Method for fabricating metal oxide semiconductor field effect transistor (MOSFET)

[0002] The present invention is related to a method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) and more particularly to a method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) on a gallium arsenide (GaAs) substrate.

[0003] For several ten years, the field effect transistor (FET) has been significantly developed in the application of semiconductor device. Due to its low-power dissipation and highly integrated technique, it plays the most important role in the field of very-large-scale integration (VLSI), particularly in ultra-large-scale integration (ULSI). There are many kinds of FET design, in which the main differences are in the gate and channel structures. In the fabrication of an integrated circuit using silicon as a material, a metal oxide semiconductor FET (MOSFET) has the properties of high yield rate and excellent reliability in addition to the above-described advantages of FET. Therefore, MOSFET has become the commonly used device in recent years. Certainly, there are some other designs for FET, such as a metal-insulator-semiconductor (MOSFET), an insulator-gate FET (IGFET), a junction-gate FET (JFET) using reversely biased p-n junction as a gate, a metal-semiconductor FET (MOSFET) using the metal-semiconductor schottky contact as a gate, and a modulation-doped FET (MOSFET).

[0004] In the fabrication of an integrated circuit using a GaAs material, although the electron mobility of GaAs is several times higher than that of silicon, the former is not so popular as the latter when used in the semiconductor industry. Except for its relatively expensive cost, the reason is that GaAs is unstable at a temperature greater than 650° C. so that a high-quality oxide layer can not be fabricated in the high-temperature tube. The oxide layer with bad reliability and low interface density will bring about a difficulty in the fabrication of enhancement-mode MOS and limit the design and operation of many other FET structures. Therefore, it is desirable to overcome the problem encountered by the application of GaAs material.

[0005] In our previous studies (please see Hwei-Heng Wang et al., Japanese J. of Applied Physics, Pt. 2 (Letters) Vol. 37, No. 1 AB, pp. L-67-L70, 1998), we proposed a method for selectively forming an oxide layer on a portion uncovered by the photoresist or noble netal layer near the room temperature (40˜70° C.). Thus, how to combine the above-described processes for manufacturing the field effect transistors with this method and use GaAs as a substrate of metal oxide semiconductor FET (MOSFET) is the developing purpose of the invention.

[0006] An object of the present invention is to provide a method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) on a gallium arsenide (GaAs) substrate.

[0007] In the first preferred embodiment of the present invention, the method includes the steps of (a) providing a substrate; (b) forming an epitaxial layer on the substrate; (c) defining and forming a drain and a source on a portion of the epitaxial layer; (d) forming a recess in another portion of the epitaxial layer; (e) forming an oxide layer on a surface of the recess by relatively low-temperature oxidation; and (f) forming a gate on a portion of the oxide layer between the drain and source.

[0008] Preferably, the substrate is a semi-insulating GaAs substrate.

[0009] The epitaxial layer includes a buffer layer, a moderately doped layer, and a heavily doped layer. The buffer layer is an undoped layer and has a thickness of about 5000 Å. The moderately doped layer has a thickness ranging from 2000 Å to 3000 Å, a concentration of 10¹⁶˜10¹⁷cm^{31 3}, and serves as a channel. The heavily doped layer has a thickness of 1000 Å, a concentration greater than 10¹⁸cm⁻³, and serves as an ohmic contact for the source and drain.

[0010] After the step (b), the method further includes a step (b1) of immersing the wafer in 2-propanol, acetone, and methanol in sequence and cleaning the wafer by an ultrasonic oscillator. Thereafter, a lithographic process is performed to etch the epitaxial layer for giving a mesa isolation, in which the mesa deepens to a place lower than the buffer layer to isolate all formed devices on the wafer, and a height of the mesa ranges from 6000 Å to 8000 Å. The mesa isolation is achieved by using an etching solution containing HF, H₂O₂, and H₂O (HF: H₂O₂: H₂O=1:2:25 by volume) for giving rounded edges to the mesa.

[0011] Certainly, the step (c) further includes steps of: (c1) coating a photoresist on the epitaxial layer; (c2) performing a lithographic process to pattern the photoresist for defining and forming the drain and source on the portion of the epitaxial layer; (c3) coating an alloy film on the drain and source; (c4) using a lift-off procedure to leave a drain/source metal in place; and (c5) executing an annealing process to accomplish a metallization process of the drain and source. In the step (c3), the alloy film is deposited by e-beam evaporation using Au/Ge/Ni or Au/Be. In the step (c5), the annealing process is performed at 450˜600° C. for 10˜25 seconds.

[0012] In the step (d), the epitaxial layer is partially etched by using an etching solution containing NH₃, H₂O₂, and H₂O (NH₃: H₂O₂: H₂O=3:1:50 by volume) and using the drain/source metal as a mask to form the recess in the another portion of the epitaxial layer.

[0013] In the step (e), a formed thickness of the oxide layer is controlled by using 1.15M nitric acid solution containing 0.36 mole of Ga ion whose initial pH value is adjusted to 4.3 by 2.5% ammonium hydroxide hydroxide (NH₄OH) solution. During the formation of the oxide layer, an ammonium hydroxide solution, prepared from diluting 28% ammonium hydroxide solution with de-ionized water to 300 folds, is added therein for adjusting the pH value within a range between 4.0 and 5.0 to control the growth rate and refractive index of the formed oxide layer.

[0014] The gate is formed by steps of coating a photoresist on the fabricated wafer, performing a lithographic process to pattern the photoresist for defining a gate region on the portion of the oxide layer, coating a conducting layer with a relatively high conductivity on the defined region for the gate, and using a lift-off procedure to form the gate on the portion of the oxide layer. Preferably, the conducting layer can be made of gold, aluminum, or polysilicon.

[0015] In the second preferred embodiment of the present invention, the step (e) further includes a step (e1) of performing a synchronic sulfurated passivation process by adding a processed (NH₄)₂S solution, prepared from adding sulfur powder to (NH₄)₂S solution, stirring, and filtering out the undissolved sulfur powder, to a system solution for forming the oxide layer at any time during the formation of the oxide layer.

[0016] In the third preferred embodiment of the present invention, after the step (e1), the method further includes a step (e2) of performing a rapid thermal annealing (RTA) process, wherein the wafer is heat-treated and irradiated for several to several ten seconds. Certainly, after performing the rapid thermal annealing process, the method further includes a step of immersing the wafer in 2-propanol, acetone, and methanol in sequence and cleaning the fabricated wafer by an ultrasonic oscillator to remove an impurity deposited on the fabricated wafer during the rapid thermal annealing process.

[0017] In the fourth preferred embodiment of the present invention, the method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) includes the steps of: (a) providing a substrate; (b) forming an epitaxial layer on the substrate; (c) forming a recess in a first portion of the epitaxial layer; (d) forming an oxide layer over a surface of the recess and a second portion of the epitaxial layer by relatively low-temperature oxidation; (e) defining and forming a drain and a source on a third portion of the epitaxial layer; and (f) forming a gate on a portion of the oxide layer in the recess and between the drain and source.

[0018] The present invention may best be understood through the following description with reference to the accompanying drawings, in which:

[0019] FIG. 1 is a flowchart showing the processes for fabricating a metal oxide semiconductor field effect transistor (MOSFET) according to the present invention;

[0020] FIGS. 2a˜2fare schematic diagrams showing a preferred embodiment of the method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) according to the present invention;

[0021] FIG. 3 is a schematic diagram showing a system for growing an oxide layer on a wafer according to the present invention;

[0022] FIGS. 4a˜4gare schematic diagrams showing another embodiment of the method for fabricating a metal oxide semiconductor field effect transistor (MOSFET) according to the present invention; and

[0023] FIG. 5 is a graph showing the current-voltage characteristic of the drain and source of the MOSFET fabricated by the method of the present invention.

[0024] The present invention will now be described more detailedly with reference to the following embodiments. It is to be noted that the following descriptions of the preferred embodiments of this invention are presented herein for the purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.

[0025] The present invention provides a method for fabricating a metal oxide semiconductor field effect transistor (MOSFET). Please refer to FIG. 1 which is a flowchart illustrating the embodiments of the method of the present invention. According to the different selection of the process, there are four preferred embodiments described in detail as follows.

[0026] This process can be divided into several steps below:

[0027] A. Growth of Epitaxial Layer

[0028] The epitaxial layer includes an undoped buffer layer 2, a moderately doped layer 3, and a heavily doped layer 4. The undoped buffer layer 2 with a thickness of ˜5000 Å, the moderately doped (10¹⁶-10¹⁷cm⁻³) layer 3 having a thickness of 2000˜3000 Å (acting as a channel), and the 1000 Å thick heavily doped (>10¹⁸cm⁻³) layer 4 (serving as drain/source ohmic contact) are formed on a semi-insulating GaAs substrate 1 in sequence by molecular beam epitaxy or organometallic vapor-phase epitaxy along the (100) direction, as shown in FIG. 2a.

[0029] B. Cleaning the Wafer

[0030] The fabricated wafer of Step A is immersed in 2-propanol, acetone, and methanol in sequence and cleaned by an ultrasonic oscillator.

[0031] C. Mesa Isolation

[0032] A photoresist is coated on the cleaned wafer and patterned by lithography. Then, the epitaxial layer not covered by the photoresist is etched by an etching solution (HF:H₂O₂:H₂O=1:2:25) to give a mesa isolation as shown in FIG. 2b. The used etching solution has a naturally rounded etching ability so that this is advantageous for the distribution of metal lines after mesa insolation. The mesa deepens to a place lower than the buffer layer 2 to ensure the isolation between the formed devices. The height of mesa can range from 6000 Å to 8000 Å

[0033] D. Formation and Metallization of Drain and Source

[0034] A photoresist is coated on the fabricated wafer of Step C and is patterned by lithography to define and form the drain and source. Thereafter, an alloy film is deposited thereon by e-beam evaporation (using Au/Ge/Ni for n+-type heavily doped layer; or using Au/Be for p+-type heavily doped layer) and then a lift-off process is performed to leave a drain/source metal in place. Finally, the wafer is put in the tube and annealed at 450˜600° C. for 10˜25 seconds to accomplish the metallization of the drain and source 5 as shown in FIG. 2c.

[0035] E. Cleaning the Wafer

[0036] The fabricated wafer of Step D is immersed in 2-propanol, acetone, and methanol in sequence and cleaned by an ultrasonic oscillator to remove the impurities deposited on the wafer during the annealing process.

[0037] F. Recess Etch

[0038] The wafer after treated by Step E is etched by an etching solution (NH₃:H₂O₂:H₂O=3:1:50) to form a recess in the channel 3 by self alignment, wherein at least the heavily doped layer 4 must be completely removed, as shown in FIG. 2d. Because Au/Ge/Ni or Au/Be alloy film can not be etched by the etching solution, the drain/source alloy film fabricated in Step D can be used as an etch mask.

[0039] G. Room-Temperature Selective Oxidation

[0040] Because the oxide layer 6 will be selectively grown on the portion of the epitaxial layer not covered by the alloy film by low-temperature liquid phase oxidation, there is no need to use the lithographic process. The fabricated wafer is shown in FIG. 2e. Preferably, the oxide layer is formed at 40˜70° C. The growth thickness of the oxide layer 6 can be controlled according to the optimized parameters. The formed thickness of the oxide layer is controlled by using 1.15M nitric acid solution containing 0.36 mole of Ga ion whose initial pH value is adjusted to 4.3 by 2.5% ammonium hydroxide solution. Furthermore, during the formation of the oxide layer, an ammonium hydroxide (NH₄OH) solution, prepared from diluting commercially available 28% ammonium hydroxide solution with deionized water to 300 folds, is added therein for adjusting the pH value within a range between 4.0 and 5.0. The growth system of oxide layer is shown in FIG. 3. Although the prior system proposed in Japanese Journal of Applied Physics has a pH measurement, the pH value is used as an initial monitoring value. According to the present invention, a pH-control mechanism is used to control the growth rate and refractive index of the oxide layer. The magnetic stirrer can control the pH value and increase the uniformity of the oxide layer. The thermostatic tank is used for obtaining an even temperature distribution.

[0041] H. Defining and Forming a Gate

[0042] A photoresist is coated on the fabricated wafer of Step G and patterned to define a region on a portion of the oxide layer for forming a gate. Then, a conductive layer with a relatively high conductivity, such as gold, aluminum, or polysilicon, is coated thereon by a plating process and a lift-off process is performed to accomplish the formation of the gate 7. Finally, a novel MOSFET fabricated by low-temperature liquid phase oxidation for III-V group is shown in FIG. 2f.

[0043] The difference between this embodiment and the first preferred embodiment is that the former further includes the step of synchronic sulfurated passivation. This step is a selective procedure. So-called “synchronic” means that this step and the above room-temperature selective oxidation are performed at the same time. First of all, S₈powder is added to (NH₄)₂S solution and the mixture is completely stirred at 60˜70° C. The undissolvable S₈powder is filtered out. Thereafter, an appropriate amount of the processed (NH₄)₂S solution is added to the oxidation solution for growing the oxide layer at any time during the formation of the oxide layer.

[0044] After the synchronic sulfurated passivation process, this embodiment further includes a Rapid thermal annealing (RTA) process. This process is also a selective procedure. The fabricated wafer is put into the RTA tube and is irradiated by a suitable light source. Finally, the wafer is quickly heat-treated for several to several ten seconds. Then, the fabricated wafer is immersed in 2-propanol, acetone, and methanol in sequence and cleaned by an ultrasonic oscillator to remove the impuritied deposited on the wafer during the RTA treatment.

[0045] The procedure of this embodiment is slightly different from those of the above-described embodiments. The differences are described as follows and its overall process is illustrated in FIGS. 4a˜4g. All symbols of the formed devices are identical to those shown in FIGS. 2a˜2f. After the growth of epitaxial layer and the mesa isolation, the fabricated wafer is shown in FIG. 4b. Then, the fabricated wafer is coated by a positive photoresist (or coating a negative photoresist and using the same photoresist for defining drain and source) and a lithographic process and a dry or wet etching are performed to form a recess in the epitaxial layer as shown in FIG. 4c. Subsequently, a room-temperature selective oxidation is performed to form an oxide layer over the surfaces of the recess and the epitaxial layer as shown in FIG. 4d. A positive photoresist is coated on the fabricated wafer and then a lithographic process is performed for defining the drain and source regions. Thereafter, a dry or wet etching is performed to partially remove the oxide layer and expose a portion of the epitaxial layer to obtain a structure shown in FIG. 4e. At this time, there is no need to remove the photoresist. Subsequently, the metal plating process for the formation and metallization of the drain and source and other steps which are identical to those in the first embodiment are performed to accomplish the device shown in FIGS. 4fand 4g.

[0046] Comparing the conventional MOSFET with the depletion-mode MOSFET fabricated by the method of the present invention, the experimental results show that the transconductance of MOSFET is about 60 mS/mm under the same condition (the impurity concentration of the channel is 5×10¹⁶cm⁻³and W/L=40/2). To date, the characteristics of MOSFET have been improved and the Ids-Vds characteristic of MOSFET is shown in FIG. 5. The device property of MOSFET is superior to that of MOSFET. For the MOSFET fabricated by the method of the present invention, there are a lot of space in the development and application. Relative studies of the MOSFET can be found in our previous report (IEEE Electron Device Letters V. 20, n1, P. 18).

[0047] In conclusion, the overall manufacturing processes of FET proposed by the present invention only need three-time photolithographic process so as to simplify the manufacturing process. Moreover, all steps after Step D (that is, the formation of drain and source and metallization) can be operated near room temperature (40˜70° C.) so that the influence of heat treatment on the quality of the GaAs epitaxial layer can be eliminated, especially in the channel under the recess. Finally, combined with the insulating oxide layer, the fabrication of gate on the channel does not need the Schottky contact. Therefore, there is a lot of elastic selections in the types of fabricated devices. For example, there are several selections for the layers 3 and 4 described as follows:

[0048] (1)N-type layer 3 incorporated with n+-type layer 4 is suitable for fabricating a depletion-mode n-MOSFET;

[0049] (2)P-type layer 3 incorporated with n+-type layer 4 is suitable for fabricating an enhancement-mode n-MOSFET;

[0050] (3)P-type layer 3 incorporated with p+-type layer 4 is suitable for fabricating a depletion-mode p-MOSFET; and

[0051] (4)N-type layer 3 incorporated with p+-type layer 4 is suitable for fabricating an enhancement-mode n-MOSFET.

[0052] To sum up the above discussion, the present invention discloses a method for manufacturing a metal oxide semiconductor FET, which utilizes a low-temperature liquid phase oxidation for Ill-V group. This method can be operated near room temperature (40˜70° C.) and will not cause a damage in the photoresist layer so that the noble metal or photoresist can be used as a stable mask for the selective growth of the oxide layer. At the same time, because the synchronic sulfurated passivation process and the growth of the oxide layer can be simultaneously performed together, the interface level density can be controlled for enhancing the device's property. Furthermore, the rapid thermal annealing (RTA) applied in this method can increase the reliability of the fabricated device.

[0053] While the invention has been described in terms of what are presently considered to be the most practical and preferred embodiments, it is to be understood that the invention need not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.