Process for forming and acoustically connecting structures on a substrate

The present invention describes a processes that builds an acoustic cavity, a chamber, and vent openings for acoustically connecting the chamber with the acoustic cavity. The dry etch processes may include reactive ion etches, which include traditional parallel plate RIE dry etch processes, advanced deep and inductively coupled plasma RIE processes. Three embodiments for connecting the chamber to the cavity from the top side of the substrate, e.g. by using pilot openings formed using at least a portion of the mesh as an etch mask, by forming the vent openings using at least a portion of the mesh as an etch mask, or by having the chamber intersect the vent openings as the chamber is being formed, illustrate how the disclosed process may be modified. By forming the cavity on the back side of the substrate, the depth of the vent holes is decreased. Additionally, using at least a portion of the micro-machined mesh as an etch mask for the vent holes makes the process self-aligning.

BACKGROUND OF THE INVENTION

The present invention is directed generally to micro-electromechanical Systems (MEMS) devices and, more particularly, to processes for forming chambers and cavities in a substrate and acoustically interconnecting such structures.

The ability to form moving parts measured in microns has opened up a huge range of applications. Such moving parts typically take the form of a beam or mesh that may form, for example, a variable capacitor, switch, or other component. The recent ability to seal micro-machined meshes has lead to the fabrication of microphones and microspeakers. See, for example, International Publication No. WO/01/20948 A2 published 22 Mar. 2001, entitled MEMS Digital-to-Acoustic Transducer With Error Cancellation, the entirety of which is hereby incorporated by reference.

A sealed mesh can function as a movable plate of a variable capacitor, and therefore can operate as a microphone. For a sealed mesh to operate as a microspeaker, the microspeaker needs to be able to push air to create a soundwave just as its larger counterparts must push air to create soundwaves. Traditional speaker enclosures have a port on the back to allow the speaker to move freely. In the case of a microspeaker, if the chamber beneath the sealed mesh does not have a vent or other opening to ambient, movement of the sealed mesh inward is inhibited by the inability to compress the air in the chamber while movement of the mesh outward is inhibited by formation of a vacuum. Thus it is necessary to form a vent in the chamber if the microspeaker is to create soundwaves.

Currently, such vents are formed by boring through the substrate from the rear. That requires patterning the back side of the substrate followed by an etch through the entirety of the substrate to reach the chamber. Forming of vents by this technique is slow in that several hundred microns of substrate may need to be etched to reach the chamber beneath the sealed mesh and the diameter of the vent is small compared to its depth. Additionally, there are registration problems in that it is necessary to work form the back side of the substrate where there are no landmarks, and hundreds of microns may need to be etched to reach a chamber that may measure in the tens of microns. Thus, the need exists for an easy, repeatable, fast process for forming vents in the chambers of sealed meshes that are to function as speakers.

BRIEF SUMMARY OF THE INVENTION

The present invention is directed to a process comprising reducing the thickness of a back side of a substrate in an area where vents are to be formed. A micro-machined mesh is released from a top side of the substrate. A vent opening is formed that connects the released mesh and the area of reduced thickness. Several embodiments of the invention are disclosed. In one embodiment, pilot openings are formed in the substrate in the area of reduced thickness by using at least a portion of the mesh as an etch mask. That assures proper alignment of the vent openings. Releasing of the mesh involves removal of a portion of substrate from beneath the mesh. While the mesh is being released, the vents are formed by expanding the size of the pilot openings.

According to another embodiment, the vent openings are formed after the mesh is released. In that embodiment, the mesh is released using an isotropic etch. After the mesh is released, the mesh is used as an etch mask for an anisotropic etch to form vent openings for connecting the chamber with the area of reduced thickness. Use of the mesh as an etch mask eliminates a lithography step and assures proper alignment of the vent openings.

According to another embodiment of the invention, vent openings are formed in the area of reduced thickness from the back side of the substrate using an anisotropic etch prior to releasing the mesh. When the mesh is released using an isotropic etch, during formation of the chamber under the mesh, the chamber intersects the vent openings.

A common step in the various embodiments of the invention is to reduce the thickness of the substrate in the area of the vent openings. That is accomplished by etching a large cavity; because the diameter of the cavity relative to the depth is large, this step can be carried out efficiently. Connecting to this cavity is accomplished from the top side of the substrate, e.g. by using pilot openings formed using at least a portion of the mesh as an etch mask, by forming the vent openings using the mesh as an etch mask, or by having the chamber intersect the vent openings as the chamber is being formed. Those embodiments that use the mesh as an etch mask eliminate a lithography step thereby increasing processing speeds. Those advantages and benefits, and others, will be apparent from the Detailed Description of the Invention herein below.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the present invention is illustrated in conjunction withFIGS. 1–9. InFIG. 1, a die10is received from a CMOS foundry. At the CMOS foundry, a silicon substrate12has been processed so as to form alternating layers of, for example, a dielectric material and a metal. The die10illustrated inFIG. 1has a first layer of dielectric material14carrying a first metal layer16. The first metal layer16has been patterned such that a portion thereof forms a micro-machined mesh18. Formed on the first metal layer16is a second layer of dielectric20. The second layer of dielectric20carries a second metal layer22which has been patterned to have an opening24formed therein. The second metal layer22carries a third layer of dielectric26. The third layer of dielectric26carries a third layer of metal28which has been patterned to have an opening30formed therein. A top layer of dielectric32is formed on top of the third metal layer28.

The present invention is not limited to the position and configuration of the metal layers shown in the figures. For example, the pattern shown inFIG. 1could be implemented in metal layers two, three and four such that references herein to a first, second and third layers of metal need not correspond to metal layers one, two and three, respectively. Additionally, the configuration of the layers of metal need not be as shown in the figures but rather may vary depending upon the device to be fabricated.

As previously mentioned, the die10would be received, for example, as shown inFIG. 1from the CMOS foundry. Thereafter, the die10will be subjected to post-processing fabrication steps. Although it is anticipated that the post-processing fabrication steps will take place in a facility different from the CMOS foundry which fabricated the die10, that is not a requirement of the present invention.

Turning toFIG. 2, a layer of resist34is formed on the back of the substrate12and patterned with a mask which is used to form an acoustic cavity. Those of ordinary skill in the art will realize that landmarks from the top side of the die10need to be transferred to the bottom side of substrate12. Transferring such landmarks is known in the art are therefore not described herein.

InFIG. 3, the substrate12is subjected to an anisotropic etch. The anisotropic etch may be a dry, deep reactive ion etch (DRIE) into the substrate12to create an acoustic cavity36. Etching stops at tens to hundreds of microns (dimension h inFIG. 3) before the substrate-dielectric interface.

FIG. 4illustrates the substrate12ofFIG. 3after the resist on the back side of the substrate12has been removed and a new layer of resist38formed on the top side and patterned to provide an opening40in the area of the mesh18. InFIG. 5, the substrate12ofFIG. 4is illustrated being subjected to an anisotropic RIE dry etch through the dielectric layers32,26,20and14. The patterned resist38and the first metal layer16are used to pattern the first dielectric layer14. The layer of resist38may not be necessary if it is not necessary to protect the top layer of dielectric32.

FIG. 6illustrates the substrate ofFIG. 5after the top side has been patterned with a layer of resist42to enable certain portions of the mesh18to act as an etch mask for pilot openings to be formed in the substrate12in the area of the cavity36.FIG. 7illustrates the substrate12ofFIG. 6being subjected to a DRIE anisotropic etch which forms pilot openings44extending through the silicon substrate12in the area of the cavity36.FIG. 7Aillustrates the same step of the process asFIG. 7. However, inFIG. 7Ait is assumed that the width of the mesh is much smaller (eg. 0.6 microns) then the depth of the reduced substrate (eg. 100 microns). Under these circumstances, formation of the pilot holes44may result in formation of vent openings.FIGS. 8 and 8Ashow the substrate12ofFIGS. 7 and 7A, respectively, after the resist42has been removed.

FIGS. 9 and 9Aillustrate the substrate ofFIGS. 8 and 8A, respectively, being subjected to an isotropic etch so as to form a chamber50under the mesh18. Forming the chamber50releases the mesh18from substrate12. InFIG. 9, as the chamber50is being formed, the pilot holes44are being enlarged to form vent openings52. InFIG. 9A, the already formed vent holes are enlarged. Thus in the embodiment of the invention illustrated inFIGS. 1–9, at substantially the same time that the chamber50is being formed, the chamber50is being connected to the cavity36by the formation or enlargement of vent openings52. Because the vent openings52are formed by enlarging the pilot openings44, and the pilot openings44are formed by using a portion of the mesh18as an etch mask, the vent openings52will be in alignment so as to connect chamber50with cavity36.

FIGS. 10–15illustrate another embodiment of the present invention.FIG. 10illustrates the substrate ofFIG. 2after a second layer of resist54has been deposited on the back side of the substrate and patterned in a manner, as described below, for forming acoustic cavity36and vent openings52. Those of ordinary skill will recognize that whether two separate layers34,54are used, the positions of layers34,54are reversed, or one layer is used, are not material to the present invention as long as resist is provided in any manner to provide the function described below.

FIGS. 11 and 12illustrate the progression of an anisotropic etch performed on the back side of the substrate12. The etch may be a DRIE and, as the etch progresses, the resist layer54is progressively removed. As the layer54is progressively removed, vent openings52are being formed. Eventually, layer54is entirely removed and, as the etch progresses, cavity36is formed while the depth of vent openings52continues to increase. During this time, a portion of resist layer34may be removed as shown inFIG. 12. The etch may be stopped anywhere from a few ten to one hundred microns below the dielectric-silicon interface (h inFIG. 12).

FIG. 13illustrates the substrate12ofFIG. 12after the resist34has been removed from the back side of the substrate12and the top side has formed thereon a layer of resist56having an opening58formed therein. The opening58is designed to expose the mesh18. Depending upon the need to protect the top layer of dielectric32, applying and patterning the layer of resist56may not be necessary.

InFIG. 15, an isotropic etch of the silicon substrate12is performed to remove material from underneath the mesh18to thereby release the mesh18. The removal of material from underneath the mesh18forms a chamber50. As the chamber50is being formed, it intersects the vent openings52.

FIGS. 16 and 17illustrate another embodiment of the present invention. InFIG. 16, the substrate ofFIG. 5is illustrated. However, instead of performing the anisotropic etch illustrated inFIG. 5, an isotropic etch is performed. For example, an inductively coupled plasma (ICP) RIE may be performed. The isotropic etch illustrated inFIG. 16forms a chamber50under the mesh18thereby releasing the mesh18. Thereafter, inFIG. 17, an anisotropic etch, such as a DRIE etch, is performed using the mesh18as an etch mask. The anisotropic etch ofFIG. 17forms a plurality of vent openings60connecting chamber50with acoustic cavity36. Those of ordinary skill in the art will recognize that when the mesh18is used as an etch mask, that saves a lithography step as a resist layer does not need to be formed and patterned to guide the etch process.

Completing the process, the mesh18ofFIG. 9, the mesh18ofFIG. 15, or the mesh18ofFIG. 17may be sealed using known deposition techniques to form a membrane capable of operating as a speaker or a microphone.

The present invention describes a set of dry etch processes that build an acoustic cavity36, a chamber50, and vent openings52,60for acoustically connecting the chamber50with the acoustic cavity36. The dry etch processes may use reactive ion etches, which include traditional parallel plate RIE dry etch processes, advanced deep and inductively coupled plasma RIE processes. The three embodiments of the present invention illustrate how the present invention may be modified to achieve the result of forming a chamber under a micro-machined mesh, to thereby release the mesh, and acoustically connecting that chamber with an acoustic cavity formed on the back side of the substrate. By forming the cavity on the back side of the substrate, the depth of the vent holes is decreased. Additionally, using the micro-machined mesh as an etch mask for the vent holes makes the process self-aligning.

While the present invention has been described in connection with preferred embodiments thereof, those of ordinary skill in the art will recognize that many modifications and variations are possible. The present invention is intended to be limited only by the following claims and not by the foregoing description which is intended to set forth the presently preferred embodiments.