JPS5893243A - Improvement of polysilicon thin film semiconductor - Google Patents

Abstract

PURPOSE:To improve electrical conductivity of polycrystal by annealing a poly crystalline Si film deposited on a substrate with laser or electron beam and diffusing hydrogen into the polycrystalline grain boundary using the activated atoms or molecules of hydrogen excited by a micro wave under the particular conditions of reaction temperature and pressure. CONSTITUTION:An ordinary polycrystalline Si thin film deposited on a substrate using monosilane is annealed by the laser or electron beam in order to make large the crystal grain size, the activated atoms or molecules of hydrogen activated by the microwave is diffused deeply into the Si in order to activate the grain boundary and a barrier height of grain boundary is reduced. At this time, the hydrogen activating treatment is carried out at the reaction temperature of 200-500 deg.C and pressure of 0.2-2.0 Torr. Thereby, electrical characteristic of polycrystalline Si thin film is improved and a semiconductor substrate having excellent quality for industrial use can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of annealing polycrystalline silicon to grow crystal grains and then diffusing activated hydrogen excited by microwaves to improve the characteristics of a semiconductor device.

When forming a thin film transistor on an insulating film substrate, there is a device using polycrystalline silicon (hereinafter referred to as polycrystalline silicon). Polysilicon has superior reliability compared to other thin film materials, but when using polysilicon as deposited to form a thin film I-transistor, the threshold voltage is large and the field effect mobility is small. Sufficient characteristics cannot be obtained. The reason for this is that the density of defect levels is 1018 cm in grain boundaries at the boundaries of silicon, which is an aggregate of microcrystals.
Therefore, the defects on the surface and depletion layer move along the Fermi, and the gate voltage fills the charge in the non-conducting region rather than forming an inversion layer or storage M layer. Even after channel formation, gate voltage interacts with defects and reduces conductance. Potential barriers at grain boundaries impair conductivity and reduce field-effect mobility.

For this reason, countermeasures are being considered to improve the grain boundaries of polysilicon. The main methods include a method of annealing using lasers, electron beams, etc. to grow crystal grains and reducing the number of grain boundaries per unit volume, and a method of reducing the localized level density of grain boundaries by hydrogen plasma treatment. There is.

In conventional plasma hydrogen treatment methods, the hydrogen diffusion layer is less than a few microns, the reaction layer is tingling, and no growth in crystal grain size can be expected. The mobility of thin film transistors is also 1~
10Cm2/v-8, and in this plasma hydrogen treatment method, the grain size is about 50A, and the potential barrier at the grain boundary depends on the impurity concentration, but for example,
In the case of QCm, 0.5 eV decreases to 0.3 eV. Next, when polysilicon is improved by an annealing process using a laser or the like, it depends on the laser output, but in general, higher output tends to improve the properties of grain boundaries where crystal grains are not large. However, as far as we can see from the temperature dependence of the characteristics of polysilicon thin film transistors subjected to laser annealing, it is possible to reduce the growth of crystal grains, lattice misalignment at grain boundaries, neutral point defects, charged point defects, and improve the potential barrier. However, the grain boundary scattering effect remains even after laser annealing, indicating that the material has a higher density of point defects than single-crystal silicon. A large number of these point defects exist inside polysilicon, and it has been found that the properties can be further improved by applying plasma hydrogen treatment to reduce this density. However, with conventional plasma hydrogen treatment methods, hydrogen sales are only achieved in the surface layer, and the effect is still not sufficient.

In view of the above points, it is an object of the present invention to provide a method for effectively improving the electrical characteristics of polysilicon thin films.
.

In the present invention, ordinary polysilicon formed from monosilane is annealed with a laser, an electron beam, etc. to increase the crystal grain size, and then hydrogen activated atoms excited by microwaves are added to the polysilicon. It is characterized by deep diffusion to activate grain boundaries, reduce barrier hydride at grain boundaries, and improve the electrical characteristics of polysilicon thin films.

A further feature of the present invention is that in the hydrogen activation treatment of polysilicon thin films, the reaction temperature and pressure are optimized, or hydrogen atoms excited by microwaves are introduced into the reactor to reduce the particle size. The purpose is to activate hydrogen through expansion and hydrogen segregation at grain boundaries.

In the present invention, a polysilicon film is deposited and formed to a desired thickness on a substrate such as silicon or glass. Polysilicon made from ordinary monosilane has a very small grain size of 50A or less, but it is annealed. Annealing uses an energy beam that causes lattice vibrations in the crystal.
Any of ordinary heat, laser, electron beam, etc. may be used.

After the grain size has grown sufficiently, hydrogen atoms excited by microwaves are diffused into the crystal.

Hydrogen mainly diffuses and segregates within the grain boundaries and contributes to improving the electrical properties of the grain boundaries. The reason for diffusing hydrogen after annealing is that annealing increases the crystal grain size, expands the grain size and activates the grain boundaries without losing hydrogen at the grain boundaries, and the synergistic effect of these increases the efficiency of conventional laser annealing. A reduction in the grain boundary potential barrier 11a and a reduction in defect density, which could not be obtained in the conventional method, can be expected. In addition, in ordinary plasma hydrogen treatment, the penetration depth of hydrogen is several microns or less, but due to this synergistic effect, the reaction temperature #200-500C1 pressure 0.2-2.
When the pressure is set to 0 Torr, the penetration depth of hydrogen is 0.
We found that it can be as high as 5 flM. The reason why the reaction temperature is set within the above range is that at temperatures below 200C, diffusion is insufficient, and at temperatures above 500C, hydrogen is blown off and cannot be introduced. Also, the pressure is 0.2 TOr
Below r, the diffusion effect is small, and above 2.0 TOrr, the discharge effect is small. Regarding the method for hydrogenating polysilicon, introducing microwave-excited atomic hydration in front of the reactor can improve point defects existing inside laser-annealed polysilicon. This plasma treatment may be performed after gate oxidation during device fabrication.

Next, details of the present invention will be explained with reference to examples and drawings. A 400 μm thick silicon substrate with a specific resistance of 2 Ωcm is used, and an oxide film of about 300 OA is formed on it. Polysilicon (thickness) made from silane, a
m) was deposited on this oxide film at 630C. Using this substrate, after annealing using thermal laser, electronic bonding, etc., the following various hydrogen plasma treatments were performed, and the electrical characteristics, diffusion depth, etc. of the substrate were measured. Furthermore, a P-channel, Al-gate TPT was manufactured using these semiconductor substrates, and its mobility was measured.

Example I CW, Ar laser 10W1 scan speed 12c
TrL/see, ΔY (Y direction feed) - 40 μm 1 spot diameter 78, μm 1 Polysilicon substrate is annealed at a substrate temperature of 500 C, and plasma hydrogen treatment is performed at a pressure P of 1 to 2,0 Torr and a temperature T of 300 to 450 t:' Ta. FIG. 1 shows the relationship between hydrogen penetration depth and temperature in this substrate, FIG. 2 shows the relationship between penetration depth and output, and FIG. 3 shows the relationship between resistivity and ion implantation concentration. Figures 1 and 2
■ in the figure indicates only plasma hydrogen treatment (P = 0 in Figure 1)
．． 5 Torr, 15 hours, bait 2, T = 400? ? 1
4 hours), and (2) is one in which IOW, CWAr radiation annealing, and plasma hydrogen treatment were performed.

It is clear from FIGS. 1 and 2 that the penetration depth in case Q) is larger than in case ①. Also, in Figure 3, ■
1 is for plasma treatment only (300 tl?), 2 is for laser 7 annealing only, 2 is for laser annealing and hydrogen plasma treatment, ) is particularly improved in the low impurity concentration range.

In this example, the particle size of the sample subjected to laser annealing and plasma hydrogen treatment was 2 to 10 μm, and the particle size of sample 1, which was subjected to only plasma treatment, was 50A.

・1: Example 2 Above substrate (10WCWA r laser annealing,)'
= 0.5 Torr, temperature 1350C)
A channel AA' gate FET was fabricated. That is, 51 (
100) An oxide film of 300 OA was applied to the substrate, and a polysilicon film of 800 OA was applied thereon by low pressure CVD. P ion implantation was performed at 130 K e Vll Q cm, and laser annealing (IOW) was performed. Using this as a substrate, an oxide film 2700A is first formed, and 700 OA of PSGIII is formed thereon. Using this as a mask for forming the source and drain, P+ deposition is performed to form a junction. After that, the entire surface is scrubbed and an oxide film is formed again.
P and B leak prevention layers were formed on the field.

After that, a field oxide film of 1.1±0.
A gate oxide film of 1000IZ', ]050A is formed by etching the gate region and using the HCl oxidation method. After gate oxide film formation and channel implantation, Nt annealing was performed at 1000C, and plasma hydrogen treatment was performed at 350t'? , 0.5'['orr. Al
After forming electrodes by vapor deposition, sinking was performed to fabricate an FET. The channel length of this device is 8μ and the channel width is 16μ.
and = 95 Cm2/V-see.

Also, the temperature dependence of mobility is To・5 for the former and rr for the latter.
It was found that it has a temperature dependence of I. For a base plate with only plasma processing that does not require laser processing, a h = 1~
It was 10 Cm2/v-geki.

As described above, the processing method of the present invention is effective in processing FET substrates, and an industrially excellent semiconductor substrate can be obtained. Of course, this processing method can be applied not only to FETs but also to substrates of polysilicon solar cells.

[Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between hydrogen penetration depth and treatment temperature in the treatment of the present invention, Figure 2 is a diagram showing the relationship between hydrogen penetration depth and pressure, and Figure 3 is also a diagram showing the relationship between specific resistance and impurity. FIG. 3 is a diagram showing the relationship with concentration. Applicant's agent: Patent Attorney Tosuzu E Nin Hikosai 3-B P-Nogi (cm')

Claims (3)

[Claims] (1) After depositing a polysilicon thin film on a substrate, it is annealed using laser, electron beam, etc., and hydrogen activated atoms and molecules excited by microwaves diffuse hydrogen into the grain boundaries of polysilicon, forming polysilicon 11. A method for improving a polysilicon thin film semiconductor, characterized by improving the electrical conductivity of a conductor. (2) The hydrogen activation treatment is performed at a reaction temperature of 200C~
500C1 pressure 0.2-2. The method for improving a polysilicon thin film semiconductor according to claim 1, which is carried out under the condition of Q'i'orr. (3) The method for improving a polysilicon thin film semiconductor according to claim 1, wherein the hydrogen activation treatment involves exciting hydrogen with microwaves and guiding atomic hydrogen to a reactor for treatment.