Chang-Jin Sohn

Successful application of angular scatterometry to process control in sub-100-nm DRAM device

As DRAM (Dynamic Random Access Memory) device continuously decreases in chip size, an increased speed and more accurate metrology technique is needed to measure CD (critical dimension), film thickness and vertical profile. Scatterometry is an optical metrology technique based on the analysis of scattered (or diffracted) light from periodic line and space grating and uses 2θ angular method (ACCENT Optical Technologies CDS-200). When a light source is irradiated into the periodic pattern, the scattered intensity signal of zero-th order as a function of incident angle is measured. By analyzing these scattered signals, various parameters of the periodic pattern such as CD, vertical profile, mapping of substrate structure, film thickness and sidewall angle can be determined. Advantages of scatterometry are that drastic decreased measuring time and acquirement of CD, vertical profile, film thickness and sidewall angle by just one measurement. In this paper we will discuss various applications of scatterometry to sub-100nm DRAM structures of straight line and space and curved line and space patterns. Details of the correlation with CD-SEM (Scanning Electron Microscope) of standard metrology tool and repeatability of measured CD values will be discussed. As diverse applications, results of in-field, in-wafer and wafer-to-wafer CD monitoring, STI (Shallow Trench Isolation) depth monitoring and matching of vertical profile with V-SEM (Vertical SEM) will be also presented.

Optical proximity correction using a transmittance-controlled mask (TCM)

When small feature delineation is considered using existing exposure tools, special techniques might be needed such as phase shift mask, oblique illumination, top surface imaging, etc. When different types of patterns exist simultaneously or island patterns exist predominantly, optical proximity effect will become more important to be controlled. In this study, six different mask types were prepared and evaluated in view of a pattern fidelity and process latitude for 256 mega bit DRAMs storage node patterns. The masks used for this experiment were conventional transmission mask, serif patterned mask, square patterned Transmittance Controlled Mask (TCM), horizontally rectangular TCM, vertically rectangular TCM, and cross patterned TCM. The cross patterned TCM had three different transmittance on it and was evaluated also. In view of both pattern fidelity and process latitude, cross-TCM showed the best result. The vert-TCM also showed fairly good result. But the worst results always came from the conventional mask. From plane surface area point of view, once serif mask or TCMs are used, the areas always improved ranging from 120% to 145% at the best focus condition compared to the convention mask. There was not so much difference among three different transmittance in view of pattern fidelity and process latitude. As one of candidates for optical proximity correction, since small serif delineation on mask level is not easy for devices with small features such as 1 giga bit DRAM or beyond, TCM is more promising which has much bigger and easily writable gray area.

Optical proximity correction of bit line pattern in DRAM devices

In the bit line patterns of high density DRAM, there has not been enough to process latitude because of the optical proximity effect. To correct this problem, we suggest TCM (transmittance controlled mask) as a sort of optical proximity correction which has the same pattern of mask with the controlled transmittance. The parameters of established mask including transmittance and bulge size were decided by simulation. After evaluating the aerial image measurement system, wafer was evaluated to exposure tool with i-line exposure source. As a result, application of TCM can improve the overlay margin more than normal mask and DOF with 0.4micrometers as compared with normal mask.

Fidelity comparison of island patterns with different types of illuminations and phase shift masks

Since island patterns are subject to optical proximity effect, it is not easy to maintain island patterns as good image as same sized periodic lines with a conventional lithographic method such as a chrome transmission mask and on-axis illumination. Five different approaches, including a conventional mask with a conventional illumination, an alternating PSM, a half-tone PSM, a quadruple illumination, and a quadruple illumination with a half-tone PSM, were evaluated to compare pattern fidelity, meaning overlay tolerance, as well as process latitude. Conventional critical dimension (CD) measurement proved not to be a proper method as the criteria of the pattern fidelity. It is found that measurement of the shortest length of the island pattern, regardless of measurement orientation, is more meaningful. Under the condition that the dot size should be as small as possible without generating of any bridging between dots, the alternating PSM showed the widest process latitude. None of the methods except for the alternating PSM showed better result than the conventional method. From this study it is found that only the alternating PSM can substitute the conventional technique.

Overcoming global topography and improving lithographic performance using a transmittance controlled mask

As device density increases, topography gets more severe and optical proximity effect becomes worse. If light intensity can be controlled for individual patterns, linewidth variation over topography and optical proximity effect can also be minimized. A new method to solve these problems named transmittance controlled mask (TCM) is proposed. TCM is such a mask that thin absorptive films remain on the areas where light attenuation is necessary. In this paper, TCM is prepared and evaluated in view of topography and optical proximity effect improvement as well as process latitude improvement. Greatly improved process latitude was observed over 1.5 micrometers aluminum topography with TCM, while no process latitude was obtained with conventional masks even over 0.9 micrometers topography. Good optical proximity control is also possible with TCM.

Circuit design: emphasis on mask design and specification

In the past, the wafer pattern dimension was same as the mask pattern dimension because 1x exposure tool was widely used until 64KDRAM era. With the introduction of 5x stepper, required mask pattern dimension became five times larger and writing area was reduced as much. Recently, 64MDRAM and 256MDRAM whose feature sizes are smaller than 0.4μm and required patterning area is wider, require finer geometry and larger patterning area especially considering 4x exposure tools. In order to lighten both lithography and mask process burdens, new design techniques are considered for new DRAM generations such as COB (capacitor on bit line), IDB(inter-digit bit line) and SWD(split word line driver). Especially with the improvement of lithography technology using techniques such as phase shift mask and off axis illumination, optical proximity effect is unavoidable by nature. In order to correct this proximity effect, square cornered fine patterns well below the resolution limit of exposure tool are desirable. In this review, detailed discussion of new technology will not be discussed but related mask making requirement will be discussed instead.