Abstract:
There is a method for making a strontium titanate based Grain-Boundary Barrier Layer Capacitors (GBBLC) with improved dielectric properties by introducing an intermediate step of heat-treatment in an oxidizing atmosphere between sintering of a strontium titanate based powder in a reducing atmosphere and an infiltration of the obtained sintered body with a metal oxide.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method for making a strontium titanate based Grain-Boundary Barrier Layer Capacitor (GBBLC). More particularly, it relates to a method for making strontium titanate based GBBLC with improved dielectric properties by introducing an additional heat-treatment step during the fabrication process. 
     2. Description of the Prior Art 
     There have been efforts to develop capacitors to meet high capacitance and small size needed for low voltage circuitry. Strontium titanate based GBBLC has a high dielectric constant because the thin grain boundary barrier layers formed between semiconductive grains function as dielectric layers. Thus strontium titanate based GBBLC is a passive component widely applied to electronic circuits. Strontium titanate based GBBLC is prepared by sintering a powder mixture of strontium titanate and n-type dopants, such as Nb 2  O 5  and La 2  O 3  [See, M. Fujimoto and W. D. Kingery, J. Am. Ceram. Soc., 68[4] 169-73 (1985)] in a reducing atmosphere, and coating the sintered bodies with powdery metal oxides with low melting points, such as PbO, Bi 2  O 3 , and CuO, and then annealing the coated bodies in an oxidizing atmosphere to form a thin insulating layer between SrTiO 3  grains [See, Franken et al., J. Am. Ceram. Soc., 64[12], 687(1981)]. This fabrication process leads to a very high effective dielectric constant. The electronic properties of strontium titanate based GBBLC thus strongly depend on the sintering and annealing conditions. Previous methods of making strontium titanate based GBBLC consists of sintering power compacts in a reducing atmosphere and infiltrating an oxide liquid glass into the sintered compacts in air. 
     SUMMARY OF THE INVENTION 
     The purpose of the present invention is to provide an advanced process of making strontium titanate based GBBLC with high effective dielectric-constant. The present invention is directed to an additional heat-treatment step of the sintered body in an oxidizing atmosphere before liquid infiltration. 
     DETAILED DESCRIPTION OF THE INVENTION 
     According to the present invention, the effective dielectric constant of GBBLC has been improved by introducing an intermediate step of heat-treatment in an oxidizing atmosphere between the sintering in a reducing atmosphere and the oxide infiltration in an oxidizing atmosphere. 
     The intermediate heat-treatment can be accomplished by reheating and annealing the specimen which was sintered in a reducing atmosphere in an oxidizing atmosphere. Another method of the intermediate heat-treatment is to replace the sintering atmosphere from a reducing atmosphere to an oxidizing atmosphere and then to anneal the compact at the end of the sintering cycle. After the intermediate heat-treatment in an oxidizing atmosphere, oxide infiltration is done in air according to conventional methods. 
     The process of the present invention is illustrated in more detail as follows: 
     1) To make compacts of doped strontium titanate powder with dopants such as Nb 2  O 5 , and to sinter the compacts at about 1400° C. to about 1500° C. for about 2 to about 7 hrs in a reducing atmosphere, such as a mixture of hydrogen and nitrogen, 
     2) After cooling the sintered specimen to room temperature, to reheat the specimen above about 1200° C. for up to about 4 hrs in an oxidizing atmosphere comprising air or oxygen. Alternatively, after sintering, to replace the reducing atmosphere with an oxidizing atmosphere comprising air or oxygen in the condition of at above about 1200 ° C. and for up to about 4 hrs before cooling to room temperature, 
     3) To infiltrate a low melting point oxide glass, such as CuO, PbO, Bi 2  O 3 , and MnO 2 , at about 900° C. to about 1300° C. in air into the heat-treated specimen. 
    
    
     The present invention will be illustrated in great detail by way of the following example. The example is presented for illustrative purpose only and should not be construed as limiting the invention which is properly delineated in the claims. 
     EXAMPLE 1 
     A group of specimens were prepared by the following procedures. Strontium titanate powder compacts doped with 0.2 mol% Nb 2  O 5  were sintered at 1480° C. for 5 h in a 5H 2  +95N 2  atmosphere. After cooling to room temperature, the specimens were reheated at 1200° C. to 1480° C. and annealed for 1 rain to 4 hrs in air. For the purpose of comparison, a standard specimen without heat-treatment was prepared by infiltrating CuO into the specimen sintered at 1480 ° C. for 5 hrs in a 5H 2  +95N 2  atmosphere. The heat-treated specimens and a standard specimen were infiltrated with CuO at 1200° C. for 1 hr in air. CuO-infiltrated specimens were pasted with an In-Ga electrodes alloy, and dielectric properties were measured by an impedance/gain-phase analyzer from 1 KHz to 1 MHz. 
     The results are shown in Table 1 below. 
     EXAMPLE 2 
     The second group of specimens were prepared by the following procedures. Strontium titanate powder compacts doped with 0.2 mol% Nb 2  O 5  were sintered at 1480 ° C. for 5 hrs in a 5H 2  +95N 2  atmosphere. Afterwards, the sintering atmosphere of 5H 2  +95N 2  was replaced with air or oxygen at 1200° C. to 1480° C. and annealed for 1 min to 4 hrs before cooling to room temperature. The heat-treated specimens were infiltrated with CuO at 1200° C. for 1 h in air. CuO-infiltrated specimens were pasted with an In-Ga electrodes alloy, and the dielectric property was measured by an impedance/gain-phase analyzer from 1 KHz to 1 MHz. 
     The results are shown in Table 1 below. As can be seen from Table 1, the effective dielectric constant was increased remarkably and the dielectric loss decreased by introducing the heat-treatment, compared with that of the standard specimen without heat-treatment. 
     
                                           TABLE 1__________________________________________________________________________Dielectric Properties of Strontium Titanate Based GBBLC withIntermediate Heat-treatmentHeat-Treatment        Heat-Treatment                  Effective Dielectric Constant, k.sub.eff                                      Dielectric Loss,                                      tanδ(%)Method       Condition 1 KHz                       10 KHz                            100 KHz                                 1 MHz                                      1 KHz                                           10 KHz                                                100                                                     1__________________________________________________________________________                                                     MHzafter        1200° C., air, 4 hr                  22,700                       20,600                            18,700                                 16,800                                      6.36 6.66  7.77                                                     13.07Sintering Cycle        1300° C., air, 2 hr                  23,700                       24,800                            22,300                                 20,400                                      7.68 7.74  7.60                                                     11.98        1400° C., air, 1 hr                  34,300                       30,200                            25,800                                 21,700                                      9.30 9.38 12.62                                                     22.74        1480° C., air, 5 min                  24,400                       21,200                            18,200                                 13,300                                      9.82 9.81 14.57                                                     16.56        1480° C., air, 0 hr                  23,900                       20,500                            17,500                                 12,900                                      9.16 10.70                                                14.69                                                     15.77during       1200° C., air, 4 hr                  21,900                       19,100                            18,200                                 16,400                                      8.67 9.84 10.88                                                     14.89Sintering Cycle        1400° C., air, 1 hr                  38,800                       34,700                            32,200                                 30,500                                      8.38 6.35  5.55                                                      8.89        1400° C., air, 1 hr                  28,800                       25,400                            23,000                                 20,200                                      11.22                                           8.73  7.99                                                     16.58        1480° C., air, 1 hr                  31,100                       28,400                            25,500                                 22,300                                      9.89 10.29                                                12.05                                                     14.52        1480° C., air, 0.5 hr                  36,600                       32,100                            29,500                                 27,800                                      7.97 6.85 6.04 11.55        1480° C., air, 0 hr                  15,500                       12,800                            11,300                                  9,800                                      8.24 11.12                                                16.07                                                     16.67Standard Specimen      13,800                       11,600                             9,100                                  7,300                                      10.00                                           14.12                                                16.45                                                     16.01without Heat-Treatment__________________________________________________________________________