Method for forming SOI film by laser annealing

A method for making a ULSI MOSFET using silicon on insulator (SOI) principles includes masking regions of an amorphous silicon film on a substrate and exposing intended active regions. Laser energy is directed against the intended active regions to anneal these regions without annealing the masked regions, thereby increasing production throughput and decreasing defect density.

TECHNICAL FIELD
 The present invention relates generally to semiconductor fabrication, and
 more particularly to methods for fabricating improved ultra-large scale
 integration (ULSI) semiconductor devices such as ULSI metal oxide silicon
 field effect transistors (MOSFETs).
 BACKGROUND OF THE INVENTION
 Semiconductor chips are used in many applications, including as processor
 chips for computers, and as integrated circuits and as flash memory for
 hand held computing devices, wireless telephones, and digital cameras.
 Regardless of the application, it is desirable that a semiconductor chip
 hold as many circuits or memory cells as possible per unit area. In this
 way, the size, weight, and energy consumption of devices that use
 semiconductor chips advantageously is minimized, while nevertheless
 improving the memory capacity and computing power of the devices.
 One general method for making semiconductor chips is referred to as the
 "bulk" CMOS method, wherein well implants are formed in a bulk silicon
 substrate to promote subsequent proper functioning of the chip, and then
 transistor stacks are formed on the substrate. A newer chip making method
 referred to as "silicon on insulator" or "SOI" has also been introduced
 which does not require the formation of wells in the substrate, and which
 provides for faster transistor switching speed, improved resistance to
 soft error and latch-up, and higher transistor density. Moreover, SOI
 chips advantageously consume less power when inactive compared to bulk
 CMOS chips.
 As recognized by the present invention, however, the SOI process implicates
 complications, including the implantation of high doses of oxygen into the
 substrate. As understood herein, the high dose of oxygen that is required
 can lead to a relatively high defect rate in the SOI film, consequently
 requiring high temperature annealing for prolonged periods to alleviate
 the defects. Unfortunately, this in turn makes it difficult to precisely
 control the SOI film thickness, which is undesirable because a uniform SOI
 film thickness promotes optimal chip functioning. Also, because of the
 prolonged annealing, manufacturing throughput is lower than might be
 desired. Fortunately, the present invention has recognized the above
 problems and has provided the solutions herein.
 BRIEF SUMMARY OF THE INVENTION
 A method is disclosed for forming a silicon on insulator (SOI) device. The
 method includes depositing an amorphous silicon film on a substrate, and
 establishing protective stacks in the film. Active region windows are
 established over first regions of the film between protective stacks.
 Laser energy is then directed through the active region windows against
 the first regions to anneal the first regions. As disclosed further below,
 the first regions establish active SOI regions after annealing and
 cooling.
 In one preferred embodiment, the protective stacks are made of an oxide. On
 the other hand, the substrate is made of a material selected from the
 group including: sapphire, silicon oxide, and silicon nitride.
 The preferred method of establishing the active region windows includes
 masking the first regions of the film to establish stack windows over
 second regions of the film. The second regions of the film are then
 removed, and an oxide material is deposited to fill the stack windows.
 Next, the first regions are unmasked to establish the active region
 windows, prior to laser annealing.
 In another aspect, an SOI semiconductor device includes a substrate and
 active regions of recrystallized silicon on the substrate disposed between
 inactive regions of oxide.
 In yet another aspect, a method for making an SOI device includes providing
 a substrate, and depositing an amorphous silicon film on the substrate.
 Moreover, the method includes masking intended active regions of the film.
 Also, the method contemplates removing unmasked regions of the film to
 establish stack windows, and then depositing an oxide in the stack
 windows. The intended active regions are unmasked and activated using
 laser annealing followed by cooling.
 In a preferred embodiment, the method includes heating the intended active
 regions to at least nine hundred degrees Celsius (900.degree. C.). Indeed,
 the intended active regions can be heated to at least nine hundred fifty
 degrees Celsius (950.degree. C.). Preferably, the activating act is
 accomplished by pulsing a laser beam against the intended active regions
 to melt the regions.
 Other features of the present invention are disclosed or apparent in the
 section entitled "DETAILED DESCRIPTION OF THE INVENTION".

DETAILED DESCRIPTION OF THE INVENTION
 Referring initially to FIG. 1, a semiconductor device embodied as a chip 10
 is shown incorporated into a digital processing apparatus such as a
 computer 12. The chip 10 is made in accordance with the below disclosure.
 Now referring to FIGS. 2 and 3, as indicated at block 14 in FIG. 2 and as
 shown in FIG. 3, to make the device 10 an amorphous silicon
 (".alpha.-silicon") film 16 is deposited on a substrate 18 using
 appropriate deposition principles, e.g., low pressure chemical vapor
 deposition (LPCVD). The thickness "t" of the film 16 can be from one
 hundred Angstroms to five hundred Angstroms (100 .ANG.-500 .ANG.) or more.
 In any case, the substrate 18 has a melting temperature that is higher
 than the melting temperature of the .alpha.-silicon film 16. In one
 preferred embodiment, the substrate 18 can be made of sapphire, silicon
 oxide, or silicon nitride, or other appropriate, relatively high melting
 point substance.
 Moving to block 20 of FIG. 2 and referring to FIGS. 3 and 4, a protective
 cap layer 22 is deposited onto the .alpha.-silicon film 16. The cap layer
 22 can be made of silicon nitride and can have a thickness "tt" of, e.g.,
 three hundred Angstroms to five hundred Angstroms (300 .ANG.-500 .ANG.).
 As also indicated at block 20, a photoresist layer 24 is deposited over
 the cap layer 22, and then as indicated at block 26 the photoresist layer
 24 is patterned as shown in FIG. 4 by, e.g., exposing the photoresist
 layer 24 to ultraviolet light to expose regions of the cap layer 22. These
 exposed regions of the cap layer 22 are removed at block 28 in FIG. 2 by,
 e.g., anisotropical plasma etching to define stack windows 30 over
 unmasked regions 32 of the .alpha.-silicon film 16. It is to be understood
 that the unmasked regions 32 of the .alpha.-silicon film 16 overlay
 intended field regions 34 of the substrate 18, as shown best in FIG. 4.
 After establishing the stack windows 30, the process moves to block 36 in
 FIG. 2 to remove the photoresist layer 24. Next, at block 38, the unmasked
 regions 32 (FIG. 4) of the .alpha.-silicon film 16 that overlay the
 intended field regions 34 of the substrate 18 are removed to render the
 configuration shown in FIG. 5. The unmasked regions 32 (FIG. 4) of the
 .alpha.-silicon film 16 can be removed by, e.g., wet or dry etching.
 Proceeding to block 40 of FIG. 2 and now referring to FIG. 6, a layer 42
 preferably made of TEOS oxide is deposited over the substrate 18 as shown
 and polished down to the nitride cap layer 22 as shown in FIG. 7 to
 establish protective stacks 44 over the intended field regions 34. The
 layer 42 of TEOS can be polished down to the cap layer 22, which acts as a
 polish stop, using chemical mechanical polishing (CMP) principles.
 Next moving to block 46 and referring to FIG. 8, the remaining areas of the
 cap layer 22 are removed to form active region windows 47 that expose
 intended active regions 48 of the .alpha.-silicon film 16. As shown, the
 intended active regions 48 alternate with the protective oxide stacks 44.
 In accordance with the present invention, the intended active regions 48
 are annealed at block 50 of FIG. 2. In one intended embodiment, the
 regions 48 are annealed by directing laser energy, represented by arrows
 52 in FIG. 8, through the active region windows 47, against the regions
 48. Thus, only the intended active regions 48 are annealed by the laser
 energy, with the intended field regions 34 of the substrate 18 being
 effectively shielded by the oxide stacks 44.
 The laser energy preferably is an excimer laser beam that is pulsed at a
 period of a few nanoseconds to achieve a temperature in the exposed
 intended active regions 48 of at least nine hundred degrees Celsius, and
 as high as nine hundred fifty degrees Celsius or more. In this way, the
 .alpha.-silicon of the intended active regions 48 is melted, with the
 oxide stacks 44 masking the intended field regions 34 of the substrate 18
 from the laser energy. The regions 48 are then cooled to room temperature
 to permit the silicon to recrystallize, establishing active regions of the
 device 10. During recrystallization, defects in the silicon are
 effectively removed.
 While the particular METHOD FOR FORMING SOI FILM BY LASER ANNEALING as
 herein shown and described in detail is fully capable of attaining the
 above-described objects of the invention, it is to be understood that it
 is the presently preferred embodiment of the present invention and is thus
 representative of the subject matter which is broadly contemplated by the
 present invention, that the scope of the present invention fully
 encompasses other embodiments which may become obvious to those skilled in
 the art, and that the scope of the present invention is accordingly to be
 limited by nothing other than the appended claims, in which reference to
 an element in the singular is not intended to mean "one and only one"
 unless explicitly so stated, but rather "one or more". All structural and
 functional equivalents to the elements of the above-described preferred
 embodiment that are known to those of ordinary skill in the art are
 expressly incorporated herein by reference and are intended to be
 encompassed by the present claims. Moreover, it is not necessary for a
 device or method to address each and every problem sought to be solved by
 the present invention, for it to be encompassed by the present claims.