METHOD FOR FORMING AN ALUMINUM NITRIDE LAYER

Abstract

A method for forming an aluminum nitride layer (310, 320) comprises the provision of a substrate (100) and the forming of a patterned metal nitride layer (110). A bottom electrode metal layer (210) is formed on the exposed portions (101) of the substrate. An aluminum nitride layer portion (320) grown above the exposed portion (101) of the substrate (100) exhibits piezoelectric properties. An aluminum nitride layer portion (310) grown above the patterned metal nitride layer (110) exhibits no piezoelectric properties (310). Both aluminum nitride layer portions (320, 310) are grown simultaneously.

Claims

1. A method for forming an aluminum nitride layer, comprising the steps of: providing a substrate; forming a metal nitride layer on the substrate; patterning the metal nitride layer to form a residual metal nitride layer and to expose a portion of the substrate; forming a metal layer on the exposed portion of the substrate; and forming aluminum nitride on the metal layer and above the metal nitride layer.

2. The method according to claim 1, wherein the step of forming aluminum nitride comprises depositing an aluminum nitride layer in the area of the exposed portion of the substrate with a higher level of piezoelectric property than in the area of the residual metal nitride layer with a lower level of piezoelectric property.

3. The method according to claim 1, wherein the step of forming aluminum nitride comprises simultaneously depositing aluminum nitride in the area of the exposed portion of the substrate and in the area of the residual metal nitride layer.

4. The method according to claim 1, comprising forming the metal layer on the residual metal nitride layer and on the exposed portion of the substrate and then forming aluminum nitride on the metal layer.

5. The method according to claim 1, wherein the step of patterning the metal nitride layer comprises a photolithography process comprising one or more of coating the metal nitride layer with a photoresist layer, exposing the photoresist layer with a pattern of radiation, developing the exposed photoresist layer, removing portions of the developed or the undeveloped photoresist layer to expose portions of the metal nitride layer, etching the exposed portions of the metal nitride layer and stripping the residual photoresist layer.

6. The method according to claim 1, wherein the step of forming aluminum nitride comprises depositing aluminum nitride in the area of the exposed portions of the substrate with crystalline properties and/or columnar properties and/or C-axis orientation and in the area of the residual metal nitride layer with amorphous or non-crystalline properties.

7. The method according to claim 1, wherein the forming of a metal nitride layer comprises forming of a titanium nitride layer.

8. The method according to claim 1, wherein the forming of a metal layer comprises forming of a sandwich of aluminum and tungsten or forming of a sandwich of an aluminum and copper alloy and tungsten or forming of a composition of one or more of molybdenum, ruthenium, iridium and platinum.

9. The method according to claim 1, wherein the step of providing a substrate comprises providing a workpiece having a top layer of a dielectric material.

10. The method according to claim 9, wherein the step of providing a substrate comprises providing a workpiece having a top layer of silicon dioxide.

11. The method according to claim 9, wherein the step of providing a substrate comprises providing a workpiece including a bragg mirror arrangement, wherein the bragg mirror arrangement comprises a top layer of silicon dioxide.

12. The method according to claim 1, before the step of forming aluminum nitride further comprising removing oxygen from the metal layer.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0026] In the drawings:

[0027] FIG. 1 shows a workpiece according to a first step of treatment comprising a patterned metal nitride layer and an exposed substrate portion;

[0028] FIG. 2 shows another step of treatment of the workpiece with a metal layer deposited for a bottom electrode;

[0029] FIG. 3 shows yet another step of treatment of the workpiece having aluminum nitride deposited with a section of piezoelectric properties and a section without piezoelectric properties;

[0030] FIG. 4 shows a top view on the workpiece of FIG. 3;

[0031] FIG. 5 shows rocking curve measurements of AlN thin-films grown on different material layers; and

[0032] FIG. 6 shows a BAW resonator with piezoelectric AlN surrounded by amorphous AlN for a thermal conductor.

DETAILED DESCRIPTION OF EMBODIMENTS

[0033] The present disclosure will now be described more fully herein after with the reference to the accompanying drawings showing embodiments of the disclosure. The disclosure may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that the disclosure will fully convey the scope of the disclosure to those skilled in the art. The drawings are not necessarily drawn to scale but are configured to clearly illustrate the disclosure. The same elements in different figures of the drawings are denoted by the same reference signs.

[0034] FIGS. 1 through 3 show cross-sectional views of consecutive steps of a workpiece treated in a process of forming an aluminum nitride (AlN) layer. Turning now to FIG. 1, a substrate 100 is shown on which the below-described structure is formed. The substrate may be a dielectric layer that is the top layer of a more complex layer arrangement. In the described embodiment, the substrate 100 is made of silicon dioxide (SiO.sub.2) that is a portion of a bulk acoustic wave (BAW) resonator as described in more detail in connection with FIG. 6.

[0035] A layer of an orientation-disturbing or orientation-prohibiting material such as titanium nitride (TiN) is deposited. The TiN layer is structured according to a photolithography process so as to obtain a residual layer of TiN 110. The structuring comprises coating of the TiN layer with a photoresist, exposing the photoresist with a radiation pattern, developing the exposed photoresist, removing portions of the developed or undeveloped photoresist. The exposed portions of the TiN layer are dry etched relative to the remaining photoresist mask portions. The dry etch process may involve chlorine chemistry such as BCl.sub.3 and Cl.sub.2. In areas where the TiN layer is removed, the top surface 101 of the substrate 100 is exposed. The residual TiN layer 110 serves as an orientation-disturbing layer that prevents a C-axis oriented growth of a later to be deposited AlN layer.

[0036] FIG. 2 shows the workpiece having a metal layer 210 deposed thereon. Metal layer 110 may be the bottom electrode of an electronic device such as a BAW resonator of the solidly mounted resonator type as described in connection with FIG. 6. Metal layer 210 comprises a sandwich of aluminum including a small amount of copper and tungsten, consecutively formed on the surfaces of the residual TiN layer 110 and the exposed surface 101 of substrate 100. Metal layer 210 comprises a portion 210a disposed on patterned TiN layer 110 and portion 210b disposed on exposed portion 110 of substrate 100. The TiN layer portion 110 has substantially no internal preference orientation and may comprise a mixture of a multitude of grains of different size and orientation. Specifically, TiN layer 110 has no crystalline structure. Metal layer portion 210a is a substantially non-oriented bottom electrode structure that has inherited the non-crystalline multigrain structure of TiN layer portion 110. Metal layer portion 210b is a well-oriented bottom electrode structure, because it has no underlying orientation-disturbing layer, due to the fact that it is directly disposed on substrate 100.

[0037] Turning now to FIG. 3, a layer of aluminum nitride is simultaneously deposited on both portions 210a and 210b of the bottom electrode metal layer 210. The deposition parameters are such that portion 320 of AlN grown on well-oriented bottom electrode 210b on exposed portion of substrate 100 grows with good C-axis orientation so that AlN portion 320 exhibits good piezoelectric properties. The deposition of AlN on a bottom electrode structure such as 210b is well-known for the fabrication of a BAW resonator or any other electroacoustic components. Portion 310 of AlN is simultaneously grown on non-oriented portion 210a of the bottom electrode. Portion 310 of AlN inherits the structure caused by non-oriented bottom electrode portion 210a so that this AlN portion 310 has substantially no internal orientation. The AlN portion 310 has no C-axis orientation or columnar orientation or crystalline structure, because it is grown above orientation-disturbing TiN layer 110. Instead, AlN portion 310 exhibits a polycrystalline or amorphous structure. By use of a patterned titanium nitride layer 110 and a bottom electrode 210 disposed on the workpiece, piezoelectric layer AlN portion 320 and non-piezoelectric AlN portion 310 can be grown simultaneously without changing the deposition parameters of the deposition chamber. Bottom electrode layer 210a is deposited on the TiN layer 110 in a way that it impedes proper growth of a piezoelectric layer while good growth of piezoelectric AlN is promoted in areas that are directly on the substrate 100. The AlN film on top of exposed surface 101 of substrate 100 inherits the orientation of substrate 100 and hence has piezoelectric behavior where it is grown on the bottom electrode regions that are not on the orientation-disturbing layer 110.

[0038] In order to achieve a good adhesion of the AlN layer on the bottom electrode 210 and allow the forming of a C-axis oriented nucleation in area 210b and non-oriented growth in area 210a, it is useful to remove oxygen from the surface of metal layer 210 immediately before the deposition of the AlN to obtain an oxygen free surface. Oxygen removal may be performed in a hydrogen plasma.

[0039] While piezoelectric layer portion 320 can be used to manufacture an electroacoustic component exploiting the piezoelectric properties of layer 320, the adjacently deposited AlN layer 310 of the amorphous or polycrystalline type may be used to produce a capacitor having AlN layer portion 310 as the dielectric. AlN layer portion 310 can also serve as a thermal conductor to transport the heat generated in the piezoelectric component away to a heatsink so that the electrical specifications of the electroacoustic component have tolerable or substantially no temperature drift.

[0040] It is to be noted that the metal layer portion 210a disposed above the residual TiN layer portion 110 may be omitted. In this case, the AlN portion without piezoelectric properties is grown directly on the residual TiN layer portion 110.

[0041] FIG. 4 shows an SEM image which is a top view on the cross-sectional representation of FIG. 3. The lower right portion 320 shows AlN deposited with good C-axis orientation. The distinct C-axis columns can be individually identified in the image. The upper left portion 310 is the AlN deposited without C-axis orientation that is amorphous. As can be gathered at the border between portions 320, 310, the two areas have a defined transitional contact surface. There is a relatively sharp border line between piezoelectric portion 320 and non-piezoelectric portion 310. The portion 320 is generated by bottom electrode 210b directly disposed on surface 101 of substrate 100. The portion 310 is caused by non-oriented bottom electrode 210a disposed on orientation-disturbing TiN layer portion 110 disposed on substrate 100.

[0042] Accordingly, the present disclosure achieves the growth of an aluminum nitride thin film in good and poor C-axis orientation in a controlled way and to achieve portions of AlN with and without piezoelectric properties adjacently and next to each other having a common border surface.

[0043] FIG. 5 shows an X-ray analysis using a rocking curve measurement of AlN films grown on different underlying materials. The horizontal axis represents the incident angle of the X-ray beam performing the rocking curve measurement in degrees. The vertical axis represents the linear intensity in cps units.

[0044] Curve 510 results from an AlN layer deposited on a PVD-generated TiN layer. Curve 510 is flat which indicates that the surface has no major orientational structure so that it is very irregular such as is shown in portion 310 of FIG. 4. Curve 520 results from an AlN layer grown on CVD-generated SiN (silicon nitride). Curve 530 results from an AlN layer grown on CVD-generated SiO.sub.2. Curve 540 results from an AlN layer grown on PVD-generated SiO.sub.2. Curves 520, 530, 540 include a maximum which indicates that the surface exhibits a preference orientation and has a regular structure such as is shown in section 320 of FIG. 4.

[0045] FIG. 6 depicts a cross-section of a BAW resonator. The structure is built on substrate 620 which may be a silicon wafer. Deposited thereon is a Bragg mirror arrangement 610 which is composed of an alternating sequence of material of high acoustic impedance such as tungsten 612 and material of low acoustic impedance such as SiO.sub.2 611. The top layer of Bragg mirror arrangement 610 is SiO.sub.2 layer 611. The function of Bragg mirror 610 is to limit the propagation of an acoustic resonating wave into the substrate.

[0046] Disposed on SiO.sub.2 Bragg mirror top layer 611 is a pattern of a TiN layer 630. Thereon disposed is a bottom electrode metal layer 640 having a portion 640b disposed directly on SiO.sub.2 layer 611 and portions 640a disposed on patterned TiN layer portion 630. Thereon disposed is an AlN layer that has portions 650 on patterned TiN layer portions 630 and a portion 660 where the bottom electrode layer 640b is directly disposed on the SiO.sub.2 layer 611. AlN portions 650 above orientation-disturbing TiN layer 630 exhibit no piezoelectric properties. Portion 660 disposed on bottom electrode portion 640b disposed directly on SiO.sub.2 layer 611 has a strong C-axis orientation so that AlN layer portion 660 has piezoelectric properties. On top of AlN portion 660 is disposed a top electrode metal layer 670.

[0047] The BAW resonator of FIG. 6 of the solidly mounted resonator type establishes an acoustically resonating wave within piezoelectric layer 660 and between bottom and top electrodes 640b, 670. The acoustically resonating wave generates substantial heat that must be transported out of the acoustically active region to avoid a temperature drift of the electrical parameters of the resonator. For this reason, AlN layer portions 650 serve as a thermal conductor to transport the heat to a heatsink such as ambient air, the bulk portion of the Bragg mirror 610 or solder bumps (not shown in FIG. 6) or other heat-sinking elements thermally connected to AlN layers 650. As an advantage of the use of orientation disturbing TiN layer 630, the AlN portion 650 is amorphous and has no piezoelectric properties so that it does not disturb or affect the acoustically active regions 640b, 660, 670. Unwanted resonances are avoided in that the structures outside of the resonator do not exhibit resonating behaviour. On the other hand, AlN layer portion 650 benefits from the good thermal conductivity of AlN. The structure can be efficiently manufactured in that the AlN layers 650, 660 can be grown in one common process generating piezoelectric portion 660 and non-piezoelectric portion 650 simultaneously.

[0048] In a resonator such as the BAW resonator shown in FIG. 6, specific structures may be provided surrounding the acoustically active area 640b, 660, 670. Such structures are configured to exhibit a defined frequency and velocity profile that causes the acoustic wave to be substantially confined to the active area. The structures are configured to cause a reflection of the acoustic wave and substantially avoid a leaking from the active area. In this regard it is to be noted that piezoelectric AlN and non-piezoelectric AlN have different acoustic impedance and different acoustic velocity characteristics. A suitable layer stack design may be conceived by the skilled person using piezoelectric AlN and non-piezoelectric AlN that forms a lateral energy barrier for the acoustic wave so that the acoustic wave is substantially prevented from escaping from the active area. These lateral structures forming a lateral energy barrier can be achieved with alternately depositing piezoelectric and non-piezoelectric AlN using the orientation-disturbing metal nitride layer concept of the present disclosure. The quality factor of the resonator is thereby increased.

[0049] It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosure as laid down in the appended claims. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the sprit and substance of the disclosure may occur to the persons skilled in the art, the disclosure should be construed to include everything within the scope of the appended claims.