Die Separation Method for Silicon on Diamond Circuit Structures

This invention relates in general to silicon on diamond circuits, and in particular, to a method for separating dies of silicon on diamond.

Diamond has valuable thermal and electrical properties which are highly desirable in microelectronic circuits. Diamond has the highest thermal conductivity of any known material. At room temperature, diamond is about five times as thermally conductive as copper. Diamond is also an excellent electrical insulator and has a superior dielectric strength. As such, diamond makes an excellent material for heat sinks in microelectronic circuits. The ability of diamond to insulate the circuit and rapidly remove heat from the circuit increases the overall performance of microelectronic circuits because their speed deteriorates with heat. Moreover, heat also contributes to the early failure of microelectronic circuits.

Until recently diamond has not been a useful material due to the difficulty of integrating it into existing circuit fabrication techniques. However, recent advances have enabled those skilled in the art to deposit layers of diamond using chemical vapor deposition techniques. One such technique is described in "Chemical Vapor Deposition of Diamond for Electronic Packaging Applications," David J. Pickerel and David S. Hoover, Proceedings, First International High Temperature Electronics Conference, Jun. 16-20, 1991.

Another well-known property of diamond is its hardness. This property renders it difficult to separate the dies of wafers having diamond layers. In a typical die separation process, a circular saw having a diamond-coated blade is used to remove silicon from the area between dies. This operation is normally carried out on a wafer dicing machine. The wafer is affixed to an adhesive surface of a film material that is mounted on a frame. The frame and wafer are placed beneath the saw and the saw cuts through the wafer and partially into the membrane of the film. Thereafter, the die are removed from the film with a pick and place machine that mounts the die into a chip carrier or other package.

However, such traditional methods of die separation are unsatisfactory for wafers with diamond layers. Due to the hardness of diamond, circular diamond saw blades are rapidly worn out. Thus, those skilled in the art have resorted to a scribe and break method for separating die. Using that method, a saw partially penetrates the diamond layer. Thereafter, the die are separated along the scribe marks in the diamond layer by mechanically snapping the layer over a surface disposed beneath the partial scribe line. This method is akin to the method used to cut glass with a scriber. The problem with such method is that it often applies mechanical stresses across the wafer so that the risk of damage to the electronic circuits on the die is high.

In view of the above problems, it would be desirable to have a reliable method for separating silicon on diamond dies.

The invention includes a method that applies manufacturing processes which are well characterized in order to separate die from the wafer. In its more general aspects, the invention provides for patterning a layer of diamond material on a support or sacrificial layer of a wafer. The diamond material is patterned into a pattern corresponding to the scribe lines of the final wafer. Thereafter, the silicon on diamond devices may be separated from the wafer by following the scribe pattern made in the diamond layer.

More specifically, the invention includes one embodiment for providing a patterned diamond layer. This embodiment includes a process having a first step of laser machining or reactive ion etching of a silicon layer that carries the diamond layer. The silicon is ablated or reactive ion etched to have a pattern of scribe lines therein. So, portions of the diamond layer in the form of a scribe pattern are exposed by the ablation or reactive ion etching process. The exposed diamond layer is removed by a beam of high energy, such as a laser or by reactive ion etching. In the preferred embodiment, the exposed diamond layer is exposed to a laser beam of frequency doubled Nd:YAG laser radiation. Having thus removed the diamond layer from the scribe lines, the device wafer carrying the active microelectronic die may be sawed using a traditional diamond wafering saw that follows the scribe line pattern formed by the laser ablation or reactive ion etching.

In another embodiment of the invention, the diamond layer is ablated by a laser following the deposition of the diamond layer on its sacrificial substrate and prior to joining a device water to the substrate carrying the diamond layer. In this embodiment it is also possible to remove portions of the diamond layer using reactive ion etching (see SE-932, Ser. No. 08/042,299, filed Apr. 2, 1993, assigned to the same assignee as this application mid incorporated herein by reference), or ion beam etching (see Rothschild, et al., J. Vac. Sci. Technol. B 4 (1), Jan/Feb 1986).

A still further embodiment of the invention relies upon selective deposition of the diamond layer onto areas of a sacrificial layer that are patterned with a diamond resist layer. For example, it is known that diamond does not readily deposit on silicon dioxide. Hence, a sacrificial silicon wafer with a silicon dioxide scribe pattern on its surface will resist the deposition of diamond onto the silicon dioxide and allow the diamond to be deposited on the other areas of the wafer.

As described in the following embodiments, the invention may be practiced at various stages of formation of microelectronic devices. So, the diamond layer may be formed and scribed or patterned on a substrate before the diamond layer is joined to a device wafer. As an alternative, the diamond layer may be scribed or patterned after it is joined to a device wafer but before any devices are formed. A further alternative is to scribe the diamond layer after complete formation of the microelectronic devices.

FIGS. 1a-c are a series of partial schematic figures illustrating diamond removal and scribing after formation of the microelectronic devices.

FIGS. 1a.1-e.1 are a series of partial schematic figures illustrating two embodiments of the invention where diamond removal and scribing is accomplished after joining the device wafer to a diamond film but before formation of the microelectronic devices.

FIGS. 2a-c are a series of partial schematic figures illustrating diamond removal and scribing before joining a device wafer to a diamond film.

FIGS. 3a-k are a more detailed series of partial schematic views illustrating diamond removal and scribing before joining a device wafer to a diamond film.

FIGS. 4a-g are a series of partial schematic views illustrating diamond removal and scribing before joining a device wafer to a diamond film.

FIGS. 5a-e are a series of partial schematic views illustrating patterning a diamond film prior to joining the patterned film to a device wafer.

With reference to FIG. 1, those skilled in the art will appreciate that there are numerous steps in fabrication of microelectronic circuits on diamond insulating layers. As such, the steps depicted in FIGS. 1a-c are shown for purposes of illustrating the invention and not for describing the detailed manufacturing processes which will be discussed later. As such, the figures vary from the actual sequence of manufacturing events. For example, the devices 5 shown in FIG. 1 may not be formed until after the sacrificial or handle layer 2 is fully removed. Hence, FIG. 1 is given for purposes of highlighting the particular steps of the invention and is not be deemed a full description of a silicon on diamond process.

In FIG. 1a, a composite wafer 1 has a handle or sacrificial wafer 2, a diamond layer 3 and a device wafer 4. The diamond layer 3 has been previously deposited on the sacrificial wafer 2 by methods well known in the art. Likewise, the device wafer 4 bonded to the diamond layer 3, and microelectronic circuits 5 have been formulated within a layer or layers on one surface of device wafer 4 in a manner well known to those skilled in the art. Details about oxide bonding can be found in SE 898/900, Ser. No. 921,197, filed Jul. 28, 1992 assigned to the same assignee as this application and incorporated herein by reference. The devices 5 are separated from each other by a plurality of device-free areas 6 known as streets or scribe lines. In a normal die separation process, a circular saw would remove silicon from the streets 6 and thereby separate the die. However, with the diamond layer 3 disposed on the back surface of the device wafer 4, alternate techniques including those in the invention are needed to separate the die. As a first step, portions of device wafer 4 bounded within device-free region 6, are removed by laser machining to expose portions of diamond layer 3 thereunder, as shown in FIG. 1b. For the case where device wafer 4 is silicon, laser machining may be accomplished using a 248 nm KrF excimer laser. Next, as shown in FIG. 1c, the surface of wafer 4 with exposed streets 6 is exposed to a laser or reactive ion etch (RIE). The laser or reactive ion etch (for RIE using oxygen, see for example SE-932, Ser. No. 42,299, filed Apr. 2, 1992, assigned to the same assignee as this application and incorporated herein by reference) removes the diamond from the areas defined by the scribe line 6.

In the preferred embodiment, a beam of high energy is used to remove the diamond layer 3. This beam can be a laser, in particular a Nd:YAG laser, more particularly, a frequency doubled Nd:YAG laser, or a 193 nm ArF excimer laser.

After the portions of diamond layer 3 are suitably removed, as shown in FIG. 1c, the sacrificial wafer 2 may be removed to separate wafer 4 into individual silicon dies 5, suitable for packaging. Alternatively, after portions of diamond layer 3 are suitably removed, as shown in FIG. 1c, the sacrificial wafer 2 may be sawn with a circular diamond saw blade through scribe street 6 to thereby separate wafer 4 into individual silicon on diamond dies 5 suitable for packaging.

In another embodiment, described in FIGS. 1a.1-1e.1, formation of scribe areas 6 may precede formation of microelectronic circuits in regions 5. In FIG. 1a.1, a composite wafer 1 has a handle or sacrificial wafer 2, a diamond layer 3, and a device wafer 4. The diamond layer 3 has been previously deposited on the device wafer 4 by methods well known in the art. Likewise, a handle wafer 2 has been bonded to the diamond layer 3 in a manner well known by those skilled in the art. As a first step, device wafer 4 is masked by suitable means with silicon dioxide or silicon nitride or a combination thereof, to define the pattern where street areas 6 will be formed. Portions of masking layer 8 are removed by suitable etching means, such as RIE, to expose surface 40 of device wafer 4, FIG. 1b.1. Next, as shown in FIG. 1c.1, street regions 6 of device wafer 4 are removed by RIE to expose portions of diamond layer 3. Street areas 6 of exposed diamond layer 3 may then be removed as previously described by laser ablation or RIE (for example with oxygen), FIG. 1d.1. Microelectronic circuits 5 may then be formed within a layer or layers of device wafer 4. The completed structure is shown in FIG. 1e.1. The handle or sacrificial wafer 2 may optionally be removed prior to die separation (not shown).

Those skilled in the art will understand that there are numerous steps in the fabrication process between the embodiment shown in FIG. 1d.1 and FIG. 1e.1. In further processing of the wafer 2 shown in FIG. 1d.1 the scribe streets 6 may be left void or filled with a suitable material. The decision to leave a void or fill the street 6 will depend upon the particular subsequent steps of the chosen process. Voids may remain so long as the diamond layer is not exposed to an oxidizing process. If the layer would be exposed to an oxidizing process, then the voids are filled with a suitable oxidation barrier material to prevent oxidation of the diamond layer 3. Such materials include a layer of silicon nitride which prevents the oxidation of the diamond layer 3. Other materials include polysilicon either alone or in combination with a silicon nitride layer.

Turning to FIG. 2, an alternate embodiment of the invention is shown. In FIG. 2a, a diamond layer 3 is mounted on a sacrificial silicon substrate 2. Prior to joining a device wafer 4 to the substrate 30, comprising diamond layer 3 and substrate 2, the diamond layer 3 is patterned and portions of it are selectively removed in order to define a series of scribe lines 6 as shown in FIG. 2(b). As mentioned above, the diamond layer 3 may be removed by a suitable laser or by reactive ion etching or by ion beam etching. Thereafter, a device wafer 4 is joined to the diamond layer 3. As shown in FIG. 2c, microelectronic circuits 5 are subsequently formed within a layer or layers of device wafer 4. A pattern of scribe streets 6.1 disposed between microelectronic circuits 5 correspond to the pattern of scribe streets 6 within diamond layer 3.

Turning to FIGS. 3a-k, there is shown a more detailed process flow of a typical silicon on diamond manufacturing process that is adapted to include the invention. As shown in FIG. 3a, a sacrificial substrate 2 has a diamond film 3 deposited on an upper surface 11. The upper surface of the diamond film 10 is then deposited with polysilicon bonding layer 22, as shown in FIG. 3(b). The polysilicon bonding layer 22 is then planarized. It is approximately one to two microns in thickness. Next, a silicon handle wafer 20 is bonded to the planarized surface 12 of polysilicon layer 22 as shown in FIG. 3(c). The handle 20 is attached to the polysilicon layer 22 using an oxide bond process. Additional details about oxide bonding can be found in SE-898/900, Ser. No. 921,197, filed Jul. 28,1992, assigned to the same assignee as this application and incorporated herein by reference. As an alternative, the bond may be a silicide bond, i.e. the polysilicon layer 22 and the silicon handle wafer 20 may be bonded by reacting the layers in a silicide process to form, for example, tungsten disilicide. Further details about this type of bonding can be found in SE-906, Ser. No. 939,786, filed Sep. 3, 1992, assigned to the same assignee as this application and incorporated herein by reference.

Once the handle wafer 20 is suitably bonded, the sacrificial substrate 2 may be removed by any suitable means, usually chemical etching. The surface 11 of diamond layer 3 is now exposed as shown in FIG. 3(d). In the next step, the diamond layer 3 is patterned and portions are selectively removed to define a pattern of scribe lines or streets 6. This step is shown in FIG. 3(e). The diamond layer in the streets 6 is removed as discussed above by a suitable high energy electromagnetic beam including a laser beam or a reactive ion etch or ion beam etch. As an alternative, the sacrificial substrate as shown in FIG. 3(b) may be removed as shown in FIG. 3(c.1), resulting in a freestanding diamond film with polysilicon layer 22. Then the handle is placed upon the polysilicon layer 22 as shown in FIG. 3(d.1). The diamond layer 3 may be suitably patterned as shown in FIG. 3e. Further details are taught in SE-911, Ser. No. 29,860, filed Mar. 11, 1993, assigned to the same assignee as this application and incorporated herein by reference.

In order to bond a device wafer 4 to the diamond layer 3, a second polysilicon layer 32 of about 500 angstroms is deposited on the surface 11 of the diamond layer 3 and the exposed portions of lower polysilicon layer 22. The layer 32 may also comprise silicon nitride. If silicon nitride is used, it is deposited on surface 11 before the second polysilicon layer. Its thickness may be 300 to 500 angstroms. See FIG. 3(f). SE-911, Ser. No. 29,860, filed Mar. 11, 1993, assigned to the same assignee as this application and incorporated herein by reference teaches the utility of a thin nitride layer as an oxidation and diffusion barrier.

In the next step as shown in FIG. 3g, a device wafer 4 is bonded to the second polysilicon layer 32. It will be noted that the scribe lines 6 appear as a series of patterned voids between the handle 20 and the device layer 4. The device wafer 4 is suitably processed to form microelectronic circuits 5 on the upper surface 38 thereof. The microelectronic circuits 5 are covered with die passivation layer 14, FIG. 3h, consisting of, for example, silicon nitride or silicon oxy-nitride. The microelectronic circuits 5 will be separated by areas of silicon that comprise scribe lines 6. It is understood that the scribe lines 6 comprise a pattern that corresponds to the pattern of the voids 36. As such, the handle 20 may be removed and the dies 5 may be separated by sawing through the scribe line 6 and voids 36.

Prior to die separation, the device layer 4 containing microelectronic circuits 5, covered by passivation layer 14, may be mounted with wax onto a rigid substrate, FIG. 3i. The handle 20 may then be removed using a wet etch of potassium hydroxide or nitric acid/HF which will stop at the bonded oxide or silicide surface of layer 22. An oxide surface is then removed with HF acid; a silicide surface is removed with a wet etch of HCl:HNO, in 1:1 proportions. That will expose the polysilicon layer 22. At this point the die may be separated by sawing through the scribe lines 6 and removing individual die from the rigid substrate, FIG. 3j. Alternatively, the exposed polysilicon layer 22 may be attached to sticky tape, FIG. 3j.1, and the rigid substrate removed by heating the wax, FIG. 3k.1. At this point die separation can be accomplished by either sawing the scribe lines 6 from the device side or flexure of the sticky tape which causes cleavage along the scribe lines 6. An alternative die separation could proceed from FIG. 3h. The handle 20 may be attached to sticky tape and the wafer sawn along scribe lines 6, through handle 20, FIG. 3h.1. The separated dies 5 would retain their handle portions.

Alternate manufacturing processes may also use the invention. For example with reference to FIGS. 4a-g there is shown another embodiment. As shown in FIG. 4(a), a diamond layer 3 is formed over a sacrificial substrate 2. The diamond layer 3 is coated with a silicon nitride layer 24 of thickness between 300 and 500 angstroms and a polysilicon layer 22 of thickness about 1000 to 2000 angstroms (see FIG. 4(b)). The polysilicon layer 22 is patterned and completely reacted with oxygen to form a matrix pattern of silicon dioxide patterns 26. See FIG. 4(e). As shown in FIG. 4(d), a plan view, the silicon dioxide patterns 26 are randomly patterned in a dot matrix on the surface of silicon nitride layer 24 independent of the scribe lines 6. No alignment of the silicone dioxide pattern 26 is required with the scribe lines 6. The silicon dioxide pattern 26 is patterned to be 2 to 5 microns in lateral size and spaced 10 to 100 microns apart. Next, as shown in FIG. 4(e), a handle substrate 20 is bonded to the silicon dioxide patterns 26 and the sacrificial substrate 2 is removed. The scribe lines 6 are formed in the diamond layer 3 by either laser beam ablation or reactive ion etching, but are independent of the alignment of the silicon dioxide pattern 26 (see FIG. 4(f)).

A device wafer may be attached to the surface 12 of the diamond layer 3. Microelectronic circuits are subsequently formed in a layer or layers of the device wafer. Prior to die separation, the device layer or layers containing the microelectronic circuits are mounted in wax 45 onto a rigid substrate 47, FIG. 4(g). The silicone dioxide pattern 26 is completely removed by wet etch with HF, releasing the handle substrate 20 and exposing silicon nitride layer 24. The microelectronic circuits of the device wafer can then be separated by a diamond saw that removes silicon from the device wafer by sawing through the diamond scribe lines 6 visible through silicon nitride layer 24.

Another embodiment of the invention using selective diamond deposition is illustrated in FIGS. 5a-e. Details for selective deposition of diamond films is described in J. of Electronic Materials, Vol 18, No. 6, 1989, pages 711-715, J. L. Davidson et. al. In FIG. 5a a sacrificial silicon substrate 2 is first coated and patterned to have a silicon nitride mask 24 deposited thereon. In the spaces between the silicon nitride portions 26, a silicon dioxide pattern of lines 26 is grown. See FIG. 5(b). Silicon nitride is removed as shown in FIG. 5(c) leaving a silicon dioxide pattern 26 on the surface of the sacrificial silicon substrate 2. Using a chemical vapor deposition process, diamond material 3 is deposited onto the sacrificial wafer 2 but not onto the silicon dioxide pattern 26. See FIG. 5(d). The unevenness of the two patterns is reconciled by depositing a polycrystalline layer 22 on top of the diamond 3 and silicon dioxide 26 and thereafter planarizing the polysilicon layer 22. See FIG. 5(e). Thus, with the above described method the diamond layer portions 3 are separated by silicon dioxide portions 26. So long as the silicon dioxide portions 26 are patterned in a pattern corresponding to scribe lines, then the device wafer (not shown) may be affixed to the polysilicon layer 22 by bonding methods previously described and further processed to form microelectronic circuits. During further processing, the scribe lines on the device wafer will be aligned with the scribe lines 6 in the diamond layer 3.

It should be understood that although the disclosed embodiments have been directed to silicon on diamond, the methods are also applicable to other semiconductors suitable for microelectronic circuits, such as GaAs, InP, or combinations of the above. Those skilled in the art will further understand that the invention can be used at different times in the process of fabricating a microelectronic device. When the invention is used will depend upon the process selected and the thickness of the active layers and the diamond layer. For example, high speed CMOS processes used very thin active layers. As such, one may choose to wait until after the complete formation of the CMOS devices to use the invention. The embodiment of FIG. 1a-c and FIG. 1a.1-e.1 are especially suited for such devices. On the other hand, high power bipolar devices have relatively thick active layers. For such devices it is more desireable to use the embodiments of the invention that scribe the diamond layer before formation of the final devices. The embodiments of FIGS. 2-5 are more suitable for devices with thick active layers. The selection of the embodiment will also depend upon the desired thickness of the diamond layer. So, the embodiment shown is FIGS. 3c.1 and d.1 are especially useful for thick diamond film capable of providing a free standing diamond film. For free standing diamond layers of 200 microns or greater, the embodiment of FIG. 3c.1 and d.1 is preferred.

The foregoing has disclosed several embodiments of the invention. One of the technical advantages of the disclosed embodiments is that they provide practical techniques for separating silicon on diamond circuit dies. These techniques are compatible and integratable with existing manufacturing processes and assembly techniques including wafer dicing saws. Those skilled in the art will appreciate that various modifications, changes, additions and alterations may be made to the above embodiments without departing from the spirit and scope of the invention as defined by the following claims.