer of GaN, AlN, or a mixed crystal of GaN and AlN on the second layer; and a third step of forming a comb-shaped electrode on the third layer. A surface acoustic wave filter having a process, wherein when the first layer is formed, a supply amount of a nitrogen raw material and a supply amount of an oxygen raw material are changed over time, and the first layer is formed. A method for producing a surface acoustic wave filter, comprising forming the first layer, wherein the value of x in the composition formula of the layer increases from the substrate side toward the second layer side. Sapphire or Ranaru on a substrate _{(AlN) x (Al 2 O} 3) a first layer consisting of _1-x, and a second layer of AlN, this order an intermediate layer of _Al 2 _{O 3} And a second step of forming a third layer made of GaN, AlN, or a mixed crystal of GaN and AlN on the intermediate layer, and a step of forming the third layer on the third layer. A method of manufacturing a surface acoustic wave filter having a third step of forming a comb-shaped electrode, wherein when the first layer is formed, a supply amount of a nitrogen raw material and a supply amount of an oxygen raw material are passed over time. The surface elasticity is characterized in that the first layer is formed such that the value of x in the composition formula of the first layer increases from the substrate side toward the second layer side. Wave filter manufacturing method.   5. The method according to claim 4, wherein, in the second step, the third layer is formed by adding at least one element of magnesium, zinc, beryllium, carbon, and iron to the third layer. 6. A method for producing a surface acoustic wave filter according to 5.   7. The method of manufacturing a surface acoustic wave filter according to claim 4, wherein an ECR plasma film forming apparatus is used for forming the first layer and the second layer.   7. The method of manufacturing a surface acoustic wave filter according to claim 5, wherein an ECR plasma film forming apparatus is used for forming the first layer, the second layer, and the intermediate layer.   The surface acoustic wave filter according to any one of claims 4 to 8, wherein the layer formation in the first step is performed without using a heating means for raising the temperature of the substrate. Production method.

Images