Semiconductor structure with airgap

FIELD OF THE INVENTION

0001The invention relates to semiconductor structures and, more particularly, to a field effect transistor (FET) with an underlying airgap and methods of manufacture.

BACKGROUND

0002RF switches are significantly easier to make on silicon on insulator (SOI) substrates than on bulk substrates because all junctions are bounded by oxide (STI laterally and the buried oxide below) which eliminates the problem of dropping large voltages across well to substrate junctions. SOI also has low junction capacitances which reduces loading on RF signals. However, it is often advantageous to integrate an RF switch into a bulk process. This can be done with a triple well and very high resistivity substrates, but is a challenge as the RF voltages still must be dropped across a junction, and the large depletion layers in high resistivity substrates add substantial area to the layout.

SUMMARY

0003In an aspect of the invention, a method comprises forming an amorphous layer at a predetermined depth of a substrate. The method further comprises forming an airgap in the substrate under the amorphous layer. The method further comprises forming a completely isolated transistor in an active region of the substrate, above the amorphous layer and the airgap.

0004In an aspect of the invention, a method comprises forming at least one deep trench structure in a bulk substrate, on sides of an active region. The method further comprises forming sidewall structures on sidewalls of the at least one deep trench structure, which acts as an etch stop layer. The method further comprises forming a lateral undercut in the bulk substrate starting at a bottom of the at least one deep trench structure. The method further comprises filling the at least one deep trench structure with material to form an airgap from the lateral undercut in the bulk substrate under the active region.

0005In an aspect of the invention, a structure comprises: an amorphous layer under an active region of a substrate; an airgap in the substrate under the amorphous layer; and a completely isolated transistor in the active region, above the amorphous layer and the airgap and surrounded by shallow trench isolation regions.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

0006The present invention is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present invention.

0007<figref idref="DRAWINGS">FIGS. 1-4</figref> show respective structures and fabrication processes according to an aspect of the present invention; and

0008<figref idref="DRAWINGS">FIGS. 5-12</figref> show respective structures and fabrication processes according to additional aspects of the present invention.

DETAILED DESCRIPTION

0009The invention relates to semiconductor structures and, more particularly, to a field effect transistor (FET) with an underlying airgap and methods of manufacture. In more specific embodiments, the present invention is directed to an RF switch FET manufactured in bulk technology with an airgap underneath its transistor channel. In embodiments, the present invention provides a completely isolated, e.g., oxide isolated, switch FET integrated onto the bulk process.

0010In embodiments, the FET is a bulk CMOS transistor with an underlying airgap. The location of the airgap, e.g., top of the airgap, is determined by an etch barrier directly under and in contact with the transistor channel and source and drain regions. In embodiments, the etch barrier layer is an amorphous layer formed by an ion implantation process. In further embodiments, the location of the sides and/or the bottom of the airgap can be determined by the etch barrier layer. The airgap, on the other hand, can be formed by NH₄OH wet etch of silicon, where the etch access to the silicon is from a top surface opening. In alternative embodiments, the etch access to the silicon is from the bottom of a deep trench which has sidewall spacers, and the airgap is formed by XeF₂dry etch of silicon.

0011Advantageously, the structures of the present invention fully isolate the FET so that there is no junction which connects the transistor to the substrate. The FET of the present invention is also integrated into standard bulk silicon processing without disturbing adjacent elements. Additionally, the present invention eliminates the problem of dropping large voltages across well to substrate junctions in bulk technologies, as well as the problem of large depletion layers in high resistivity substrates which add substantial area to the layout.

0012The FET of the present invention can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form structures with dimensions in the micrometer and nanometer scale. The methodologies, i.e., technologies, employed to manufacture the level translator of the present invention have been adopted from integrated circuit (IC) technology. For example, the structures of the present invention are built on wafers and are realized in films of material patterned by photolithographic processes on the top of a wafer. In particular, the fabrication of the level translator of the present invention uses basic building blocks, including: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask.

0013<figref idref="DRAWINGS">FIGS. 1-4</figref> show respective structures and fabrication processes according to aspects of the present invention. More specifically, in <figref idref="DRAWINGS">FIG. 1</figref>, the structure <b>10</b> includes a BULK substrate <b>10</b>. In embodiments, the BULK substrate <b>12</b> is a silicon substrate which can be approximately 350 microns in thickness; although other dimensions are also contemplated by the present invention. A barrier layer <b>14</b> is formed on the substrate <b>12</b>. In embodiments, the barrier layer <b>14</b> can be a Silicon Nitride material, which is deposited using conventional deposition processes, e.g., chemical vapor deposition (CVD) process. Shallow trench isolation (STI) structures <b>16</b> are formed in the substrate <b>12</b>, through the barrier layer <b>14</b>.

0014In embodiments, the STI structures <b>16</b> can be formed from oxide, and fabricated using conventional photolithography, etching, deposition and polishing processes. For example, in embodiments, a photoresist can be formed on the barrier layer <b>14</b>, which is exposed to energy (e.g., light) in order to form a pattern. Through conventional etching processes, e.g., reactive ion etching (RIE), a corresponding pattern (vias) is formed in the substrate <b>12</b> and barrier layer <b>14</b>. The photoresist is then removed using conventional processes, e.g., oxygen ashing processes. An oxide or other insulator material is then deposited within the opening(s) and any residual material is removed from the surface of the barrier layer <b>14</b> using, e.g., a chemical mechanical process (CMP).

0015Still referring to <figref idref="DRAWINGS">FIG. 1</figref>, a photoresist <b>18</b> is formed on the barrier layer <b>14</b> and the STI structures <b>16</b>. The photoresist <b>18</b> is patterned to form an opening <b>18</b><i>a </i>between adjacent STI structure(s) <b>16</b>′. An ion implant process (as representatively shown by the arrows in <figref idref="DRAWINGS">FIG. 1</figref>) is then performed to create an amorphous layer <b>20</b> at a certain depth of the silicon layer <b>12</b>, abutting the STI structure(s) <b>16</b>′. The amorphous layer <b>20</b> is formed below the channel, e.g., active region, of transistors which will be formed in later fabrication processes. In embodiments, the implant process can be Ar or Ge or other species, e.g., Boron, which will form an amorphous layer <b>20</b> from the single crystalline substrate <b>12</b>, e.g., silicon substrate.

0016In embodiments, the amorphous layer <b>20</b> is bounded by the STI structure(s) <b>16</b>′ and has a depth of about 500 Å to about 2000 Å; although other depths are contemplated by the present invention as determined by the energy level of the ion implantation process. It should be understood by those of skill in the art that the depth of the amorphous layer <b>20</b> may be a function of the transistor, e.g., in order to provide sufficient space for a transistor channel, and based on the energy level of the ion implantation process. A typical amorphising dose for Ar or Ge will be about 2×10¹³to 1×10¹⁵ions per square cm. On the other hand, the dosage of the ion implantation process will determine the quality of the amorphous layer <b>20</b>. Both the dosage and energy level can be selected using known look-up tables.

0017In <figref idref="DRAWINGS">FIG. 2</figref>, a trench <b>22</b><i>a </i>and undercut region <b>22</b><i>b </i>is formed on sides of the STI structure(s) <b>16</b>′ and underneath amorphous layer <b>20</b>, respectively. In embodiments, the undercut portion <b>22</b><i>b </i>extends laterally below an active region of the yet to be formed transistor, and is of such a depth as to provide sufficient spacing to form an airgap under such transistor, e.g., about 0.5 microns to about 10 microns. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, in an alternative or optional embodiment, an amorphous layer <b>20</b><i>a </i>can be provided under the undercut region <b>22</b><i>b </i>by performing a second, higher energy ion implantation process that the formation of the amorphous layer <b>20</b>. In embodiments, the amorphous layer <b>20</b><i>a </i>is an optional structure which can be formed prior to or after the amorphous layer <b>20</b>.

0018To form the trench <b>22</b><i>a </i>and undercut region <b>22</b><i>b</i>, a photoresist <b>24</b> is formed on the barrier layer <b>14</b> and the STI structures <b>16</b> (<b>16</b>′). The photoresist <b>24</b> is patterned to form an opening <b>24</b><i>a</i>. A reactive ion etch process is then performed to remove the silicon nitride later and the silicon material, thereby forming the trench <b>22</b><i>a </i>and undercut region <b>22</b><i>b</i>. In embodiments, the undercut region <b>22</b><i>b </i>is formed under an active region, e.g., channel, of a yet to be formed transistor. In embodiments, the wet etch process uses a chemistry which is selective to silicon, e.g., which will not attack the oxide material of the STI structure or the amorphous layer <b>20</b> (or amorphous layer <b>20</b><i>a</i>, in optional embodiments). For example, the wet etch process can be performed using NH₄OH. In this way, the amorphous layer <b>20</b> and oxide of the STI structures will act as an etch-stop layer. In addition, the photoresist <b>24</b> will protect the top portion of the wafer, e.g., nitride layer <b>14</b>.

0019In <figref idref="DRAWINGS">FIG. 3</figref>, the photoresist is removed and the structure is subjected to an oxidation process to form a passivated surface <b>26</b>. In embodiments, the passivated surface <b>26</b> is an oxidized surface of the substrate <b>12</b>, e.g., surface of the undercut region <b>22</b><i>b</i>, and of the opposing amorphous layer <b>20</b>.

0020In preferred embodiments, the passivated surface <b>26</b> is formed by a growth process using an annealing process. For example, the structure of <figref idref="DRAWINGS">FIG. 3</figref> can be subjected to a high temperature anneal process at about 800° C. to about 900° C. In alternate embodiments, the structure of <figref idref="DRAWINGS">FIG. 3</figref> can be subjected to a rapid thermal anneal process to form the passivated surface <b>26</b>.

0021In <figref idref="DRAWINGS">FIG. 4</figref>, the trench <b>22</b><i>a </i>is closed to form an airgap <b>30</b>. In embodiments, the trench is closed by the deposition of a material <b>28</b>, e.g., polysilicon material. After deposition of the material <b>28</b>, any residual material on the surface of the structure can be removed by a CMP process. A conventional transistor (FET) <b>32</b> is then formed over the airgap <b>30</b> using conventional deposition, lithography, etching and source/drain formation (diffusion regions on sides of a channel) processes, already known to those of skill in the art. In this way, the transistor <b>32</b> is completely isolated, e.g., oxide isolated, from portions (undoped portions) of the substrate by the STI structures and passivated surface <b>26</b>, with an underlying airgap. The location of the airgap <b>30</b> is directly under and in contact with the transistor channel and source and drain regions (diffusion regions), shown representatively at reference numeral <b>34</b>. The diffusion regions <b>34</b> are thus electrically isolated from the silicon substrate <b>12</b>.

0022In alternative embodiments, additional implants can be performed at multiple energies to set a perimeter of amorphous material which will limit the extent of the undercut etch outside of trenches <b>22</b><i>a. </i>

0023<figref idref="DRAWINGS">FIGS. 5-12</figref> show respective structures and fabrication processes according to additional aspects of the present invention. In <figref idref="DRAWINGS">FIG. 5</figref>, the structure <b>10</b>′ includes a pad oxide layer <b>50</b> formed on the substrate <b>12</b>. In embodiments, the pad oxide layer <b>50</b> can have a thickness of about 80 Å; although other dimensions are also contemplated by the present invention. A pad nitride layer <b>52</b> is formed on the pad oxide layer <b>50</b>, which can have a thickness of about 1700 Å; although other dimensions are also contemplated by the present invention. An oxide hard mask <b>54</b> is formed on the pad nitride layer <b>52</b>, which can have a thickness of about 4500 Å; although other dimensions are also contemplated by the present invention. In embodiments, the layers <b>50</b>, <b>52</b>, <b>54</b> can be other materials, any of which are formed using conventional deposition processes, e.g., CVD, followed by a planarization process, as appropriate, e.g., CMP, as should be understood by those of skill in the art.

0024In <figref idref="DRAWINGS">FIG. 6</figref>, a photoresist <b>56</b> is formed on the oxide hard mask <b>54</b>, which is patterned by exposure to energy (e.g., light). Opening(s) <b>58</b> are then formed in the layers <b>52</b>, <b>54</b>, <b>56</b>, through the pattern, using conventional etching processes, e.g., RIE.

0025Thereafter, with an appropriate chemistry, deep trenches <b>60</b> are formed in the substrate <b>12</b>, as shown in <figref idref="DRAWINGS">FIG. 7</figref>. In embodiments, the deep trenches <b>60</b> can be about 5000 Å to about 25000 Å in depth; although other depths are also contemplated by the present invention. For example, the depth of the deep trenches <b>60</b> are provided deep enough to isolate a channel layer (formed in the substrate) of a transistor.

0026In the case of using the deep trenches <b>60</b>, continuing with <figref idref="DRAWINGS">FIG. 8</figref>, the resist is removed and sidewall structure(s) <b>62</b> are formed in the trenches <b>60</b>. In embodiments, the sidewall structure(s) <b>62</b> can be an oxide material. More specifically, in embodiments, the sidewall structure(s) <b>62</b> can be formed by an oxide deposition followed by a TEOS (Tetraethyl Orthosilicate) deposition process. In alternative embodiments, the sidewall structure(s) <b>62</b> can be formed using eDRAM collar deposition processes. In embodiments, the wall thickness of the sidewall structure(s) <b>62</b> is dependent on the dimensions of the deep trench <b>60</b>; that is, the wall thickness of the sidewall structure(s) <b>62</b> should not pinch off the deep trench <b>60</b>. The oxide process can be followed by an annealing process.

0027As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the material of the sidewall structure(s) <b>62</b> at the bottom of the deep trench <b>60</b> is removed by an etching process. In embodiments, this etching process will also remove the sidewall material from a top surface of the structure, e.g., layer <b>54</b>. In embodiments, the etching process is an anisotropic etching process, in order to remove the material on horizontal surfaces, leaving the sidewalls on the vertical portions on the deep trench(es).

0028In <figref idref="DRAWINGS">FIG. 10</figref>, an etching or venting process is performed to form a lateral undercut <b>64</b>, removing material under an active region, e.g., channel of a transistor. In embodiments, the etching process is a XeF chemistry.

0029In <figref idref="DRAWINGS">FIG. 11</figref>, the trench <b>60</b> is closed to form an airgap <b>66</b>. In embodiments, the trench is closed by the deposition of a material <b>68</b>, e.g., polysilicon material. After deposition of the material <b>68</b>, any residual material on the surface of the structure can be removed by a CMP process. A conventional transistor (FET) <b>32</b> is then formed over the airgap <b>66</b> using conventional deposition, lithography, etching and source/drain formation processes, already known to those of skill in the art. In this way, the transistor <b>32</b> is completely isolated.

0030<figref idref="DRAWINGS">FIG. 12</figref> shows a top view of <figref idref="DRAWINGS">FIG. 11</figref>. As shown in this view, the filled trenches, e.g., material <b>66</b>, surround the transistor <b>32</b> such that the transistor <b>32</b> is completely isolated, with an underlying airgap. The location of the airgap <b>30</b> is directly under and in contact with the transistor channel and source and drain regions.

0031The method(s) as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor.

0032The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.