Magento – Fatal error: Class ‘Mage_Screen_Helper_Data’ not found in /app/Mage.php on line 546

I am developing a small custom module. While creating an admin screen under system->configuration, I am getting following error Fatal error: Class 'Mage_Screen_Helper_Data' not found in /app/Mage.php on line 546

I checked my code but nothing seems wrong. Below is the code of my config.xml

    <?xml version="1.0"?>
<config>
    <modules>
        <Study_Screen>
            <version>0.1.0<version>
        </Study_Screen>
    </modules>
    <global>
        <models>
            <screen>
                <class>Study_Screen_Model</class>
            </screen>
        </models>
        <helpers>
            <screen>
                <class>Study_Screen_Helper</class>
            </screen>
        </helpers>
        <blocks>
            <screen>
                <class>Study_Screen_Block</class>
            </screen>
        </blocks>
    </global>
    <adminhtml>
        <acl>
            <resources>
                <admin>
                    <children>
                        <system>
                            <children>
                                <config>
                                    <children>
                                        <screen translate="title" module="screen">
                                            <title>Screen Settings</title>
                                        </screen>
                                    </children>
                                </config>
                            </children>
                        </system>
                    </children>
                </admin>
            </resources>
        </acl>
    </adminhtml>
</config>

Next I created Data.php in Helper folder. The code for Data.php is

 <?php
class Study_Screen_Helper_Data extends Mage_Core_Helper_Data
{
}
?>

and at last i created system.xml in etc folder. Code for system.xml is

<?xml version="1.0"?>
<config>
    <sections>
        <screen translate="label" module="screen">
            <label>Screen</label>
            <tab>sales</tab>
            <frontend_type>text</frontend_type>
            <sort_order>1000</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>1</show_in_store>
        </screen>
    </sections>
</config>

Everything seems to be fine. I can't figure out the error. Any help will be appreciated.